The Difference between Reproduction and Replication

Usually, in everyday speech, we may use certain words interchangeably but in science words must have precise meaning in order to be scientifically useful. Two such words I’m referring to are “replication” and “reproduction” and at first these words may mean alike, that is both deal with increase in numbers.


However, in biology, these words refer to two different processes that occur in different levels. Indeed these terms do refer to increase in numbers but when you focus on a feature of life such as a cell, replication and reproduction are actually two different processes in that one occurs at the molecular level which is replication, and reproduction occurs at the level of the cell, whether it is an unicellular organism such as a bacterium or amoeba or a whole, multicellular organism such as a cat.


Now that we have a feel for the differences, let use look into detail of the two different processes of replication and reproduction.




All life on earth share a common biochemistry such as the twenty amino acids which form into proteins which can perform a variety of biochemical processes needed for the survival of life forms and a genetic code where the sequences of DNA specify an amino acid needed to form a protein. To survive, the instruction for building a cell must be passed on to the next generation of life forms, and this is true for all species of life, small and large.


Ever since 1953, with the discovery of the double helix of DNA, it was now possible to explain how traits for survival could be passed on and if we focus on the molecular level, that is the level of DNA, can we then see the difference between replication and replication.


What exactly is replication and what does it have to do with molecular systems such as DNA?


Replication involves making an exact copy or replica of a pre-existing template. This requires the original template plus components needed to be arranged to take an exact copy of whatever the template is.


Of all the biological molecules, it is the DNA or deoxyribonucleic acid that has this capability. DNA consists of nucleotides and these are the building blocks of DNA. A nucleotide consists of three parts; a phosphate group bonded to a ribose, a five carbon sugar, which is bonded to one of four nitrogen containing bases called adenine, thymine, cytosine, and guanine.


A nucleotide can bind to another nucleotide in what is called a phosphodiester bond and so along one side of a DNA molecule, is a uniform sequence of  phosphate and riboses but a base can form a bond called a hydrogen bond to another base but each base can pair with a specific base. An adenine, which is one of the nitrogen containing bases that I’ve mention in the previous paragraph, can bond to a thymine, and a guanine can bond to a cytosine.


When one polynucleotide or the many nucleotides in one strand of DNA meets another polynucleotide, the two strands, along with the specific base pairs, will wind around each forming the double helix.


The fact that DNA is a double helix allows it to replicate and in the sequence of bases which is the part of the DNA molecule that is variable, contains all the information for the synthesis off all the various kinds of proteins including the proteins that are needed to help the double helix make more double helices.


To do that, we must understand how this is done, even though in reality it is a complex process, but I will only present a simplified version of how DNA replicates and to show why replication is something that occurs at the molecular level.


Recall that to make a replica requires a template and a set of components. For DNA, the template is the sequence of bases present in the original DNA molecules and since the molecule is occurring in the basic fundamental unit of life, the cell, there are plenty of nucleotides available.


Here is where replications comes into play. To replicate a DNA molecule, first a set of enzymes or protein molecules that speed up biochemical reactions are needed. One such enzyme involved in DNA replication is aptly named the DNA polymerase and it’s function is to help DNA replicate into two DNA molecules.


Once the DNA polymerase is attached to the DNA, it begins to unravel apart the polynucleotides and carefully the enzyme takes in surrounding nucleotides and with each nucleotide, the enzyme carefully pairs the nucleotide along with each base to one of the bases in the template strand. For example, if the DNA polymerase encounters a thymine, it will select an adenine and pair and if it finds a cytosine then it will pair it with a guanine.


The DNA polymer continues in this fashion until another DNA molecule with the sequence from the original template strand is completed.


Thus we see that replication such as the DNA polymerases and DNA is something that occurs at the molecular level and this fulfills the definition of replication. The template in this case is the sequence of the parent DNA and the components are the available nucleotides. Indeed this process of molecular replication can continue indefinitely, even though some errors in base pairs do occur, but the DNA polymerase is very efficient that in a billion replications only one DNA molecule may have a base pair slightly different than the original.




Reproduction, generally defined, involves producing likeness from the original, just like the sequence of nucleotides from the original polynucleotide sequence but this is something that occurs in a whole and complex system and it is much more involved than molecular replication.


Reproduction then occurs at the fundamental unit of life and what exactly the fundamental unit of life for reproduction to occur? That would be the cells and all processes of life such as reproduction occur at the level of the cell.


A cell is enclosed by a thin plasma membrane and inside the cell is the cytoplasm, a jelly like water based substance with all the molecules of life such as amino acids for proteins, nucleotides for DNA and a similar nucleic acid, RNA, as well as glucose for sugars and fatty acids for fats.


Cells come in two main forms. There are the prokaryotic cells and these are cells that only have a cell membrane and another outer covering called a cell wall that is made out of a protein carbohydrate structure and inside a prokaryotic cells are all the molecules of life and the DNA is arranged in a localized region of the cell in the form of chromosomes. The other kind of cell are the eukaryotic cells and these are more complex than prokaryotes in that a eukaryotic cell consists of a nucleus which is the region of the eukaryotic cell that houses the cell’s genetic information and the chromosomes are bound by proteins called histones. The inside of the eukaryotic cell has a variety of subcellular structures called organelles which carry out various kinds of biochemical functions. There are the mitochondria which are found in plant and animal cells and are both eukaryotes which release biochemical energy in a process called metabolism. There are the lysosomes which house enzymes to degrading parts of the cell, the Golgi bodies which package proteins for transport out of the cell, as examples.


Reproduction then occurs at the cellular level for all the biological molecules of many kinds making up the cell are involved. To make an two cells, for example, requires the cell membrane to pinch into two and at the same time both of the cells receives an equal amount of cytoplasm and an equal amount of all the molecules.


The new cells then are alike from the original cells but like replication it is not a perfect process and indeed there can be a new cell that is slightly different than the original but it will in some respects resemble the original cell.


Since there are two main kinds of cells, the process of reproduction will be different. In prokaryotes, they reproduce by binary fission and this is a “simple” form of cellular reproduction where both the cell membrane and cell wall split in two while at the same time the cytoplasm along with all the molecules and the chromosomes are parceled into two new cells, resulting in two cells, identical to the original more or less.


In eukaryotes a much complicated form of cellular reproduction called mitosis is where cell divide in a controlled step by step fashion, In mitosis, the chromosomes in the nucleus begin to replicate (at the molecular level of DNA) and condense while the nuclear membrane breaks apart. There are specialized organelles called centrosomes that produce protein strand that attach to each chromosome and there are two sets of centrosomes at both ends.


An eukaryotic chromosome consists of two pairs of chromosomes and each centrosomes attaches its protein strand to each end of each chromosome. The whole cell then divides at the cell membrane and as this division continues two new cells with identical chromosomes as well as all the molecules and organelles are partitioned into two daughter cells.


As you can see, there is indeed a difference between replication which only occurs at the level of molecules while reproductions requires the whole cell.


Also in eukaryotic cells, reproduction is taken to another level and there is another form of cell division called meiosis. Like mitosis, this involves cell division but there is a huge and crucial difference.


In meiosis, a cell has a pair of chromosomes called the maternal and paternal chromosomes and in the process of meiosis, each pair of chromosomes pairs with another pair and undergoes what is called recombination where parts of one of the chromosomes exchange another part from the other pair resulting in a different combination. These chromosome hybrids are then distributed into other cells but the number of chromosomes is halved, unlike in mitosis where the exact number of chromosomes is identical to the parent cell.


The result is cells called gametes which have half the number of chromosomes and only in meiosis is where recombination and halving of chromosomes and this kind of cellular division is only present in multicellular eukaryotic organisms.


I’ve mentioned that in both reproduction and replication, these are imperfect processes and a different kind of resultant cell can form. This kind of change is known in biology as a mutation and a mutant cell, or multicellular organism may be different than its parents and if this difference, affects the survival of the organism in such a way that it would have an advantage in comparison to those offspring that are identical, it may be able to survive in a varying environment and will end passing its new survival characteristics to its offspring.


This is the basis of Darwinian evolution and this depends on the reproductive process being imperfect.




Replication occurs at the molecular level while reproductions involves the organism as a whole. Of course in a reproducing cell, the molecules have to replicate whether in binary fission or mitosis in order for the two cells to receive (almost) identical copies of molecules. Replication and reproduction, both different ways of increasing numbers, are involved in allowing life to propagate, survive, and adapt.




Fleishaker, G.R (1994).Self-Production of Supramolecular Structures (eds.). The Netherlands: Kluwer Academic Publishers.








What is the Minimal Size of Life?

Life comes in many shapes and sizes. There are the insects that scurry over the ground and fly through the air. The fishes that swim in the sea, various kinds of birds, and large mammals ranging from lions to whales. There are also small organisms such as bacteria present in the water, air, and soil.


What are the ranges of size of organisms? Starting from the big to small, and to be more scientific, measurements are done using the metric system and the unit of measurement for length is the meter. Organisms are measured, starting with the smallest in micrometers, a unit of measurement that is used for microorganisms which can be seen only with a microscope. Going up, in terms of size, are the organisms that can be measured in millimeters and centimeters and these include small insects, small mammals, and birds and these are the kind of organisms we see with our eyes and then we proceed up to the bigger organisms which includes the blue whale and the biggest plant which is a tree called the redwood.


Still the question to ask is what is the smallest organism known to biology that can perform all life functions? Before we find the smallest organism we must first review the characterstics of life , regardless of the size of organisms and I have written some blogs about what all life has in common and for those of you haven’t read my earlier blogs, I will summarize all that life has in common before we find the smallest organisms that is capable of carrying out all these functions.


The Many Characteristics Shared by all of Life




From a human being composed of many organ systems , each of which has a specific function such as the circulatory system delivering oxygenated blood and nutrients to all body cells, a digestive system for breaking down large food molecules into their simpler components, and  a respiratory system taking in oxygen to all the way down to unicellular organisms such as an amoeba where all the functions of biochemistry are in one cell, life has a degree of organization and indeed there is nothing random at all in the organization of the components for each of them has a specific function or else it would cease being what we would recognize as life.




Living things are constantly surviving, and depending on the species, it could be the birth, life, and death of an individual multicellular organism or a single cell organism. Both of which have a variety of biochemical processes from the cellular to organ level that involves taking in food and producing wastes, maintaining a body temperature, and coping with changes in the environment. Collectively these various process define homeostasis or maintaining a core set of physiological processes to deal with changes in the environment.




One of the most important characteristics of life is its ability to form another (near) perfect copy of itself and life has been doing this, in what is called reproduction, for 3 billion years of the earth’s history. There are two forms of reproduction. One ( that we all know too well!) is sexual reproduction and this involves, at the cellular level, a process called meiosis where two cells, or gametes, are formed with half the number of chromosomes which are subcellular structures consisting of DNA and protein and these gametes with half the number of chromosomes are combined together through fertilization to form the complete organism that will develop into an organism that is almost like the parents. I say “almost” because there is no way that the process of replicating DNA is perfect and indeed such imperfections are what are called mutations and mutations can either be harmful or beneficial and such tiny changes could gradually result in a population of organisms different from the ancestral population ( I will talk about this in detail shortly).




It is accurate to consider life more as a verb than a noun. Life can do a lot such as in animals that swim, fly, or run and also of plants that break through the soil, spread  leaves and in the case of angiosperms or flowering plants, unfurl their petals. All of this requires energy. Organisms do not create energy out of nothing. The energy comes somewhere and in another blog where I talk about life and energy, there are two sources of energy, one is sunlight coming from the sun and the other geochemical energy near hydrothermal vents. These two forms of energy are used by those organisms called autotrophs and they are organisms that can one form of energy to combine simple inorganic compounds into complex molecules. Plants are considered autotrophic because they carry out photosynthesis where energy in the form of sunlight is use to synthesize carbohydrates, a complex organic molecule using water and carbon dioxide. In the case of life near hydrothermal vents, this would be chemical energy using minerals such as iron sulfide in what is called chemosynthesis.


In addition to autotrophs, there are the other class of organism called heterotrophs and these organisms need organic molecules to survive. Animals are the perfect example of heterotrophs in that they actively search for food which consists of carbohydrates, proteins, and fats and derive energy by breaking them down and utilizing the building blocks of the molecules which in carbohydrates is glucose, amino acids from proteins, and fatty acids and glycerols from fats.


Photosynthesis is an example of a form of metabolism called anabolism and this involves building up complex molecules from simpler components. Another example that is important and universal in all of life (as we know it, so far) is protein synthesis and this is where protein molecules are built up from amino acids. Both of these processes involve an input of energy and for the former this is electromagnetic radiation in the form of visible light and the latter a source of chemical energy called adenosine triphosphate or ATP, a unit of energy used for every anabolic process.


The opposite of photosynthesis, respiration takes carbon and hydrogen rich molecules and in a complex series of processes together with oxygen, a waste gas released from photosynthesis,  produce carbon dioxide and water as waste and energy locked up in ATP. With the ATP, which is found in all of life, is the source of energy needed for anabolism. The breakdown of molecules is known as catabolism and respiration is a form of catabolism.




Many organisms are observed to show an increase in size and this is apparent in the plant and animal kingdom. A chick hatches from an egg which developed from an embryo inside the egg as a result of sperm and egg uniting together and the chick then grows up to a chicken. Likewise , a seed from a tree, in the soil, together with water and nutrients will grow up to become a tree with leaves. This depends ultimately on the cells and for eukaryotic organisms or cells with nuclei, one cell becomes two in a complex process called mitosis and even though it is a splitting of cells, the process anything but simple and there are several stages that a cell must go through in order to divide. This is the main reason for the growth of an organism from small to large.


Response and Stimuli


Any organism is never in complete isolation from its environment. An organism such as a bacterium must be able to sense what is present outside of it, whether there is glucose, a food source, or an antibiotic, a poison. These are the stimuli that will allow the cell to move towards for the former or away if the latter is present. Whatever stimuli the single cell senses, it will execute an appropriate series of behavior which are the responses. The behaviors and methods of sensing stimuli is much more sophisticated for multicellular organisms such as animals with a nervous system for taking in various kinds of senses such as sight and smell and with a muscular system for movement.





When life appeared on the planet about around 3.8 billion years ago, it was simple and unicellular but as the fossil records, many different life forms appeared throughout the history of planet earth, many of which became extinct while other species survived. Why is that? It is because organisms have a set of characteristics that allowed them to live in their environment and as long as the environment is stable, it is likely but by no means certain, that any species of life form will survive in its environment. This is know as adaptation and there is a mechanism, first realized by Charles Darwin, called natural selection where the environment selects those characteristics for survival and if in each reproductive cycle of each species, an organism has features that allow it to survive in its native environment, then natural selection will allow the organism to survive into the next generation whereas if in each generation , the offspring does not have these kinds of adaptations, then they will likely die. It is this process of filtering out those that can adapt, that gradually many organisms were able to exploit whatever natural resource which by this I mean, the presence of food and space for living and mating and many , if not all organisms, become more complex in organization and even behavior than there predecessors resulting in different species, some of which have survived but others, because of changing environments went extinct.



From the Large to the Small


Now that we have reviewed the major characteristics of life, we know begin to ask a question that is central to this blog and the question is “What are the kinds of life forms that are the largest to the smallest?” By asking this question, we eventually get to the main point of what the smallest organism is that has all the characteristics of life.


Starting from the largest organism, I will also explain why the laws of nature can permit as well constrain what the biggest organism can be and likewise I will do the same for the smallest organism as well.


To start, and keeping with the accuracy and precision that is characteristic of science, I will use the measurement of length, the meter, when expressing size.


Let’s begin with the kingdom of life forms that we all know so well and that’s the animal kingdom. We must ask “What is the largest animal that we know?”


The answer is a marine species adapted to life in the oceans and it is the mammal, the Blue whale. What is the average size of the blue whale? The average size is 24 meters or 79 feet. Compare this size to other whale species and the second largest below that of the blue whale, the North Pacific right whale. It’s size is 15.5 meters or 51 feet. The third largest below that of the North Pacific whale, the Southern Right whale, measures close to15.25 meters or 50 feet.


I have compared the sizes of three whale species , all of which live in the ocean. What about land animals? What is the largest land animal? That turns out to be the African Bush elephant with a length of 6 meters or 19.7 feet.


So much for large living animals. What about plants? What is the largest plant on record? That would be the aptly named giant sequoia and it measures on average to 70 to 85 meters in height or 230-280 feet.


Starting from the largest life form, is the giant sequoia and the second largest life form, the blue whale all the way to the African bush elephant. We then proceed to the small organisms that includes small fish to small frog down to small plants all the way down to life forms so small you need a microscope to see them and then we proceed from meters to centimeters, millimeters and down to the unit of measurement that is used in the field of microscopy, the micrometer which is a millionth of a meter.


At this level, we see the cells, and depending on the cell, we may come across an amoeba which is a unicellular organism known for its ability to shape shift because an amoeba is an active hunter and must consume bacteria in order to surviving. Another is the paramecium and it is an oval shaped cell surrounded by tiny hairs called cilia and with these cilia, this allows the cell to move quickly through water. Both amoeba and paramecia are fine examples of unicellelular eukaryotes and these are cells with nuclei and specialized structures called organelles which carry out different kind of biochemical functions. Amoeba and paramecia belong to the kingdom Protista and this is the kingdom of unicellular eukaryotes but recent methods of molecular biology have revealed that are even multicellular protists as well but we won’t get into that.


Eukaryotic cells, because of their complex structure tend to be much larger than another set of cells, which are the prokaryotes. Prokaryotes are the opposite of eukaryotes and that is because they have no nucleus and no organelles. The DNA is arranged in chromosomes like eukaryotes but the difference is that in eukaryotes the DNA is wrapped around in special proteins called histones and that results in a complex organization of the chromosomes whereas in prokaryotes it is only DNA. In both eukaryotes and prokaryotes, both of these cells have what are called ribosomes and these are protein RNA complexes where proteins needed by cells are synthesized and even there is difference in size. Ribosomes in eukaryotes are much larger than ribosomes in prokaryotes.


Despite their different organization, prokaryotes are nonetheless capable of all the characteristics of life so what is the smallest prokaryote, the one capable of all life functions?


From Mycoplasma to Nanoarcheum


Since prokaryotes are much simpler in terms of cell structure compared to eukaryotes, the class of bacteria that have the smallest size are the Mycoplasmas, more specifically a bacterium called Mycoplasma genitalium, a disease causing bacteria, was at first reported to be a smallest organism known to biology capable of carrying all the properties of life and there is one way of quantifying the size in terms of the organism’s genome.


Recall that in the cell structure, all cells possese DNA as the carrier of genetic information. DNA is present in both eukaryotes and prokaryotes and it is the sequence of base pairs or when adenine pairs with thymine, and guanine pairs with cytosine , is what determines the genetic information.


Indeed there is a quantifiable unit, the base pair, which can be used to determine the size of the genome and that is all the organism’s genes. There is even a rough if not exact correlation between the complexity of an organism and the size of the genome although there is no exact correlation between the size of genome and the complexity of the organism. An example is that some organisms have even more DNA than humans. In fact, in the case of eukaryotic DNA, much of the DNA doesn’t code for any proteins and is aptly named “junk DNA”.


If we consider DNA that codes for proteins while ignoring the rest that does not, then as we proceed from eukaryotes where the DNA in chromosomes are divided into regions called introns which are the genes that do not code for proteins and the exons where the genes do, then as we move towards the prokaryotes there are more exons and less , if any introns.


The size of the genome can be measured in terms of the number of base pairs and one such unit of measurement is called megabase pairs or Mbp for short and it is equal to a million base pairs.


If we use that criteria for judging the size of prokaryotes then what is the size of the genome of Mycoplasma or rather we should be asking: What is the smallest genome size that an organism posseses that is still capable of performing all the key requirements for the definition of life?


The answer turns out to be within the range of 525 genes and since each gene that codes for proteins requires no more than 3 bases, which are any of the four bases, adenine, thymine, guanine, and cytosine, the total number of base pairs is 580, 070 or the total number that codes for all proteins needed for protein synthesis, DNA replication, in short all that is needed to satisfy the requirements of life , so from the perspective of base pairs, this would make  this life form the smallest form of life but is it really the smallest life form known so far?


In 2003, another smaller microbe with a somewhat smaller number of genes and still with the capacity to perform all the seven characteristics of life was discovered and named Nanoarchaeum equitans. The base pairs for Nanoarchaeum is 490,885 base pairs. Given the relatively small number of base pairs than Mycoplasma genitalium, and like Mycoplasma genitalium, it is the only Archaen or prokaryote adapted to extreme environments that is a parasite and must depend on another Archaen, Ignicoccus, for survival since because of its small genome, it doesn’t have enough genes to carry out most of other vital functions but must be dependent on another.


What about organisms with even smaller genomes? They wouldn’t be free living organisms but they would be known as viruses and these organisms live between the living and non living in that on their own, they are incapable of carrying out all life functions but depend on other organisms to do so.




It seems that organisms within the base pair ranges of 587,070 to 490,885 is the limit by which they can carry out the necessary biological functions but they are more likely to be parasitic in that due to the size of their genomes, they do not have enough genes to live a fully independent life and for Mycoplasm genitalium with a total of 525 genes, must resort to living on cells of another species, in this case human, and for Nanoarchaeum, an even smaller genome size forces it to depend totally on another living cell.


Since all of life can be (almost) traced to a single universal ancestor and if it was a small unicellular organism composed of a single cell, its genome would likely to be much larger than 587,070 base pairs  but no larger and through evolution, the base pairs would up increasing whenever each new species evolved into various kinds of habitats. Life had small beginnings.





Eigen, M (1992) Steps towards Life: A Perspective on Evolution. (Wooley, P, trans). Oxford, England: Oxford University Press


Fraser, C, Gocayne, J.D, White,O, Adams, M, Clayton, R, et al. (1995). The Minimal Gene Complement of Mycoplasm Genitalium. Science, 270, doi:10.1126/science.270.5235.397


Genome size (n.d). Retrieved August 21, 2017, from


Largest organisms. (n.d). Retrieved August 8, 2017, from


Morowitz, H. J. (1992) Beginnings of Cellular Life: Metabolism Recapitulates Biogenesis. Binghampton, NY: Yale University Press


Mycloplasma genitalium (n.d). Retrieved August 21, 2017, from


Waters, E et al. (2003). The genome of Nanoarchaeum equitans: Insights into early archael evolution and derived parasitism. Proceedings of the National Academy of Science, 22, 12984-12988. doi: 10.1073/pnas 1735403100





Antoine Laurent Lavoisier: The First Biochemist


Antoine Laurent Lavoisier and his wife. Lavoisier helped established the science of chemistry and he even made contributions to biochemistry (ArtGallersErgArt-By Ergs Art)



Chemistry which studies how atoms and molecules interact to form new and different substances is subdivided into various fields and one of which biochemistry studies how living organisms synthesize and use carbon and hydrogen based molecules needed for various kinds of life processes.


All the fields of chemistry depend on emphasis of precise quantitative measurements and these include measuring substances before and after a chemical reaction and this includes measuring the weights of the substances, finding the temperature if heat is released and/or absorbed, and even determining the molecular structure of the substances involved.


Chemistry, as a science, began with the work of a brilliant French scientist Antoine Laurent Lavoisier and he not only established the quantitative nature of chemistry while emphasizing the qualitative,  he even did experiments involving organisms which, according to this blog, makes him the first biochemist.


Antoine Laurent Lavoiser was born in Paris, France to wealthy parents and  because of his parent’s wealth, he had no trouble receiving a fine education. Originally he was meant to study law but he found science more to his taste and he pursued it with passion.


After completing his education in science, Lavoisier turned to chemistry and in the course of his experiments he emphasized careful weighing of chemical substances called reagents which are any substances before a reaction and also he weighted the products which are those after a chemical reaction.


It was his meticulous observations of accurate measurements that helped established the quantitative nature of chemistry and it was thanks to his determined effort that he had helped pushed chemistry as an quantitative science and on par with that other quantitative science, physics.


Thanks to his method of emphasizing careful weighings, he was able to solve two problems of chemistry, one involved the process of combustion and the other that water was not a chemical element as was once previously believed but a chemical compound or a combination of two or more different chemical elements.


Not surprisingly, chemistry under Lavoiser along with his British contemporaries, Joseph Priestly, who was one of the first to discover oxygen and Henry Cavendish, who isolated hydrogen was becoming a science in what is known as the chemical revolution.


Shortly after Lavoisier completed his science education, he was elected to the French Academy of Science on the grounds of excellent notes detailing his experiments and after that, his scientific researches were put to practical as well as academic use and being of noble birth, he was granted access to what was then prominent organizations prior to the French Revolution and one such organization was the Ferme générale  which was a private organization that collected money for the government under France’s monarchy. Lavoisier used the money to help fund his research and because this organization was part of the government’s tax collection, this made him and others involved a target of France’s Reign of Terror and it was because of this that Lavoisier’s career in science ended when he lost his head to the guillotine.


While as a member of the French Academy of Science and the Ferme générale , Lavoiser was involved in various projects and one such project involved implementation  of street lighting and Lavoiser showed no hesitation in leading the project.


To improve street lamps , which in those days before electricity, were the only source of light then, Lavoiser had to know about the process of combustion where certain substances burn releasing light and heat in the presence of gases, Lavoiser began the task of investigating combustion and through his experiments, he found that combustion involved the gas oxygen and he also found out a similar process occurring in animals but much more slowly and with less heat.


What were the steps that Lavoiser took to prove that oxygen is responsible for combustion? To understand how he arrived at that conclusion we must first emphasize that Lavoisier discovered an important principle in chemistry as well as in physics which is called the conservation of mass and this is simply that mass is always constant in a chemical reaction that is the properties of whether elements like oxygen and hydrogen may be different before and after a reaction where with oxygen and hydrogen, the result is water, the masses of the atoms remain constant.


An example to illustrate this is when wood burns and produce ashes and gas. If a piece of wood is placed in a closed vessel in the presence of oxygen and is allowed to produce ashes and gases, the wood may be different but since this reaction occurred in a closed vessel where no other gases were allowed to enter and escaped, the total amount of molecules and atoms are the same before and after. It is just that the molecules are arranged differently resulting in substances with different properties.


Lavoisier not only measured the weights of liquids and solids undergoing reactions he even measured the weights of gases. Prior to Lavoisier and beginning with ancient Greek speculation, it was thought that the matter making up the physical universe could be reduced to just four simpler “elements” which are fire, water, air, and earth.


Any combination of these resulted in the various kinds of substances we see but Lavoiser challenged this old theory and proved that “earth” is really a combination of various kinds of solid elements such as copper, iron, and sulfur and that water, which was first revealed by Lavoisier is a combination of two gaseous elements, oxygen and hydrogen.


For “proof” behind the ancient Greek idea, if a glass vessel of water is heated to boiling, some visible material may be noticed and according to the four element model, water was turning to earth. Lavoisier disproved this with his principle of mass by accurate weighing, and in an experiment where a vessel with a measured amount of water was heated for 101 days, he found that the glass vessel weighed the same before and after. Had it been true that water turns to earth, there would have noticeable changes in the vessel’s weight where the glass vessel would have a different weight after the heating.


The conclusion? The weight of the vessel was equal to the weight of the amount of visible material being released and it was from this that the visible material precipitated from the glass itself after long periods of heating.


Lavoisier challenged another theory, one that was previously put forward to explain combustion and that was the phlogiston theory.


Phlogiston theory states that substances such as wood burns because the wood is releasing a substance called phlogiston in the process of combustion and in addition, phlogiston theory was applied to the rusting of metals, or in modern terms to the formation of oxides ( this process, at the time was called “calx formation”)


As far as rusting goes, when a metal loses phlogiston, there should be a decrease in weight but when measurements are applied to rusting metals, the opposite is seen to occur. Metals gain weight but for those chemists who were working under phlogiston theory, they had to assume that phlogiston had negative weight!


Lavoisier approached this problem by first performing experiments using a combination of careful weighing and with the use of closed vessels. Before Lavoisier, phlogiston theory only created more confusion when interpreting the results regarding the increase of weight of metals after rusting (or “calxing”) and it was very likely that the confusion stem because the experiments were rather sloppy to begin with.


In 1772, Lavoisier approached the problem of combustion by first heating a sample of phosphor and he repeated the same for sulfur and found that after combustion, both solids increased in mass. The increase in mass was observed for the two metals lead and tin.  How was that possible? Lavoisier came to the conclusion that since phosphor, sulfur, tin, and lead were  were burning in air and not in a vacuum, and that like the sulfur and phosphour after combustion, tin and lead increased in mass , the only way to plausibly explain this was that the two metals and non metals were reacting with something in the air. Just what it was in the air that resulted in the increase in mass where the phlogiston theory should predict the opposite?


The solution to the problem occurred when doing his experiment on calx formation of tin, that the gain of weight observed occurred after Lavoiser opened the vessel. As air rushed in, the amount of mass gained resulted because of outside air rushing in! This was the case for the amount of mass observed after combustion. Recall the conservation of mass, where matter before and after a reaction is constant. If that principle is correct, then how can there be an increase of mass after the reaction? That is because when the vessel is open after burning, do you have the increase in weight.


From the conservation principle, there should be no increase in weight as well as a decrease in weight so when the vessels are closed and measured before and after, the total weight is constant and that is what Lavoisier found.


From these studies, Lavoisier concluded that combustion and calx formation or in modern terms oxide formation are the same processes since both metals and non metals produce new substances when burned in air.


What was in the air that supported both combustion and oxide formation? He proceeded to determine the composition of air and his first clue came by studying the research of two British scientists, first by the chemist Joseph Black.


Black did analysis of chemical reactions involving chalk or calcium carbonate and that after a reaction, it gave of a gas that Black called “fixed air” which is now known as carbon dioxide, an end product of combustion. In regards to fixed air , this gas is a constituent of the atmosphere and it does not support combustion. Lavoisier at first thought that carbon dioxide may explain the weight gain observed in calx formation.


Lavoisier did his experiments using closed vessels with air and metals. There was no doubt that in the presence of air, there is that weight gain but the question was , was it all of the air and some part of the air? Lavoisier  came across the work performed by another British scientist, Joseph Priestly. Priestly noticed that when a sample of a substance called mercury oxide or back then, red calx of mercury, was heated to extreme temperature , it gave a gas that supported burning.


To generate this gas requires heat that was pure and more intense and Lavoisier knew this so he used magnifying glasses to generate intense heat in the course of his combustion experiments. He repeated Priestley’s work and found out that that this released gas (which Priestly under the influence of phlogiston called “dephlosgisticated air”) does support combustion.


If it was proven to support combustion then it should also support oxide formation. Lavoisier’s experiment with this gas, released from mercury oxide did support combustion and oxide or calx formation much more so than just air.


Lavoisier even gave it the name, oxygen, which in Greek meant “acid former” mainly because when he carried out combustion of phosphorous , he produced phosphoric acid, and even though we now know that not every product of combustion results in an acidic product but the name of course is still with us.


Also, he found that the fixed air of Black, was not the beginning but the end after a reaction with the fixed air or carbon dioxide the gas released after the reaction, which he observed with the red calx of mercury.


Lavoisier proved that it was this part of the air called oxygen that supports combustion and when mixed with other gases such as carbon dioxide, only the oxygen, under the influence of heat, will react with whatever substance is present whether it is wood, resulting in heat and flames or if it is a metal, rust.  If these same substances were in pure oxygen, the results would be more quick and vigorous.


In 1778, after six years of rigorous research on combustion, Lavoisier summarized his findings in what is called the Easter Memoir to the French Academy of Science and this important paper detailed his careful experiments on combustion and oxide formation. These two processes are indeed related because metals, such as iron and lead, and non metals such as sulfur react with oxygen, the gas that supports combustion while nitrogen ( which Lavoisier originally called “Azote” ) and carbon dioxide ( or fixed air) did not. When wood is burned, it reacts with oxygen giving off carbon dioxide while nitrogen, another component of air, remains inert or unreactive.


Needless to say, not everyone accepted Lavosier’s findings and some thought that he may have misinterpreted the phlogiston theory. Recall that if there is such a thing as phlogiston, then in the process of burning, phlogiston goes away which means that the burnt substance should weigh less. This would also be true for oxides.


In the course of his experiments, no such decrease as well as increase in weight was observed and as long as the reactions were done in closed vessels, the mass of the reactants and products will be equal.


An experiment is as effective in proving or disproving a theory if the same experiment produces the same result, under the same conditions, if repeated in the same way many times. In time, chemists performed the same experiments and found the same results. Metals and non metals combine with oxygen. It was because of this that the phlogiston theory was disproved.


After the Easter memoir, Lavoiser turned his attention to the composition of water. It was reported that when oxygen and hydrogen ( it was called “inflammable” discovered by another British scientist, Henry Cavendish ) combines under the presence of electricity, water is formed.


Lavoisier repeated the experiment and found the same results. He also did other experiments proving that water is a chemical compound of two different gases, oxygen and hydrogen. In collaboration with the physicist Pierre Simon de Laplace, Lavoisier devised an experiment where two jets of the gases oxygen and hydrogen were allowed to react under the presence of mercury.


Another method involved passing water through a red hot metal tube where it was hot enough for a reaction with the oxygen combining with the metal forming an oxide and the hydrogen being released from another end of the tube. The observation that water can be decomposed and formed from oxygen and hydrogen was finally revealed in the presence of thirty scientists using a different set of measuring equipment which included a thermometer for measuring temperature changes, a barometer for measuring pressure, balances, and a pneumatic trough for collecting gases.


The experiment was performed by Lavoisier in collaboration with a colleague named Jean Baptist-Meusner who helped him with the red hot iron tube experiment previously. The result of the demonstration was enough to convince the skeptical colleagues the correctness regarding the composition of the water along with the final demise of the phlogiston theory.


In 1789, Lavoisier published the Elementary Treatise of Chemistry( Traité élémentaire de chimie), which is no doubt, the first textbook devoted to chemistry. Most of the chapters deals with his work on combustion but he puts emphasis on quantitative measurements which is the foundation for modern chemistry. He even introduces a nomenclature or system of naming chemical elements and an element, in the modern definition of chemistry, is any simple substance such as oxygen   that cannot be broken down any further while a compound is a combination of two or more elements such as water which is a compound composed of oxygen and hydrogen.


Since oxygen supports combustion, it did not escape Lavoisier’s attention that perhaps oxygen may be responsible for animal and human life. Since I reviewed the necessary background of Lavoisier’s work, it is here in the work on combustion that lead to Lavoisier performing experiments that would rightly make him a biochemist in modern terminology.


In 1774, Priestly isolated oxygen from burning mercury oxide using a magnifying glass and found that this gas increase activity in mice. It seemed that the gas was responsible for the mice’s frantic activity.


It was known even before Priestly and Lavoisier that in addition to food and water, air is vital to both terrestrial animals and humans and if deprived of water and air, humans and land animals die. Air is then vital to the survival of many organisms.


Throughout his research, Lavoisier devised new methods and even devices to aid his investigations. In 1783, Lavoisier invented a device called an ice calorimeter which measures heat given off from any substance.


Along with Laplace, an ice calorimeter was developed which consisted of two metal cylinders nested within one another. An outer cylinder enclosed a smaller cylinder and in between was snow and in the smaller cylinder is where a chemical reaction would take place. By measuring the amount of gases such as carbon dioxide being released and also the melt water released, he could arrive at an estimate of the temperature of a combusting substance.


The ice calorimeter was put to use when Lavoisier used a healthy live guinea pig in his ice calorimeter and he estimated how much carbon dioxide and body heat realesed by the live guinea pig. He compared the results with a sample of carbon burning in oxygen and by taking into account the amount of carbon dioxide released as that of the guinea pig, Lavoisier concluded that animals do take in oxygen and give of carbon dioxide but at a much slower rate and with the release of a small amount of heat. This form of combustion, is now called respiration.


Lavoisier proceed much further with his experiments in respiration and he moved from guinea pigs to even using a human subject and once again, Lavoisier applied his rigourous method of combining accurate measurments with careful reasoning , as is the core property of science, to proving that oxygen supports animal life. It was this kind of experiment that he rightly considered a biochemist since biochemistry does depend on quantitative reasoning in formulating hypotheses and supporting theories. These experiments were carried from 1789 to 1790 but came to an end when Lavoisier was arrested, tried, and sentenced to death.


Although, it wasn’t called biochemistry then but Lavoisier proceed with the foundation of quantitative reasoning with the use of meticulous observations and accurate weighing which in the end disproved old theories such as the phlogiston theory while creating new ones such as the fact that oxygen supports combustion and respiration and without, there would be never have been an established link between oxygen and animal life and hence no biochemistry.





Antoine Lavoisier (n.d). Retrieved July 7, 2017, from


Asimov, I.  (1964) Breakthroughs in Science. Eau Claire, W.I. Houghton and Mifflin Company


Joseph Priestly (n.d). Retrieved July 7, 2017, from



Image Credit


Art Gallery ErgsArt-By ErgSap’s photostream  Public Domain Mark 1.0




The Three Questions of Astrobiology

Planet earth, an oasis of life in outer space. Earth is the only planet known so far that has life. Is life only unique to this world or is life a property of the cosmos? (NASA Goddard Space Flight Center)



Life. Is it a property of this planet in this solar system or is it common throughout the cosmos? What is life and what is so unique about it? What is life’s origin or origins? What is it about our planet that sets it apart and how and why our planet is in the only one that can support life? If life is common in the universe is there something life on our earth and life elsewhere have in common? Does life share some universal laws that would enable us to know just how common life may be? How would we know which planets have life or not? Can we ever find much less recognize is something alive especially if it is from another planet?


All of these questions are actually scientific questions and there is one field of biology or the scientific study of life that can now answer these questions and that field is astrobiology or the field of biology that studies the possibility of life in the universe.


What is astrobiology and how, as a science, can it be used to answer whether or not life is something universal? The definition of astrobiology is that it is a field of biology or the science that studies life but it is a field of biology that studies the nature, origin, and distribution of life on other planets as well as the origin and evolution of life on earth.


Astrobiology, as a science and field of biology, can be structured according to the scientific method of proposing hypotheses, determining the predictions of hypothesis, designing the suitable kinds of experiments, obtaining, collecting, and analyzing of data, and forming conclusions but astrobiology with it goals of determining whether or not life is common in the universe requires other fields of biology such as biochemistry or the study of the chemical properties of living things and how living organisms use matter and energy to produce the specific kinds of molecules needed for growth and reproduction, biophysics or the field of biology that takes into account principles and methods of physics for determining how life, as a physical phenomenon, use energy and what kind and what principle of physics allow for the survival and maintenance of life in a given environment. Cell biology which studies cells, the fundamental unit of life and how cells can adapt, process nutrients, reproduce and flourish in whatever environmental conditions is present. Evolutionary biology and ecology which studies how life evolves and how life integrates itself with its environment and genetics or the mechanism of heredity that makes reproduction possible. Each field of biology has it own specific research goals and agendas but they can be combine together to assese the likelihood of the origin and evolution of life that makes up a biosphere and to determine whether there are plenty of biospheres in the cosmos and since astrobiology is concerned about the question of life and the cosmos other fields of science must come into play and these fields include geology or the study of earth and how the structure and evolution of the earth has had a profound influence on the evolution of life but geology can include studies of other planets besides earth and this has been made possible by space probes in which we now have a better if still growing understanding of the different kinds of planets and for determining whether or not they could support life.  In addition the science of chemistry must also be taken into account which studies what kind of atoms interact molecules and how out of a possible range of reactions and with each chemical element what specific set of reaction can form the molecules of life and where they are likely to be found and lastly the science of astronomy which helps put the search for life in a cosmic context by first considering the evolution of stars and solar systems, the range of electromagnetic energy emitted by stars and how chemical elements are synthesized and also the number of galaxies other than our own that may or may not have the conditions for life to occur.


Astrobiology then depends on a multitude of various kinds of scientific disciplines for determining the likelihood of life and is a fine example of the definition of conscilience or according to E.O Wilson as the unity of knowledge from various kinds of disciplines coming together and astrobiology represents the synthesis of various disciplines which are absolutely crucial for the goal of determining if life is universal.


To see how conscilience of every scientific field combines together into the multidisciplinary science of astrobiology, we must approach the problem by asking three main questions that can be put into scientific form and these are



1. What is the origin, evolution, and distribution of life on earth?


2.What is the origin, evolution, distribution of life in our solar system and other solar systems?


3 Is there extraterrestrial intelligence?


We will examine each question separately and we will see how, according to the definition of conscilience, various fields of science come together to give meaningful answers to each questions which highlights the interdisciplinary nature of astrobiology in answering , in what I think is the most profound questions of our time.


What is the origin, evolution, and distribution of life on Earth?


Of all the eight planets in orbit around the sun, there is only one planet that is known to harbor life and that is the planet earth. Based on what we know so far, life on earth, as varied as it is , shares certain fundamental attributes , however different it may be and that life has been on earth for as long as the earth has existed which formed out of gas and dust like all the other planets 4.6 billions of years ago and as each planet condensed, each planet became different in terms of environment depending on the amount of matter, the presence of various kinds of gases in atmosphere, or none at all, distance from the sun, and so on but for reasons that are still unknown, earth is the only planet that is capable of supporting life, both simple and complex, and is earth the only planet in our solar system with qualities that are so unique that other planets in our solar system and other solar systems allowing this planet to support life as we know it or are there processes that can, under a range of certain conditions that are universal, that there can be life forms present in other planets? Before we can answer, with some degree of certainty, whether life is something of a universal phenomenon that is bound to arise whenever conditions permit, we must at first consider what life on earth has in common and how it originated but before we get in that, I must let you know that since as of 2016, there is no unambiguous evidence of extraterrestrial life, which we will discuss later on how we could recognize any extraterrestrial life, so there is no way we can compare what life on other planets may be like since the only life we know of is of this earth so in science which is concerned with experimentation and observation which involves data collecting, so without any hard evidence whatsoever, we are forced to make generalizations based on key properties of terrestrial life, past and present, of what life , assuming to be a universal phenomenon, may have in common elsewhere. This kind of bias would ignore other possible evolutionary pathways of life forms that are based on chemistries that do not use carbon and water and other ecosystems that use energy other than starlight but without such evidence, it would be difficult if not impossible to make any correct generalizations but until evidence is found, we will have to limit our search on life that uses three characteristics which are 1. carbon, 2. water, and 3. Energy. Indeed as Lucas-Mix (2009) has clearly stated, in terms of the scientific method underlying the goals of astrobiology “When humans construct models about how the universe operates, we come with certain assumptions and biases. They may be built in and they may simply reflect the positions we may hold of the universe, but it has become clear that humans cannot be entirely impartial.” (pg. 58).


To understand how our biases could influence how we observe we must at first know something about the scientific method and how the use of this method could help answer the question whether or not we are alone and we also see how this method, unlike other forms of human inquiry can end up reducing bias to such an extent that we can be very sure that life may be a cosmic phenomenon and also how various fields of sciences such as astronomy, physics, chemistry, geology, and biology can contribute to the science of astrobiology in providing answers that can be checked through observation and experiment.


Just what is the scientific method? It is nothing more than a process of understanding a part of our universe by first asking questions that are testable and by testable I mean asking a question that can be put in the form of a experiment which is a series of step by step procedures performed under controlled conditions  and that is what I meant by testable. In the case of astrobiology, the three questions that I’ve mention can be subjected to the scientific method and starting with the first question, life on earth, it’s origin and distribution, there are a series of questions about terrestrial life, which forms the core basis of biology or the “study of life” and questions that have been put to experimental tests are


1 What is it that distinguish  life from nonlife?


2. Despite all different forms of life that we can observe, what is that all of life has in common?


3. Why is life so different in appearance and function?


To see how the scientific method comes into play, we will first review in general how it works. As I have mentioned the method begins with asking questions that can be put to the test and that test is known as the experiment. In an experiment, which is of course carried out in a laboratory and the purpose of experiment is to prove whether or not the questions we ask about one part of the universe is true or not. These kinds of questions are known as an educated guess that is either right or wrong and that is known as an hypothesis. A hypothesis can make predictions which can be proven or disproven by experiment and if a hypothesis is proven wrong by an experiment then we can modifiy it to take into account what went wrong and set up a new course for an experiment which can either confirm the modified hypothesis or if an hypothesis, after several attempts has been proven to be wrong, then that hypothesis must be discarded in favor of a new hypothesis.


As for the experiment, once an experiment is underway, we must collect data such as temperature readings, the presence or absence of a particular chemical, how much energy is being absorbed and so on. Collecting data can then lead to a pattern which can suggest that this may be a fact of nature that may be or may not be universal or rather it may tell us something about what it is we are observing and if an hypothesis predicts that such a pattern should exist then the hypothesis can not only be confirmed but additional observations may make it clear that the conditions for something to be true has been confirmed under the same set up as part of the experiment. At this point, such additional testing has elevated the hypothesis into the status of theory and a theory is that which predicts what is true and a theory , like a hypothesis, can be subjected to various test and what theory predicts can be confirmed , be modified, or rejected in favor of another alternate theory.A theory can also make predictions as well as explain separate facts into one coherent whole.


In addition, technology has been an invaluable tool in enlarging our view of the universe and by that, I mean that experiments can be broadened by the use of instruments that can analyze what chemicals are found in organisms, the use of microscopes to see the inner working of life and with the development of each instrument, has not only enlarged our view of what is out there but it has been used to ask a set of new questions that once again can be proven or disproven. An example is with the microscope, and with the presence of microorganisms along with the inner details of cells, new questions were posed and our view of the biological world has broadened thanks to improvements in microscopy along with other techniques of studying life forms made possible with other fields of science such as physics which studies the fundamentals of forces, energy, and matter and with physics applied to problems in biology or biophysics, instruments such as the electron microscope which takes into account the fact that the electron, or negatively charged particles, can behave like a wave but with very short wavelength and this has made possible the study of the structure and hence function of cells and through chemistry or biochemistry, we now know that there are four kinds of chemical important to life and there are techniques, developed from chemistry applied to biological problems which have proven to be fruitful. Notice that different fields of science can be applied to one field such as the application of chemistry and physics to biology for answering questions specific within the field of biology, a fine example of consilicience.


Of course, in regards to experimentation not every question we asked can be rigourously tested but some questions that were asked were at one time unprovable and one such question was the origin of the universe. At first this would be more a question for theology which specializes in the existence of a God or gods and indeed it seemed that this would be the kind of question that would be in the realm of the unprovable which theology asks but  ever since the theory of gravity was modified into form by Einstein as the property of space time instead of as a force between objects, it was then found that the universe of a whole could behave as it spranged from an initial beginning and in addition the fact that galaxies in every direction are moving away could only be explained as if the universe is expanding from some point in space and time and since then additional observations have confirmed that indeed our universe did have a beginning so this is an example of a question that at first was a unprovable question is now part of the science of cosmology which studies the universe as a whole. The same would be true for the science of astrobiology and at one time the idea of life on other planets was also an unprovable question (in the scientific sense ) but as the science of astrobiology matured with findings in biology, along with findings made in other fields such as astronomy together with the development of space technology for sending space probes to other planets, there is a high degree of confidence that the questions of life on other planets can be solved.


I have mentioned in the last part of the paragraph about confidence when we find that a theory can explain at once were previously separate facts. Just what is it that we mean by confidence?


When practicing science, a theory is supported by many experiments that confirm the predictions and suppose a theory predicts that in the outcome of an experiment, let the theory be considered X and X predicts that under a experiment, a outcome Y is to be expected. If we do the experiment , and we do find Y what this would indicate ( this is a simplified description of what real experiments can confirm) is that theory X is sound and one experiment confirms it’s predictions. If we could do the experiment under the same exact conditions a thousand times and get the same answer, then by the definition of confidence in the field of science , that is out of a 1000 experiments, we are willing to accept that theory X is true and so we put our faith in the truth of theory X. Of course, in the history of science, theories can be subject to modification or even rejected and that is true for every field of science and suppose  that we repeat the same experiment for another 1000 rounds of experimentation but out of a 1000 outcomes there is one single outcome that resulted in a different outcomes not identical to the other 999 observations. What then? We can either conclude that a mistake in observation or experimental design has occurred which can be checked out and if we have confidence in the original claim of theory X, then we redo the experiment and found out that indeed an error was found and correct for that one particular observation which does agree with theory X. With the correction taken into account, our confidence in theory X is restored. However there is the possibility that no error in part was detected and that the observation was carried exactly as in the other observations, then what? Well, we would now have to question the underlying assumptions of theory X and we would have to find another factor that was not accounted four and so our confidence in theory X will not be the same as it was before.


We will have to consider modifying the theory based on the unexpected outcome of that result by considereing the fact that in each experiment performed in the laboratory there are what are called variables and these are things that you change in an experiment and suppose that in theory X there are just two variables, call them x_1 and x_2 that interact with one another and during a certain procedure, the interaction of these two variables define the expected outcome that is Y and in our experiment, repeated 1000 times only one gave a different result. We find out that we ruled out errors carried out in the experiment and must consider the possibility of finding another variable and we then find another variable that was not present in this one particular experiment and we call it x_3 and with the discovery of this new variable, we then have to modify our original theory to take into account this unexpected result but not by first changing our confidence in light of this new fact.


What I have attempted to show is that even if a theory is proposed to explain a certain kind of phenomena, our confidence or our degree of faith we have in a theory which is also alternatively defined by Lucas-Mix  as ” Confidence has to do with changing your mind” (pg. 20). Every theory can explain at least a portion of a phenomenon being studied but sooner or later it will be either modified or rejected entirely and so another theory has to be proposed and this is what we meant by confidence


Now that we understand, about the scientific method and now we understand that astrobiology is a field of science that specializes in determining whether life is present or rare in the cosmos, there is one important principle to state before we see how astrobiology can answer questions in regards to life.


From what we now know, the same laws of physics and chemistry are universal that is the same kind of chemical elements are present throughout all of the universe and in addition through recent findings in astronomy we now know that planets are pretty common. There is a principle of science that guarantees that the chances of life, even intelligent life should be a common property of the universe, and that principle is known as the principle of mediocrity.


What is the principle of mediocrity? It basically states that there is nothing unique and special about our species and life for that matter and that whatever resulted in life on earth, should be the same as any other solar system. After all, it has been revealed that earth is just one planet out of eight that orbit a star, which is just one of billions in a galaxy and that this galaxy is one of hundreds of millions or more in a vast universe.


The atoms and molecules that can be observed on earth are also universal, in that hydrogen, for example, the most simplest chemical element known is present in all of outer space, if you have seen one hydrogen atom, then you’ve literally have seen them all!


In a nutshell, the mediocrity principle simply states that because there is nothing special or even magical about life, then given that the atoms that make up life are universal, then there is no reason think that life on earth is something unique, but rather life based on a carbon and water biochemistry or even a different kind of biochemistry so in conclusion astrobiology is valid if the mediocrity principle holds true.


Even though astrobiology studies the possibility of extraterrestrial life, however, not every environment in space may be conducive to life and indeed because of findings from astronomy, there are places in space where life is not likely to be appear and these include the surface of stars, the strong magnetic fields around dense neutron stars, supernova explosions, and of course black holes, as well as large planets that are in close orbit near their parent stars, unless there are exotic life forms that could somehow adapt to these very extreme conditions and although scientifically these kinds of life forms have not been ruled out, and for the moment I will not talk much less speculate about how such life forms could exist



These three questions have been answered through the application of the scientific method to biology. For the first question, there is a subfield of biology, biochemistry, which studies the chemical composition of life and how life utilizes molecules for survival. To study the chemical composition of life, one must take an organism for study, and break it up into its basic components and isolated each chemical present and if need be, study the atoms that make up the biological chemical. What is has been found is that all of life is based on a chemical element which turns out to be present in the earth and even in outer space and that is the element carbon. What is so special about carbon? From the study of chemistry and physics, carbon is one of the few naturally occurring elements that can form bonds with itself and with other chemical elements such as oxygen and nitrogen and in a variety of conditions, carbon can form many molecules including those molecules associated with life. In addition, at a most fundamental level, there is no difference at all between life and nonlife and as far as chemistry goes, carbon can be present in a non living form as well as in a living form so there is no difference but as far as that question of the properties of life, chemistry by itself tells us nothing of the key differences between that which is alive and that which is not. To answer such a question is to require other different fields of biology such as physiology which studies life processes and genetics which studies how information in life forms are passed from one generation to another. These fields have made some strides in understanding the difference between life and non life.


As for the second question, there are so many different kinds of life form that at first it may not seem that there is anything that a human, a bird, a rat, a worm, and a bacterium have anything in common but it turns out that there is something in common and that is all life on earth are composed of cells. This incredible fact was only made possible by one invention that has become indispensable to the science of biology and that is of course the microscope.


Cells are the basic components of life and as the science of microscopy progressed, it was found that all life forms, however different, share one thing in common and that is all of life is either made up of one cell such as a bacterium or several kinds of cells that perform specific biological functions such as the cells that make up a worm, a rat, a bird, and a human. Complementing microscopy in the study of cells is the science of biochemistry and it has been proven that all of cells and hence of life is made up of four classes of biological polymers, the proteins, nucleic acids, lipids, and carbohydrates.


In addition, the science of molecular biology which focuses on the two most important molecules, the proteins and nucleic acids has also revealed that all of life at the cellular level, shares a common ancestry that lived around 3.8 billions of years but as time went on, different life forms became diversified and that leads us to the last question.


The reason why life forms are different is because life evolves into many forms in each generation and the theory of evolution by natural selection is what explains the reason for the diversity of life


Now that we aware of biases in our search for life elsewhere along with the objective nature of science itself, we can the proceed to the biology of life on earth and what clues it may hold for the search for life.


On planet earth, and as everyone knows whether we live in the countryside, in a neighborhood or in the city, one cannot escape the fact that this is a planet that has life and in fact it is impossible not to encounter any form of life wherevere we go on earth. What is about life that is common, however different from one life form to another? Can we categorize life forms into recognizable categories and by that I mean categories that reflect similarities between one form of life to another? Would such categories reveal that all life have a common origin, whatever that may be?


First, start at the one field of biology that analyzes the chemical composition of life which is biochemistry and what all of life has in common is that it is based on the element carbon which is a pretty common chemical element that is ubiquitous not just on earth but on other planets in combination with other chemical elements such as the Jovian planets which are the planets that are composed of hydrogen but also significant amounts in the form of methane which is composed of carbon and four hydrogen atoms and methane is also present in a moon of Saturn, another jovian planet,  Titan which is common to its atmosphere as well as lakes of methane in liquid form. In addition, carbon is present in interstellar clouds and also in the presence of atmosphere in stars but still, when approaching this problem from a scientific perspective, we must ask: Why carbon and what is so special about this element?


Carbon, like all chemical elements, is composed of a nucleus of protons and neutrons and protons are those particles with positive electric charge while neutrons are composed of electrically neutral particles, all held together by a short range but strong nuclear force and in all elements each is one distinguished by the number of protons or the atomic number so the element with atomic number 1 has one proton and that is hydrogen, second the element with two protons is helium and so on up to carbon with six protons.


Surrounding the nucleus are the negatively charged particles, the electrons and the negative charge of a electron balances the positive charge of a proton making the atom neutral, otherwise if an atom lost an electron or two or more and likewise gained an electron or more, the atom would have an overall positive charge and negative charge respectively and would be called an ion. The electrons , although they orbit the nucleus in each atom, are not arranged randomly but in a particular order and the reason why has to do with the fact that electrons, like all subatomic particles, or those particles that make up atoms like the protons and neutrons not only have an electric charge and mass but have something called spin and in some ways spin is related to the fact that an object rotates around an axis which applies to subatomic particles but the spin of the particles is not like what we think of a spinning  wheel and because there is a field of physics called quantum mechanics which studies the fact that energy and forces take on discrete values, all of which are multiples of a fundamental constant called Planck’s constant which is a very tiny but non zero number and in addition there is a principle in quantum mechanics called the Pauli exclusion principle which states that in a qiven quantum system such as electron orbiting a nucleus, no two particles with the same spin can occupy the same state or in other words if there were two electrons with the same spins, they cannot be in the same state unless the spins were opposite to one another and here is the key to why carbon can form many chemical bonds.


I previously mentioned that the orbit of electrons are not randomly distributed but are arranged in certain orders and indeed there is a term describing how electrons are arranged in atoms and these are called shells. There are several shells around the nucleus but each shell can hold only a limited number of electrons because of the exclusion principle and starting with the simplest chemical element, hydrogen, a single electron orbits in the first shell which is called the K shell and the K shell only holds no more than two electrons so after helium, which has two complete shells filled , atoms with more than three electrons up to neon which has a total of 10 electrons, additional electrons are placed in the second shell or L shell, and the  L shell can hold a total of eight electrons and here we find the secret to how carbon can form bonds and in a carbon atoms, two electrons form the K shell and four electrons form the L shell making a sum total of six electrons but since there are four electrons in the L shell, and there are four vacant spaces available for four additional electron, a carbon atoms can take in electrons from other chemical elements such as hydrogen and if a carbon atom is in contact with four hydrogen atoms, the outermost shell will take electrons from each hydrogen forming what are called covalent bonds or the sharing of electrons between two different elements, and with carbon forming four bonds with hydrogen, we have the chemical compound, methane and it is the sharing of electrons between atoms that defines that property of carbon and in chemistry language, carbon is said to have a valence of four because it can form four chemical bonds or rather four covalent bonds while hydrogen, on the other hand, can only form one valent bond and no more than one.


It is this ability that carbon is the most important molecule because out of the many compounds that exist, there are four molecules of life that I’ve previously mentioned which are the proteins, nucleic acids, lipids, and carbohydrates that are universal of all forms of life and that carbon has played a crucial role in the origin and evolution of life but in addition to carbon, life makes use of another molecule which is also present in the cosmos and that is water and like asking the question on why carbon, we will now ask, why water?


What is it about water that is also crucial to terrestrial life and possible other life forms that may exist outside of earth?


It is no surprise that water is one the most important and vital to all life forms and to date, there has been any life form, wherever it was found on earth, that did not come with water so to understand the properties of water and why it is important to life, is to once again to consider the molecular structure or water.


Water is composed of two different elements, oxygen and hydrogen. Oxygen forms two covalent bonds so it will need two electrons from two elements which are the two hydrogens and oxygen forms two bonds with hydrogen giving H_2O or more simply water but here is something about the bond with oxygen that makes the properties of water unique. An oxygen atom has a nucleus that is slightly bigger than carbon and its atomic number is eight so it has eight protons and when water forms bonds with hydrogen, the positive charge in the oxygen’s nucleus draws the electrons even closer and also in the language in chemistry, oxygen is known as “electronegative” element and that means when an atom like oxygen bonds to other elements, the nucleus, because of its positive charge will attract electrons even more and that is what oxygen and when oxygen draws in more electrons to itself, the oxygen become more negatively charged while the hydrogens wind up with a positive charge and so the water molecule has a net distribution of charge and the molecule is known as polar since it has one negative charge ( the oxygen atom) and two positive charges ( the two hydrogen atom).


The fact that water is a polar molecule is the key reason for why water is vital to terrestrial life. For one thing, water molecules, due to their polarity, will attract other water molecules because of a different type of bonding, a hydrogen bond, where the negative part of one water molecule is attracted to the hydrogen part of another water molecule and it is this reason why water can exist in a range of temperatures from freezing cold to boiling hot. At freezing temperatures, all the water molecules are held firmly in place by the same hydrogen bonds but with each molecule close to one another, but then here lies the secret on how water is vital to life.


Because of the hydrogen bonds, and when water turns to ice, the molecules aggregrate into hexagonal structures with empty spaces in between and because of the hexagonal structure , made possible only by hydrogen bonds, is the reason why ice floats in water. Indeed there are life forms on earth that can tolerate cold temperatures ranging from bacteria to fishes and in seasonal environments, where water freezes in the winter time , there is only a layer of ice and hence there is water below allowing life to thrive there.


In addition, liquid water is a good solvent in that molecules do have group of atoms that do have unequal distrubutions of electrons and hence when a molecule such as a sugar cube is in water, each water molecule will hydrogen bond with any atom with a slight electric charge so that if one atom has a positive charge, the oxygen will temporarily bond to the atom with positive charges an vice versa and together with a lot of water molecules, the sugar will slowly dissolve.


Also for biological molecules that are assembled together in cells, the biological molecules which are large molecules or polymers are composed of smaller subunits called monomers and an example are the protein molecules which are composed of amino acids and when two amino acids bind together a molecule of water is released. Also water can act to break apart polymers in a process called hydrolysis while the opposite is called condensation.


In addition, in the liquid part of water, most chemical reactions occur quickly than either in the solid phase (ice) since all the molecules are held firmly in place but hardly also in the gas phase (steam) where each molecule is widely spaced where the chance of molecules coming together is minimal.


With carbon and water, one can now see why the search for these is not only important to the science of biology but in astrobiology and to date , carbon and water are pretty common as revealed through the science of astronomy.


There are four major molecules that all life on earth share and these are the carbohydrates and lipids , both of which provide energy while carbohydrates form energy storage reserves in animals in the form of glycogen, which is a form of starch while starch is also a product of a processs called photosynthesis, a form of metabolism where energy of light , water, and carbon dioxide are combined together to form the monomers or building blocks of carbohydrates which is glucose, a building block of sugar, a source of biochemical energy, and cellulose, a complex carbohydrates that forms the cell walls of all plant life and in lipids, the formation of a cell membrane which because of it’s ability to separate water because of the hydrophobic nature of the molecule or its ability to repel water.  Also in addition to serving as a source of energy, there is a form of sugar ribose and deoxyribose present as the molecular structure of nucleic acids which will be described shortly


Nucleic acids are the molecules that carry biological information and is present in all life forms. Deoxyribonucleic acid or DNA carries the information and it is because of the fact that DNA is a double helix consisting of a repeating pattern of deoxyribose and phosphate groups bonded together and there is a variable part of four nitrogen bases called adenine, thymine, guanine, and cytosine and in sequence of these bases, each bases is weakly bonded to another series of bases in another chain of nucleotides or the building blocks of nucleic acids. Adenine bonds to thymine and guanine bonds to cytosine and it this ability that makes DNA, not only a carrier of information but also suitable for passing information in one generation to the next.


Proteins are the molecules that carry much of the work inside living organisms. They function as enzymes which are catalysts or molecules that speed up biochemical reactions, and there are the cytoskeletons which provide structural support inside cells, transport proteins have the task of ferrying vital molecules in and out of cells as well as the protein hemoglobin which carries oxygen to every cell in the human and animal body. Proteins even allow DNA molecules to replicate into the next generation of cells. All proteins are composed of amino acids which are the building blocks of proteins and there are twenty naturally occurring amino acids. The sequence of amino acids are first determined by the sequence of nucleotides , that is three bases can code for one or more amino acid and for any sequence of bases there will be a corresponding sequence of amino acids which will then determine both the structure and function of a protein, whether it will be an enzyme, a transport protein that will carry oxygen, or a DNA polymerase which is an enzyme that participates in the replication of DNA.


How does information on how to live, reproduce, and adapt, which is known collectively as the phenotype go from the genetic instructions, the genotype, to all the protein molecules in every living thing? In cells from bacteria to humans, there is a cellular structure called the ribosome which is a molecular machine composed of protein and RNA, a form of nucleic acid where the sugar is ribose and with all three bases, adenine, guanine, and cytosine but with uracil in place of thymine. In all cells, there is an enzyme called RNA polymerase which reads the sequence of a strand of DNA or gene that carries the information for a polypeptide sequence or combination of amino acids. The RNA polymerase is able to form a strand of RNA polynucleotides carrying the exact sequence of DNA but with the RNA, uracil in place of thymine and the RNA molecule is called messenger RNA and the messenger RNA then goes to the ribosome where reading each consecutive three bases, the ribosome uses another RNA molecule, a transfer RNA which is attached to the amino acid and the ribosome along with the messenger RNA aligns the growing polypeptide and the process of matching amino acids with the corresponding three bases or codons in the messenger RNA until the polypeptide synthesis is complete and the ribosome is ready for another messenger RNA. In short, information flows from DNA to RNA to protein in every cell.


In all cells in all organisms, the fact that information in DNA can be translated into the language of proteins is made possible by the genetic codes and this is an universal code ( with a few minor exceptions) in all kingdoms of life where three bases code for one or more amino acids. The fact that all of life use this code is indicative of the fact that all of life shares a universal ancestor where in the distant past, the first life form on this planet was using this genetic code for the synthesis of its proteins , necessary for survival and through evolution, life forms have diversified to every conceivable habitat, but have retained the machinery for synthesis of proteins, which is present in all life forms, more or less.


From this brief description of the fact that carbon, which is an element that is not only present in earth but is also found throughout the cosmos, and that to form all the molecules necessary for life, that is life as we know it, there has to be energy and life , through its evolution, has utilized two sources of energy, which is sunlight as well as geochemical energy, and every biochemical reaction occurs in water in the liquid phase. Still, we must ask, what is the property that sets life apart from non life? This question, really does not have a single answer and that question can be broken up into a series of related questions and I believe that these sets of questions are not just applicable to the forms of life on this planet, but are likely universal to other life forms, whether or not extraterrestrial life forms are based on carbon and water.


In a previous blog, I have talked about certain characterisal molecules in and out of cells as well as the protein hemoglobin which carries oxygen to every cell in the human and animal body. Proteins even allow DNA molecules to replicate into the next generation of cells. All proteins are composed of amino acids which are the building blocks of proteins and there are twenty naturally occurring amino acids. The sequence of amino acids are first determined by the sequence of nucleotides , that is three bases can code for one or more amino acid and for any sequence of bases there will be a corresponding sequence of amino acids which will then determine both the structure and function of a protein, whether it will be an enzyme, a transport protein that will carry oxygen, or a DNA polymerase which is an enzyme that participates in the replication of DNA.


How does information on how to live, reproduce, and adapt, which is known collectively as the phenotype go from the genetic instructions, the genotype, to all the protein molecules in every living thing? In cells from bacteria to humans, there is a cellular structure called the ribosome which is a molecular machine composed of protein and RNA, a form of nucleic acid where the sugar is ribose and with all three bases, adenine, guanine, and cytosine but with uracil in place of thymine. In all cells, there is an enzyme called RNA polymerase which reads the sequence of a strand of DNA or gene that carries the information for a polypeptide sequence or combination of amino acids. The RNA polymerase is able to form a strand of RNA polynucleotides carrying the exact sequence of DNA but with the RNA, uracil in place of thymine and the RNA molecule is called messenger RNA and the messenger RNA then goes to the ribosome where reading each consecutive three bases, the ribosome uses another RNA molecule, a transfer RNA which is attached to the amino acid and the ribosome along with the messenger RNA aligns the growing polypeptide and the process of matching amino acids with the corresponding three bases or codons in the messenger RNA until the polypeptide synthesis is complete and the ribosome is ready for another messenger RNA. In short, information flows from DNA to RNA to protein in every cell.


In all cells in all organisms, the fact that information in DNA can be translated into the language of proteins is made possible by the genetic codes and this is an universal code ( with a few minor exceptions) in all kingdoms of life where three bases code for one or more amino acids. The fact that all of life use this code is indicative of the fact that all of life shares a universal ancestor where in the distant past, the first life form on this planet was using this genetic code for the synthesis of its proteins , necessary for survival and through evolution, life forms have diversified to every conceivable habitat, but have retained the machinery for synthesis of proteins, which is present in all life forms, more or less.


From this brief description of the fact that carbon, which is an element that is not only present in earth but is also found throughout the cosmos, and that to form all the molecules necessary for life, that is life as we know it, there has to be energy and life , through its evolution, has utilized two sources of energy, which is sunlight as well as geochemical energy, and every biochemical reaction occurs in water in the liquid phase. Still, we must ask, what is the property that sets life apart from non life? This question, really does not have a single answer and that question can be broken up into a series of related questions and I believe that these sets of questions are not just applicable to the forms of life on this planet, but are likely universal to other life forms, whether or not extraterrestrial life forms are based on carbon and water.


In a previous blog, I have talked about certain characteristic properties that define life and here I will talk about each of them in regards to terrestrial biology ( the forms of all life on this planet) to extraterrestrial biology ( hypothetical life forms on planets even moons other than earth).




In order to function, an organism whether a unicellular organism or an organism composed of one cell such as a bacterium, or a multicellular organism, which is composed of many kinds of cells, such as ourselves, there must be a set of processes that are able to function within certain limits which can range from a narrow pH, the presence of a certain kind of protein necessary for synthesis of key molecules, and so on. These set of coordinated process that allow an organism function is known as homeostasis and allow an organism to function in a given environment whether that environment  is a cold region on earth, near the edge of a geothermal vent, in the canopy of a rainforest, or in subsurface layers in martian soil, and whether we are considering life on earth, life on mars, or life on some planet orbiting a star hundreds of light years away, any life form must be able to survive during each part of its life cycle from birth to death which would made possible through homeostasis.




Life forms , with their ability to grow and reproduce and survive in a range of conditions, can do these abilities mainly because of their degree of organization and that means that every subcellular structure must be able to perform either a certain specific task or have several tasks and that each component whether in a single cell or in an organ must have a high degree of sophistication where every component must be present and indeed it is to strain credulity to imagine a life form that is lacking in the degree of biological sophistication which is below a certain organizational threshold, would not satisfy any scientific definition of life that depends on that definition. To make this clear, what is the simplest organism that satisfies all the definition of life, including those that depend an organization as an important criterion? These would be the prokaryotes or cells that lack nuclei which house chromosomes and organelles or the substructures that carry out specific functions and those class of prokaryotes are the bacteria. Every prokaryote is composed of the four biological molecules , proteins, nucleic acids, carbohydrates, and lipids, and these four molecules are arranged in specific structures; for the lipids , a cell membrane encloses a given volume of the cell and functions to separate the inside from the outside and this kind of lipid, a phospholipid, which consists of hydrocarbon chains alternating with negatively charged phosphate groups forms what is called a semipermeable barrier where most materials are sequestered but some can pass in and out such as simple sugars and amino acids. There are proteins that exists as enzymes which are synthesized as enzymes, depending on the nutritional requirements of the bacterium , or when the cell must undergo division a set of enzymes for replicating DNA must also be synthesized. The nucleic acids such as DNA is in the form of chromosomes which are tightly wound within the tiny volume of the cell and carry all the information that a cell needs in order to survive. The other nucleic acid, RNA must be present to carry information from DNA to the ribosomes along with transfer RNA needed for protein synthesis. The bulk of the cell interior is composed of a fluid called cytoplasm and it a mixture of water, amino acids, proteins, nucleotides, vitamins, inorganic ions, and sugars and the cytoplasm exists in between as a liquid in that the cytoplasm can flow inside the cell and as a solid where it has a fair degree of order. In addition to having a cell membrane composed of both phospholipid and proteins in the form of transport proteins, the prokaryote consists of another external structure called a cell wall which gives the cell a fairly rigid structure. The cell wall, unlike the cell membrane is composed of a form of glycoprotein which is a combination of a carbohydrate and a protein. From this, it goes to show that even if life is found to be based on a chemistry that is not carbon based, it must still have a degree of organization that gives it the ability to reproduce , function, and evolve or else it would not be considered life in that sense.




In previous blogs, I have argued at length that living or to make a living is more of a process and to make a living requires the use of energy. Livings things that are alive as well as extinct use energy to perform their various functions and there is a set of processes that are collectively called metabolisms. From the perspective of that field of physics, thermodynamics, to carry out a process, energy must be converted into various forms and there is a difference between energy that is coherent and can do organized work and energy that is random and incoherent and that is called heat. The second law of thermodynamics states that the amount of usable energy that is known as free energy or that energy that is coherent and can be put to various kinds of uses is decreasing while the opposite, random and incoherent energy is increasing or the law of increasing entropy. I have also argued in one of my previous blogs that although the second law allows for the increase of entropy, it does not disallow for the formation of organized structures that are low in entropy and hence have a high degree of organization, which is indicative of life but to have a functioning organisms requires that more than enough wastes must be put towards the environment in order to have a high degree of order.  Just what are these set of processes that life uses in order to maintain a low even a negative amount of entropy which correlates to higher organization? These processes are collectively called metabolisms.


Metabolisms are those set of interrelated biochemical processes where energy is released from the breakdown of one of three molecules of life, lipids, carbohydrates, and proteins into their simpler components and that energy is parceled into a molecule that is universal of all life forms, ATP or adenosine triphosphate and in this form, the energy is momentarily broken down to release energy to power every biochemical function in the living cell. Metabolisms consists of two processes that are coupled together in that one process must depend critically on the other and they are anabolisms or the buildup of molecules and indeed at the level of the fundamental building block of life, cells, they are exquisite biochemical machines that manufacture complex molecules such as protein synthesis, a form of anabolism but in order to synthesize a protein not only requires information , in the form of DNA sequences, but energy and the process of releasing energy and storing it in ATP requires the opposite of anabolism, which is catabolism or the breakdown of large molecules into small molecules. In order for anabolism to proceed, energy must be released in the form of ATP and to do this requires a complex step by step set of chemical reactions beginning with either the breakdown of carbohydrates into glucose, proteins into amino acids, and lipids into fatty acids and glycerols. For organisms that use oxygen or aerobic organisms such as all the animals, plants, and fungi, oxygen is what is known as a terminal electron acceptor at the end of the steps in the process of catabolism and what this means is that metabolisms depend really on the transfer of electrons from atoms within the building blocks of organic molecules or carbon based molecules and in a biochemical reaction called respiration which is common to all animals, as well as plants and fungi, an oxygen molecule is needed to react with a molecule of carbohydrate which produces water vapor and carbon dioxide as waste and a small amount of heat which is really the sum total of a complicated set of chemical reactions where the energy is liberated only in the form of ATP.


I have mentioned that electrons are passed between atoms and this is known in a coupled series of reactions known as redox reactions or “reduction-oxidation” reactions for short and in biochemistry, oxidation is the process of losing electrons while reduction involves gaining electrons so in the process of respiration electrons from the atoms of a glucose molecule are passed on into an oxygen molecule resulting in carbon dioxide and water vapor and as long as the process of transferring electrons continue, ATP is being synthesized and with ATP the process of anabolism can proceed. If it wasn’t for this subtle but exquisite passing of electrons it would be very unlikely that there would be anything like anabolism much less the evolution of life.


Since respiration, through redox reactions, produce water vapor and carbon dioxide, then the opposite of respiration, photosynthesis, which occurs in cyanobacteria, algae, and higher plants, takes water and carbon dioxide and in the presence of light, synthesize molecules mainly in the form of carbohydrates ranging from sugars to cellulose, an abundant carbohydrate while at the same time, also generating ATP and like respiration, photosynthesis is also a complex set of interlinked biochemical reactions.


Like respiration, photosynthesis is a complex series of chemical reactions and it involves visible light, a form of electromagnetic energy, which is converted into a series of chemical reactions where as carbohydrates are synthesized from water and carbon dioxide and in the process generating ATP, oxygen is released as a waste gas. The bulk of the oxygen in the earth’s atmosphere is the result of photosynthesis. In fact it is not impossible, through the technique of spectroscopy to detect the presence of oxygen in any planetary atmosphere ( more on this later).


Both respiration and photosynthesis are in a sense, mirror images of one another and it is what has driven the evolution of life for with photosynthesis came oxygen which favored the evolution of respiration and likewise the carbon dioxide given off has also favored the evolution of organisms that could carry out photosynthesis.


Organisms that carry out photosynthesis in which simple inorganic compounds are synthesized into complex organic molecules are called autotrophs while those organisms that use organic molecules as food are heterotrophs. Many life forms , usually of the microbial kind, not only carry out photosynthesis such as cyanobacteria but can carry out a form of autotrophy known as chemolithotropy and this involves the use of mineral ions for the synthesis of organic compounds and it is common for some strains of bacteria living in subsurface conditions and that form of autotrophy would be useful for any prokaryotic cell that did not evolve to use sunlight as an energy source as would be the case for any extraterrestrial microbe living in planetary soil underground in a planet that is far away from its star.




Anabolism of a biological system allows a cell to undergo reproduction or the making of a kind that more or less identical and that would include binary fission where one bacterial cell makes two copies to mitosis, a sophisticated form of cell division that is done step by step in eukaryotic cells. For multicellular life forms, there is a process of meiosis where gametes which have half the number of chromosomes when combine together results in the division and differentiation of cells forming a fully formed organisms.It would be no different for any extraterrestrial life form even if it is not based on carbon and water, life would still need to make more of itself .




In every environment that defines the terrestrial surface as well as extraterrestrial habitats, each organism can make a living utilizing energy and carbon in its habitat which can range from full sunlight to low levels of light, habitats where there is water as well as less water, and in temperatures ranging from temperate as well as close to boiling and also close to freezing. How are organisms able to do this?


It is because organisms have the ability to adapt or to fit within their environment and adaptation is the reason why there can organisms living in extreme environments and also the same would be true for any extraterrestrial organism. Adaptation is the result of natural selection for the environment acts as a filter for determining which organism can live if it has certain phenotypes that allow survival or not, and that is the basis of adaptation in the Darwinian sense.


Response to Stimuli


In its life cycle, an organism must be able to distuingish between whether a bit of matter is something edible or not or if the organism in question has the ability to fly to tell the difference between up and down or if it an organism that is part of a species that reproduces sexually, the ability to find a mate and to determine whether or not the mate is a part of its species. All of these involve detecting and responding appropriately to stimuli, another hallmark of biological entities. Organisms even as simple as a bacterium can sense the difference between food and poison which involves the use of proteins that can discriminate in terms of the molecular structure of a food molecule and poison molecule where in the former it will execute a series a responses toward food and in the latter another series of responses to move away.




Throughout the history of planet earth , life has the ability to make more copies of itself and reproduction is that process where life forms produce another life form that is identical to the previous one, more or less. There are two ways that this is accomplished. One is asexual reproduction and it is where one organism divides into two identical organisms. The two organisms have the same genotype as well as phenotype as the previous one. Sexual reproduction, involves the fusion of two special cells, called gametes which have half the number of genes as the parent and the halving of the number of genes, is the result of a process called meiosis, and this not only results in the halving of the number of genes, but in the rearrangement of chromosomes and hence rearrangements of genes and when gametes fuse, a new organism, with a novel arrangement of genes is formed and through meiosis, there is considerable genetic diversity for natural selection to act on.


These seven properties of life are not only applicable to terrestrial life but would likely be common for extraterrestrial life forms evolving on other planets in other solar systems, even if the life forms on other planets use something other than carbon and water but what is more important is the pattern that all life share.



What are the physical conditions in the solar system that have permitted the origin and evolution of life on earth? In order to answer the question we must keep in mind that to understand  how the biosphere comes about we must broaden our perspective to not only include the whole planetary level but to include the stellar level as well . In addition whatever processes have operated that allowed the formation of the solar system are not only universal , and in the past twenty years planets have been detected in other stars in our Galaxy where we now can confidently that not only solar systems are common but the chances of planets with physical conditions that will allow independent origin and evolution of life are good. With that in mind we will see how both properties of stars and planets together will increase the chance of life appearing.


Since eight planets orbit the sun, there are two major group of planets. These are the terrestrial planets and there are four in orbit, and they are Mercury, Venus, Earth, and Mars and these are the planets that consist of a metallic core, a mantle and silicate crusts and with a degree of plate tectonics along with an atmosphere. The other group of planets the Jovian planets are Jupiter, Saturn, Uranus, and Neptune and are large planets composed of hydrogen and helium along with some organic compounds , notably methane and ammonia. Out of the eight planets, earth is by far the only planet with a biosphere and an atmosphere with oxygen and why is it that earth is one planet capable of supporting life? The main reason has to do with the fact that it is an certain distance from its star, the sun where conditions are not too hot, not too cold but just right and it is because it is in a region called the habitable zone or that is in area surrounding the star where the energy of sunlight is emitted in all directions and within the habitable zone, there is a region where conditions are within the range where water is a liquid and indeed much of the earth’s surface is covered in liquid water and given the small size of earth, it intercepts a small amount of sunlight, which has enough energy to power what is called the hydrological cycle which is a cycle where water is circulated between oceans, in the form of vapor into clouds and from clouds there is precipitation in the form of rain and snow, falling onto terrestrial surfaces to form back as liquid and/or gas returning to the seas via the rivers.


As far as the habitable zone is concerned, it may be a region around the star where a planet intercepts enough light in order for the temperature to be within the range where water is a liquid but this not the only definition for habitable zone.


It turns out that the habitable zone that I have mentioned, is applicable to the evolution of complex, multicellular life and this definition of a habitable zone is one such definition according to the scientists Ward and Brown Lee (1999). Life, at least microbial life, can exists in a wide range of conditions that does not necessarily depend on sunlight. According to these authors, they believe that the habitable zone of simple life forms like prokaryotes can be found in a habitable zone that can be outside the stellar habitable zone mainly because prokaryotes can exist in a wide range of environments and so life forms, at least the simplest should be common while multicellular life forms such as animals inhabit a narrow range imposed by the stellar habitable zone. This is the basis of a hypothesis called the rare earth hypothesis, and this states that complex life forms including intelligent life may be rare but not zero while simple life forms may be the rule than the exception, and this is one hypothesis regarding life in the universe, but we must be open to the possibility , that this being a hypothesis, can either be supported or refuted, we should be open to the possibility of life , simple and complex, being common.


To assese the probability of extraterrestrial life, we must view it in an astronomical and even in cosmological context and starting from the astronomical context, that is stars, is where we will focus followed by what we know about planets, and then finally we will see how the universe , the cosmological context, may play a role in supporting life.


The star, our sun, is composed of hydrogen and helium and in the center of the sun is the core where temperatures are very high, and in the range of millions of degrees and at these temperatures, four hydrogen nuclei are converted into a helium nucleus and gamma rays and all gamma rays move from the core and each gamma ray photon, a particle of electromagnetic energy, loses energy until it is within the wavelengths of visible light and this happens when each photon is at the surface of the sun and from the surface, every photon in the spectrum of visible light from violet to red, moves in all directions from the sun and in addition to powering all the weather of the earth’s atmosphere, the constant stream of visible light keep’s the earth’s surface warm enough for water to be liquid and also for keeping the biosphere active in the form of photosynthesis, which results in the constant production of oxygen in the atmosphere.


The size of a star determines how long it will convert hydrogen into helium. Stars begin their conversion of matter into energy first as a large cloud of gas and dust , mainly hydrogen, helium, and a few heavy elements such as carbon and oxygen and under the pull of gravity along with another physical principle, the conservation of angular momentum were objects speed up quickly as they decrease in size. Under the inward pull of gravity , a sphere of gas and dust is compressed until at the center, temperatures begin to increase and as long as gravity pulls the sphere towards the center, temperatures will continue to rise until the hydrogen atoms are broken into their electrons and protons and at even higher temperatures, protons move so fast that , despite similar electric charges since like charges repel, the protons move so fast that they end up colliding and it takes up to four protons to form a helium nuclei and a gamma ray photon. All the thermal radiation energy emitted at the center is released from the center to the surface while gravity pulls all the matter inward and at this point radiation pressure is balanced by gravity so there is no further collapse and the large spherical object reaches a point, known as the main sequence, and at the main sequence, the objects produces light and so it becomes a star at this point.


As for the region of gas and dust surrounding the nascent star, and because of the conservation of angular momentum, most of the gas and dust will end up congealing into smaller bodies, which due to their own gravitational attraction will slowly get larger and larger until, because of the release of radiation  from the newborn star where most of the hydrogen will likely get blown away while objects surrounding the star will slowly grow in size, and these objects will eventually become planets and this is the description on how our solar system formed 4.6 billions of years ago and also it is same process that is occurring within our galaxy as well many other galaxies. Planets are then the side effect of star formation.


I have mentioned that there is an inverse relationship between the lifetime of stars on the main sequence and their mass. Stars that have the same mass as the sun tend to have long lifetimes within 5-10 billions of years. Stars that are less massive, the red dwarfs, have lifetimes within 10 billions years or even longer, mainly because with a small size and less of a gravitational pull, hydrogen fusion is slow whereas for massive stars, the giants and supergiants, these stars consume hydrogen rather rapidly and can last for less than 10 million years to even a million years or less.


The age of the solar systems is dated to be around 4.6 billion years old, and all planets including earth are of this same age. Life on earth has been around roughly at 3.8 billion years so evolution of life depends so much on the lifetime of the sun and with such a long lifetime, there has been plenty of time for life on earth to evolve.


In addition to long lifetimes, there is even a inverse relationship between the size of the habitable zone and the type of star. Stars like the sun tend to have habitable zones that are within the size ranges of what is called an Astronomical Unit or the average distance from the sun to earth, which is about 93,000,000 miles or 1.A.U for short. Stars not only come in various sizes but have different surface temperatures which is related to the colors. A basic rule of thumb is that stars that are red emit cooler radiations from infrared while stars that are yellow like our sun , emit hotter temperatures within the yellow green part of the spectrums while stars that are blue and white are even hotter, radiating energies mainly in the ultraviolet to x-rays. The red dwarf stars, since they emit infrared light, which is a weak form of electromagnetic energy, have habitable zones that are much less than 1.A.U so any planet that orbits a red dwarf star must be within the range of less than 1.A.U in order to have a planet with water while giant and supergiant stars have habitable zones much larger than 1.A.U for a planet in order to have liquid water.


From the sun to the planets, the size of the planets also determines habitability. Recall that the solar system is divided into the two main groups of planets, the terrestrial planets and the jovian planets. The terrestrial planets are composed of dense matter in the form of metal such as iron and nickel which forms the basis of an core surrounded by a mantle which is a semi solid layer surrounding the core and on top of that is a thin layer of rock called the crust whereas the Jovian planets,on the other land are large and are composed mainly of hydrogen and helium, and being massive, have a strong gravitational field allowing them to have a large number of satellites. In regards to the terrestrial planets, which in the case of earth, is divided into large plates that are slowly moving and earth is the only planet with moving plates consisting of continental and oceanic plates whereas the other terrestrial planets Venus and Mars either have a less substantial degree of plate tectonics or with a form of plate tectonics that is completely different than the one earth has. The size of earth is perfect for holding on to a atmosphere which exerts pressure on the planetary surface and it is this pressure that keep water as a liquid and together with a active plate tectonics that has been shaping the earth for 4.5 billions of years, the interaction of the atmosphere and the earth’s surface, known as the geosphere, has determine the range of environmental conditions for life to evolve. As for the other terrestrial planets, Mars , is about the 1/10 size of earth so it’s atmosphere is less dense than that of earth so any standing puddle of water will instantly evaporate and for Venus, it’s atmosphere is so dense that the surface pressure would crush metal, as well as melt it, except for silicate rock and Mars is cold and Venus is hot and not surprisingly it is because Mars is at the edge of the habitable zone of the sun while Venus is close to the sun while earth is “just right” for surface temperature to allow for water to remain liquid and together with an atmosphere that is sufficient for preventing water from evaporating.


Even plate tectonics plays a key role in earth’s long term habitability. Earth’s atmosphere has carbon dioxide, which is a greenhouse gas and that it can absorb infrared radiation, the form of radiation we feel as heat, and re radiate it back into the earth’s surface. Without carbon dioxide or any greenhouse gas, earth’s surface would be frozen solid. Carbon dioxide, also reacts with water, which is abundance in the atmosphere and the reaction produces carbonic acid, a weak acid which can dissolve some silicate rocks , forming calcium carbonate. Gradually, this process of calcium carbonate formation slowly takes the carbon dioxide gas from the atmosphere and there are sedimentary layers of calcium carbonate which slowly , because of plate tectonic, is returned back to the hot mantle where it is then released through volcanoes as carbon dioxide back into the atmosphere, a slow and gradual process known as the carbonate-silicate cycle and this is just one process that has allowed earth to maintain a warm surface temperature around an average of 60 degrees Fahrenheit or about 16 degrees Celsius.


The most important factor of all are the elements themselves that are have a higher atomic number than hydrogen and these are the chemical elements themselves which are carbon, nitrogen, and oxygen. Where did these elements originate? The universe is composed mainly of hydrogen and helium, which were produced in the first few minutes of the Big Bang and millions of years later , all the hydrogen condensed into the first generations of stars. In a star, the fusion of four hydrogen nuclei produces one helium nucleus while a gamma ray photon is released and it is in the core of stars that hydrogen is converted to helium but the size of the star makes an important difference in what kinds of elements are synthesized.


Stars that are within the size range of the sun can only generate helium and when helium makes up the core, there is not enough energy to balance the inward gravitational pull of the surrounding layers so gravity ends up contracting the layers which causes the helium core to heat up even more and this end up producing nuclei up to carbon but gravity ends up producing a white dwarf and before formation of the white dwarf all that thermal energy causes the outer layers to expand forming a red giant and this will be the fate of our sun in the next 5 billion years.


What about stars that are more massive than the sun? What is their fates? It turns out that massive stars end up producing the biogenic or life forming elements necessary for the long evolution of life. A star that is 5 times massive than the sun tend to have shorter life spans that are less than 10 millions years or less. Once the hydrogen core in a giant star is converted into helium, there is no more thermal energy to balance the inward gravitational pull and with gravity contracting towards the core, helium is then converted into carbon then oxygen, then into silicon and finally towards iron so in the process of ever increasing contractions and at even higher temperatures, in a process called nucleosynthesis, elements with atomic numbers higher than hydrogen but up to iron are synthesized and with no more energy to produce elements heavier than iron, the star generates more energy which ends up causing the giant star to end in a very spectacular cosmic explosion called a supernova and the initial explosion produces enough energy to outshine all the stars in a galaxy and with such a massive, titanic explosion not only is the hydrogen return to the interstellar medium from whence it was born, but all the heavier elements are also returned so in the history of a galaxy such as our own galaxy there is a steady cycle of star birth where stars are born, along with planets as well as in association with other stars, die whether in the form of a planetary nebula or in a supernova explosion where in the latter, all the elements are returned to the interstellar medium so the medium becomes enriched with carbon and oxygen and if it weren’t for supernovas, there would be no organic chemistry and definitely no biochemistry. Thus, when seen in the context of astronomy, the origin and life then takes up a cosmic significance.


For the evolution of life, life has had to have a beginning and during the early history of the young solar systems, and since planet earth formed within the size that allowed a substantial atmosphere and active geological processes, the conditions were perfect for many interesting kinds of chemistries to occur, including the set of chemistries that would to the formation of a molecule that carried instructions on how to replicate and how to synthesize catalysts for the formation of a cell membrane along with a set of chemical reactions for breaking down organic molecules in order to release energy to how to make two copies of itself which would be the ancestors of today’s cells.


Fossil evidence indicates that the first life forms date from 3.8 billions of years, and that the earliest life forms were unicellular and prokaryotic. We may never know exactly the conditions of what the early earth was like and as far deep as we can probe beyond that, there is absolutely nothing about what the conditions were like. Nonetheless, we can speculate, within the evidence available, of what set of conditions allowed for the origin of life. Current speculations suggest that as the earth condensed from the gas and dust of the cloud, at first, it was just then condensations from dust grain then as the grains got bigger and bigger, first from electrostatic attraction, then as the grains were the size of larger objects, then gravity began to pull all the planetesimals together into larger planetary size objects. The formation of planets was a violent process of collisions and as the sun began its phase of hydrogen burning, it underwent what is called a T-Tauri where it ejected a lot of energy and matter, before it stabilized and all that energy would easily vaporized any volatiles such as water and hydrogen while farther away, larger planets began forming and having stronger gravitational fields allowing them to absorb not just dust grains but hydrogen and that is part of the reason why the jovian planets are made mostly of hydrogen.


As the early earth began to form, it was likely a violent and turbulent planet, where it was bombarded constantly by meteors and comets, and with heavy vulcanisms. Slowly as the early earth cooled, much of the heavy metals began to sink , forming the core while the silicates formed the mantle and crust respectively. In addition, there was a lot of out gassing together with infall from comets and with outgassing, which add substantial water vapor along with comets which are mostly water, the earth’s oceans slowly began to form.


What was the earth’s early atmosphere composed of? It may have been a reducing atmosphere that is an atmosphere composed of hydrogen based gases such as methane and ethane, along with ammonia and nitrogen, carbon dioxide, and carbon monoxide, and hardly any oxygen, as well as water vapor. Also the earth was bombarded by ultraviolet radiation, and the surface was full of geothermal energy along with cosmic radiation and impacts from asteroids.


It has been known that when reducing gases such as methane, in combination with ammonia and water vapor are subjected to energy flow in the form of electrical discharges, ultraviolet radiation, and even shock waves, organic compounds including sugars and amino acids are sythensized. Even molecules such as nucleotides, the building blocks of DNA and also glycerols, which form the basis of cell membranes have also been synthesized and from these synthesis using hydrogen and nitrogen based gases, the molecules of life can easily be synthesized and it seems that with an atmosphere of reducing gases which may have been the case for the early earth, and together with energy present, the chances were good that there were plenty of those chemical reactions that lead first to molecules and then from the first life forms from which evolution through natural selection could then produce the biological diversity that forms the earth’s biosphere.


Of course, this is still speculation since there is nothing in the fossil record that indicates that conditions were like this as described and despite half a century of research into the field of what is called abiogenesis or prebiotic chemistry, there is still no complete theory on how inorganic matter could transform into living matter for there is a huge degree of complexity between a mineral grain which is composed of a few chemical elements arranged in a simple but repetitive crystal structure and a bacterium which is composed of a few chemical elements but arranged in a variety of polymers with various kinds of functions. A living organisms is a complex and interacting system that can reproduce itself , and that is something that not even inorganic matter so for the moment it is safe to say that since we are here and there is life, there had to have been a narrow set of conditions that slowly but surely resulted into that complex set of interactions we can call life and once it appeared, it definitely did not appear by just random chance but something other than chance was at work that constrain the all the possibilities resulting in the biosphere that is found on the third planet from the sun.


Just what exactly all are the life forms that characterize the earth’s biosphere? Life can be conviently divided based on a sharing of certain characterstics indicating evolutionary heritages but also certain phenotypes that sets each group apart from one another. The list of all terrestrial life forms will be described briefly and they are




Shigella , bacteria. (AJC1)



Lactobacillus casei, bacteria (AJC1)


Anaerobic bacteria or bacteria that can survive in the absence of oxygen. (Iqbal Osman)


Bacillus subtilis, bacteria (felixtsao)


The life forms that are the most abundant and dominate of the earth’s biosphere are the prokaryotes and these are unicellular organisms or organisms composed of only one cell. Prokaryotes include two of the largest  hierarchial groups, called domains which are the familiar Bacteria and also the Archaea. In both of these domains are characterized by a rather simple cell structure which consists of a cell membrane and a cell wall, composed of a protein-carbohydrate complex called peptidoglycan which is present in only in bacteria but a different version is present in the Archaea. Prokaryotic cells have no specialized structures called organelles and the DNA is not found in a nucleus. Rather, the DNA is wrapped tightly in a small volume called a nucleoid and enzymes or protein molecules that function in various biochemical reactions are present in the cytoplasm as well as in the cell membrane.  Both bacteria and archaea are present in many environmental regions of the earth such as in oceans, rivers, and lakes, as well as in soils and present in the air but are also found in areas where there are inhospitable to most life forms which includes freezing deserts, hot boiling springs, and near geothermal vents underwater. Prokaryotes have a wide range of metabolic functions in that there are not only prokaryotes that use what is called heterotrophy and that is the use of carbon and nitrogen from organic compounds but also autotrophy and that is the synthesis of organic compounds using inorganic compounds such as water and carbon dioxide and an energy source such as sunlight but the prokaryotes can also use mineral compounds such as sulfur compounds and even elements such as iron along with sunlight allowing them to exploit a wider range of environmental conditions.


Prokaryotes were the first simple organisms present on the early earth and they continue to be present in the biosphere. They are indispensable to the biosphere in that they efficiently recycle two of the most important chemical elements, carbon and nitrogen and play a critical role in supporting life which includes not just recycling of carbon and nitrogen but also providing oxygen from the largest group of prokaryotes, the cyanobacteria and indeed fossil evidence indicates that ancestors to the cyanobacteria, with their ability to use photosynthesis , produced so much oxygen that the quality of the atmosphere changed from a reducing to an oxidizing atmosphere and in addition, ultraviolet radiation converted most of the oxygen into ozone and with that, less ultraviolet reached the surface, so in an indirect way, life altered the conditions for its own existence.


Having a simple cell structure, prokaryotes reproduce through asexual reproduction and that is in a process called binary fission, a single cell can reproduce into two cells with an identical cell structure and an identical genetic makeup. Bacteria can also engage in a very simple form of “sexual reproduction” called conjugation and this involves two cells , one of which donates DNA, in the form of circular ring called a plasmid to another cell.




The other life forms include the eukaryotes and these are organisms that do have a membrane bound nucleus where the DNA is wrapped around histones and within the cytoplasm there are various organelles that perform specific biochemical functions such as the mitochondria which releases biochemical energy in the form of ATP, which provides energy for all kinds of biochemical and physiological functions, from nutrients and chloroplasts which perform photosynthesis. Eukaryotes can be unicellular but also multicellular and in multicellular organisms which includes fungi, plants, and animals, a multicellular organism consists of various kinds of cells that perform specific functions such as in animals, there are cell that function in carrying electric impulses which are the neurons, cells that synthesis insulin , and so on. There can be both asexual and sexual reproduction in that in the former involves a complex process called mitosis where one eukaryotic cell divides into two cells and in the latter, meiosis is the process of forming cells that have half the number of chromosomes, or gametes, and when two gametes, mainly sperm and egg, combine together in a process called fertilization, another organism results after round of cell division followed by cell differentiation where different cells form from an identical set of cells.


Eukaryotic organisms can only carry out two forms of metabolism. There is respiration which is carried out by animals, plants, and fungi and which takes place in the mitochondria where food molecules, undergo a complex set of enzyme mediated processes where energy is released into ATP in the presence of oxygen, resulting in water and carbon dioxide as waste products. Photosynthesis which is carried out not only in cyanobacteria but in algae and higher plants is when sunlight is used to combine water and carbon dioxide to produce carbohydrates and oxygen is released as a waste gas.


Evidence indicates that in  animal plant, and fungal cells, the organelles mitochondria , present in animal, plant, and fungal and chloroplast, present only in plant cells,  have their own DNA and double membranes and this points out to the fact that in the distant past, both these kinds of organelles were once free living organisms and in a process called endosymbiosis both the ancestor of mitochondria and chloroplasts were taken up by larger cells and in endosymbiosis the large cells provided protection for these smaller prokaryotes. Neither of which could survive without the other and so the cells with mitochondria became the ancestors of fungi, animals, and plants , whereas those with chloroplasts as well as mitochondria became the ancestors of plants.




Amoeba (Tim Menzies)


Paramecium bursaria (Picturepest)


Volvox (Craig Pemberton)


Euglena mutabilis (Picture Pest)


In addition to prokaryotes, there are a variety of unicellular eukaryotic organisms that are capable of respiration and photosynthesis and there are a large variety of these microbes that inhabits various kinds of environments as well as being descendants of an endosymbiotic cell and these are collectively known as the protists.


Protists can be either photoautrophic or organoheterophic but there are species that are both.


A variety of unicellular eukarytotic organisms include the well known paramecia, amoeba, euglenas, and vorticella, as well as seaweeds and the kingdom Protista as it is called is a very diverse group of unicellular as well as multicellular organisms.




Yeast (eLife-The Journal)



Fungal Mycelia (The_Gut)


Mold (Thomas Bresson)



Mushrooms (Karen Neoh)



This kingdom of organisms includes a few species of unicellular eukaryotes which includes the yeasts and also the multicellular organisms, the molds and mushrooms. These kinds of organisms are heterotrophic or more specifically saprophytic that is they can live off on dead plant and animal tissues and also play in role in recycling organic and inorganic nutrients.



Algae (Roban Kramer)


Moss (rjp)



Trees in a forest (Nicholas A. Tonelli)


Flowers (Patrick Nouhallier)


These organisms are either unicellular as well as multicellular eukaryotic organisms that carry out photosynthesis mainly because of the presence of chloroplasts, the organelle where photosynthesis occurs. These organisms take in water, carbon dioxide, and along with mineral nutrients and together  with the energy of visible light, convert water and carbon dioxide into carbohydrates which range from sugars, starches, and cellulose.


Plants come in a variety of species and these range from mosses which reproduce using spores to conifers, broad leaves, and flowering plants , all of which reproduce by making seeds. Although plants can grow in order to reach for sunlight, these kinds of organisms are completely immobile, having to depend on a solid substrate from which water can be extracted. In the process of photosynthesis, oxygen, a waste product, is released, and it is because of plants that the bulk’s of earth’s oxygen is created. Plants form the basis of the major terrestrial ecosystems providing both food as well as shelter.




These organisms have the ability to move from one place to another, are able to sense their surroundings and actively move through three forms of motion, swimming, running, and flying. Animals live in the oceans and in land and are adapted to various kinds of ecosystems.



Jellyfish (Daniel Spiess)


lionfish (Elina Ezera)


snake (Karra Rothery)


Beetle (Katja Shulz)



Dolphin( Ed Dunes)


Monkey (Snake3yes)




If life is assumed, by the principle of mediocrity, to be anything but typical, then we can be confident in knowing that the origin and evolution of life is then universal then to the understand life, we must not focus only on terrestrial life but extraterrestrial life, how it can form depending on what conditions and if these conditions can allow or constrain the evolution of extraterrestrial life.


The evolution of life depends on the evolution of the solar system and in fact biological evolution is just one part of a process of evolution called cosmic evolution and so ultimately the small part of the matter and energy needed to form, support, and maintain life depends on the origin and evolution of the universe.


According to accepted theory, our universe had a definite beginning around 15-20 billions of years ago, and in an event called the Big Bang where a massive explosion resulted not just the origin of matter and energy but also of space and time and even though we still don’t know how or why there was a Big Bang, nonetheless the Big Bang resulted in the future evolution of the cosmos.


In the first few minutes, temperatures were so high that not even atoms such as hydrogen could exist. Rather it was a violent mix of energy and matter with energy being the most predominant. As space expanded, there was a decoupling of matter and energy and thanks to the rapid expansion of space, matter in the form of protons combined with electrons to form hydrogen atoms and along with neutrons, the isotopes of hydrogen, deuterium and tritium, along with helium, with two protons and two neutrons, and lithium, so the first three elements , hydrogen, helium, and lithium were synthesized, only when the universe expanded rapidly and indeed this would have happened had the universe undergone what is called inflation and that is the rapid expansion within a short amount of time.


As matter, in the form of hydrogen, was synthesized, the radiation was stretched from gamma rays up until microwaves and in the universe today, there is still the remnant fossil radiation , which is detected as microwaves that fill all of outer space and that is one of the evidence of the fiery origins of the universe.


As the universe expanded, there was dense regions of hydrogen gas and from these dense regions, came the first proto galaxies and with gravity causing the dense regions to collapse, came the first stars, which according to recent models, these stars would have been not only pure hydrogen, but a hundred to a thousand times the mass of our sun, and in the first generation of stars, nuclear fusion will not only synthesis hydrogen to helium but the heavy elements and it is likely that the protogalaxies had higher rates of star formations and along with many supernova explosion, enriching the surrounding medium not just with hydrogen but also heavy element beginning a cycle of death and rebirth, where each new of generation of stars has heavy elements along with the elements for planets and life, and this has happened in one corner the galaxy that we call the milky way and presumably all other solar systems in our galaxy and in other galaxies.



A star that burns hydrogen into helium for 9-10 billions of years, size of a planet with a gravitational field to hold on to an atmosphere, plate tectonics that recycle greenhouse gases, and being situated within the center of a habitable zone, and more importantly, the presence of supernova for enriching the interstellar medium with heavy elements. All of these are just some of the factors that have allowed the earth to be a perfect habitat for life and since there are a huge number of stars in the Galaxy with planets, it is not asking too much that given the large number of planets that are in orbit, some of them may have conditions like the earth where a biosphere may be evolving and a smaller number may have intelligent life which may be or even beyond the level of intelligence of Homo sapiens.


From what we know in general regarding the conditions for habitability of earth, we proceed to the next question, whether life is present in the other planets of our solar system and the probability of its occurrence in other solar systems.


What is the Origin, Evolution, and Distribution of Life in Our Solar System and Other Solar Systems?


If we are to know whether life is a common occurrence or a set of conditions that would not likely to occur elsewhere, we first had to understand why planet earth is the one planet in the solar system capable of supporting life and to fully answer the question we did not just focused on biology but had to consider geology and astronomy in order to answer that question and notice that several disciplines of science had to be required and this is the beauty of the science of astrobiology which relies on several scientific disciplines not just in answering the first question but also of the second question; What is the origin, evolution, and distribution of life in other solar systems? In order to determine if life is common, we must consider first how planets can support life by considering the lifetimes of stars, the size of habitable zones, the presence of chemical elements that are heavier than hydrogen, and the availability of energy as some of the factors for assessing the probability of life’s occurrence.


We will consider these factors first for the planets of the solar systems and also for the planets for other solar systems in assessing the probability of life as well as the probability of extraterrestrial intelligence in other solar systems. Before we proceed, however some caution must be warranted. For one thing, we are still considering the possibility that life within our solar system and elsewhere must be based on carbon and water. For one thing carbon and water are pretty abundant throughout the galaxy in some parts and possibly in other galaxies and because of the way carbon can form a variety of compounds, it is hard to imagine any living system composed of anything complex that is not based on carbon and although scientifically any biochemistry that depends on elements other than carbon have not been ruled out, it is still difficult to imagine how this is possible without considering what Sagan (1973) called “carbon chauvinisms” since obviously the only life that we are familiar with is the terrestrial kind, but until there is evidence to the contrary, which I believe in the future there will be an extraterrestrial life form discovered that uses some sort of exotic biochemistry, we must stay with the possibility that carbon and water biochemistry is universal and also there is a high degree of confidence that organic compounds are also present through the cosmos, since methane, ammonia and even the precursors of complex organic compounds such as proteins have been found in meteorites, comets, even in interstellar clouds suggesting that a biochemistry based on carbon is present but possibly not the only kind of biochemistry. With that we will then see which planetary environments are likely as well as less likely to harbor life.


As we do so, we will consider three criteria for assessing the probabililty of a biosphere and the criteria the abundance of carbon, energy needed for causing chemical reactions, and water as a solvent where the chemical reactions can occur.


The Sun


The sun (David Warrington)


Of all the stars making up the galaxy, no star is more important to the evolution of terrestial life, in terms of the habitable zone, than the sun itself.


The sun, like all stars, is composed of hydrogen and helium along with few heavy elements such as carbon. The energy released, in the form of electromagnetic radiation originates in the center or core of the sun where hydrogen is converted into helium via nuclear fusion and this is a form of nuclear reaction where at temperatures ranging in about 16 millions Kelvins, four protons, the nuclei of hydrogen, are slammed together to form a helium nucleus, which consists of two protons and two neutrons ( this is the net process, it is really a multi step process but for the sake of this blog, I will keep this description simple) while energy in the form of gamma ray photons are released.


All the gamma ray photons move in all directions from the core of the sun, to the next layer which is the radiative zone and that is the bulk of the energy released in the form of radiation. The radiative zone then ends with another zone, the convective zone where the energy of thermal radiation is transferred in the form of convection which is a process where hot gases rises and at the top where the thermal energy is released, resulting in a net cooling and from the cooling, gases become dense enough to sink. From the convective zone, there is the solar surface or the photosphere and as energy which begins at the core makes it way to the surface, the energy is in the form of visible light while there is a change in temperature ranging from 16 millions Kelvins to at the surface 6500 Kelvins and from the surface the energy for terrestrial life is available within the volume of the habitable zone and stars that are similar to the sun, have a habitable zone that is roughly around 1 A.U or the mean distance from sun to earth which is about 93,000,000 miles.


In regards to the habitable zone, the size of this region of space around the sun where water can exist in three phases (liquid, solid, and gas), the zone, like the evolution of the sun, is not static but changing through vast time scales. The sun has a large but finite amount of hydrogen in its core where hydrogen is converted into helium and will do so in the next 5 billions years. Once all the hydrogen is gone, there is no other source of energy for further nuclear fusion other than the gravitational pull of the layers of the sun and with no thermal energy to balance the inward gravitational pull, gravity ends up compressing the helium core, causing the temperature to rise higher resulting in fusion of heavy elements up to carbon and more energy is released and this energy will then cause the sun to expand into greater volume turning it into a red giant and with the sun expanded in this huge size, the sun will engulf two of the closests planets Mercury and Venus and this will likely destroy the earth as well. As for the habitable zone, it will likely expand covering the volume up to Jupiter and Saturn so the size of the habitable zone depends on the evolution of the sun.


As for the surface of the sun with temperatures ranging around 6500 Kelvins, the temperature is hot enough to allow hydrogen and helium but also hot enough to destroy complex molecules like proteins so obviously there can be no life on the surface of the sun and excluding the possibility of life forms with an exotic chemistry allowing for its survival on the solar surface, we will only consider life on the planets that orbit the sun as well as the satellites that orbit these planets.




Mercury (NASA Goddard Space Flight Center)


The innermost planet of the solar system is Mercury and one of the first of the two main groups of planets, the terrestrial planets or planets with dense metallic cores surrounded by layers of rock, usually silicate rocks  and metal and within the size and volume of the earth.


Being the closest planet to orbit the sun, Mercury experiences a form of tidal locking where as the planet rotates it shows one face towards the sun as it orbits the sun much like the moon showing the same face towards earth as it orbits the earth and Mercury makes a revolution for a total of 88 days which is the length of the year on Mercury.


For the physical characterstics of Mercury, starting with observations from Mariner 10 which did a flyby of the planet in 1975, along with a recent orbit of another space probe MESSENGER, and from data from these two space probes, Mercury is in ways similar to the moon for it has plenty of craters and signs of ancient volcanism. From measurements of the density of Mercury, the planet has a crust, mantle, and core with the core taking up a bulk of the planet’s interior. Despite this, there is no sign of any active geological activity and it has been that way for billions of years.


Mercury seldom has an atmosphere albeit a very tenuous atmosphere composed of hydrogen, sodium, helium, and argon and being close to the sun, the intense thermal energy has driven these elements away from the surface. In addition, Mercury is the smallest planet of the solar system and with a weak gravitational field , it is unable to hold on to an atmosphere where the pressure is strong enough for liquid water to exist on the surface.


Surface temperature range from 380 K to a low of 100 K and even though regions of Mercury’s surface is hot, at the poles it is cold and at the poles there is water, but only in frozen form.


From what we know so far  of Mercury, what is the probability of life existing on the surface? Aside from energy flow on the planet’s surface, there is no atmosphere to keep a surface layer of water, and without an active plate tectonics to recycle biogenic elements such as carbon and nitrogen, along with an atmosphere doing the same form of recycling, and along with temperature extreme , as a result of a fast rotation rate, the chances of life existing on Mercury is extremely low and so for the moment we can then disregard the environment on Mercury as a lifeless planet and we will then move to the next planet, that interestingly has nearly the same mass, radius, and density as Earth but with a totally different environment and well within the habitable zone of the sun and that is the planet Venus.




Venus (toneynetone)




The second planet from the sun, Venus, is in some respects similar in to earth in terms of size, mass, and density but their surface environments are vastly different. The atmosphere of Venus is composed mainly of carbon dioxide, with small amounts of nitrogen and sulfur dioxide and the atmospheric pressure is 92 times that of the earth. With intense atmosphere pressure , the surface of Venus has a hot surface temperature that is around 462 degrees Celsius , which makes it surface much hotter even than Mercury despite the fact that the latter is closest to the sun.


The hot surface temperature is the result of all the carbon dioxide trapping the infrared radiation released from the surface. Visible pours in from the sun, warming up the surface but the carbon dioxide impedes the flow of heat and since Venus is close to the sun than the earth, it receives a lot of sunlight. Venus, then has the strongest greenhouse effect, and in fact the only planet with a strong greenhouse effect.


Venus does show evidence of geological activity in that there are vast extensive volcanic plains along with two major continental masses indicative of plate tectonics but of a different kind.


What are the prospects of life on Venus? Given that the surface temperature doesn’t change much by 462 degrees Celsius which is close to the melting point of lead, it is extremely unlikely that any complex life forms can survive on the hellish surface. Any complex molecule such as DNA and proteins will immediately break down and obviously there can be no flowing water, so with the criteria of life forms that are based on carbon and water, it is unlikely that an ecosystem can survive on the surface of Venus.


Despite this, this in some ways does not necessarily mean that Venus despite its harsh environment is completely free of life. There has been some suggestions regarding the possibility of at least simpler life forms, mainly extremophilic microbes that are adapted to hot temperatures but given the extreme conditions, there would be few options for life to take a steady hold.


If there is at least microbial life on Venus, where would it possibly be found? Or rather, what reasons do we have in believing, before actually proving somehow, that there could be life on Venus, if any?


Evidence, beginning with the atmosphere of Venus, indicates that possibly shortly after Venus condensed from the original solar nebula, the planet may have had substantial water on it’s surface, perhaps even an ocean of water but unlike earth, which has kept its oceans of water, Venus was unable to hold on to its oceans since Venus is much closer to the sun and receiving much more sunlight, this started a process where the surface temperature heated the Venusian surface, resulting in gradual but massive evaporation of water and water vapor, like carbon dioxide, is also a greenhouse gas which traps infrared radiation emitted from the surface.


With more water vapor being released into the atmosphere and nothing to prevent it from going back to the oceans and as the oceans were drying up, more and more water vapor was added to the atmosphere further heating up the surface and as the surface got hot, it was hot enough for the release of carbon dioxide from carbonate rocks and together with volcanoes, which can also give of carbon dioxide, this resulted in a runaway greenhouse effect which after hundreds of millions of years later, the surface of Venus is now at a temperature around 450 degrees Celsius (800 degrees Fahrenheit).


There is still some traces of water vapor but it is confined within the upper atmosphere. Considering that if true, then if there was water, along with organic compounds and with a steady source of energy, then there is no reason to think that there would have been a prebiotic chemistry which may lead to life forms, albeit microscopic, on Venus but given the fact that a runaway greenhouse effect has occurred, such gradual but dramatic changes may have either destroyed the first life forms or some did survive but evolved into forms that required them to adapt into environments where it is less extreme. Where could Venusian microbes survive? One possibility is that there are microscopic life forms in the upper atmosphere of Venus.


At first, it would seem that this would be a place to adapt given the hellish conditions on the surface, but there is a problem with this scenario. It turns out that life forms , not even bacteria, can complete their full cycle in the atmosphere.


If simpler life forms had to either adapt or die, as the Venusian surface became close to the temperature of 800 degrees Fahrenheit, then it could evolve to thrive underground and if it is life as we know it (that is life based on carbon and water) then given that the surface was getting hotter and anything organic when heated, releases carbon dioxide along with disappearing amount of liquid water, the chances of survival on the surface would have been extremely slim so one option is to evolve underground where there is a chance of liquid water and a source of geothermal energy, which in the case of earth, there are microbes that can live of the chemical energy from hydrothermal vents but it turns out there is just one problem with this scenario. Unlike earth, which has an active system of plate tectonics where minerals with elements needed for life, are recycled, and in order to have hydrothermal vents, there must be active plate tectonics to work. What would be the case for Venus? Recent findings such as the two year study conducted by the Magellan space probe which went into orbit during the early 1990’s, mapping the Venusian surface , reveals that Venus does have evidence of some kind of plate tectonics but of a different kind that is in no way related to what happens on earth.


It turns out in order for plate tectonics to work, water is required to lubricate the massive plates covering a planetary surface and given what we know of the Venusian surface, there is no water and without water, plate tectonics grinds to a halt and with no active motion of plates, there seems to be no chance of underground vents , if any, and so the chances of microbial life, thriving underground , with a steady flow of chemical energy seems very remote, so Venus is considered to be a geologically dead planet, in that Venus does show very little, if any sign, of geological activity.


The only way for a surviving generation of microbes to find refuge is in the lower atmosphere, where there is still traces of water vapor but if life on Venus had no other choice but to remain in the lower atmosphere, there would still be problems and I’ve briefly mention that no life form, not even bacteria, can complete a full life cycle in the atmosphere.


However recent studies of the Venusian atmosphere do suggest that the lower atmosphere may actually be suitable for a thriving population or populations of microbes. Unlike earth, the weather system of venus is completely different. For one thing, because of the hot surface temperature, which is uniform on the surface, there is no wind and so there is little movement of the lower atmosphere but as one goes up the atmosphere, the speed of the clouds increase and in between there is expected to be large shear forces so any particle that is in the lower clouds would be picked up by the shear forces and up the upper atmosphere. What would this mean for microbial life? If there were photosynthetic organisms, then being caught in the circulation would be advantageous since these organisms could then be exposed to sunlight which is present in the upper atmosphere but below hardly any sunlight penetrates the lower atmosphere.


Also the clouds are more or less continuous unlike the clouds of earth which are constanly changing and are made up of water vapor. The clouds of venus are made up of sulfur aerosols along with sulfur dioxide and hydrogen sulfide. At first, it may not seem that this would be a comfy abode for life, but we are making the mistake of thinking that it is terrestrial life that we are familiar with. Rather, Venusian life would have to be similar to extremophilic life and indeed on earth, there are extremophiles that have the ability to metabolize sulfur compounds such as hydrogen sulfide and this could be the perfect environment for Venusian bacteria with the ability to live of sulfur based compounds as an energy source.


Bacteria have been showed to live in tiny droplets of fluid even in higher atmospheric altitudes and the same could be true for Venusian bacteria adapted to harness chemical energy from sulfur compounds. There is still the problem of radiation exposure since the atmosphere of venus is bombarded by ultraviolet. Venusian microbes could easily survive the assault of ultraviolet by encasing themselves in a type of sulfur called S8 so named because the sulfur atoms form a ring of eight sulfur atoms. This arrangements absorbs ultraviolet and releases the radiation via fluorescence. Anything including bacteria surrounded by S8 would end up being protected and indeed this is the kind of protection some terrestial bacteria use in order to survive in environments with high amounts of UV light and in the evolution of life on earth, some bacteria with the ability to survive in ultraviolet radiation by despositing S8 was favored by natural selection and had an obvious advantage. The same would be true for Venusian microbes since sulfur compounds are plentiful in the atmosphere.


There may be hints that this is actually occurring since the Venusian atmosphere has been thoroughly studied by space probes. Venus has a high albedo or high amount of reflectively and it reflect so much sunlight that in order to see details, you would need to the atmosphere with ultraviolet light and in ultraviolet light there are many details to be found which could otherwise not be seen in visible light, alone. Through ultraviolet light, there are various kinds of dark streaks and these are interpreted to be absorptions caused by S8. Why the absorptions caused by S8? Is it likely that this is a sign of biogenic activity or in other words life , that is microbial life using S8 as a “sunscreen” or is it also likely that the S8 is caused by volcanic activity? I will speculate about these possibilities and for one thing, venus does show signs of volcanic activity but because there is no active plate tectonics there is little or no volcanic activity giving off sulfur based compounds and if there were, it would be too little to provide a source of S8 so it is also possible the result of past volcanism which is possible.


The other possibility that this is indeed the presence of Venusian life using S8 as a protection against ultraviolet and it is nothing but S8 if interpretation of the absorption streaks are correct and indeed we are observing life but so far the evidence is taken from orbital missions and even so there is still no way to determine whether if the streaks are caused by microbes are just the result of volcanism and the only way to settle the matter is to send a probe into the atmosphere for analysis and with the ability to distinguish if the sulfur causing the absorption streaks is the result of biological activity or geology.




Earth (Kevin Gill)


Earth is the one planet that is within the habitable zone and in orbit around a star that will last on the main sequence for 5-10 billions years into the future. What is it about earth with it’s ability to support life? In addition to being in the habitable zone, other factors contribute to the planet’s ability to support life in the history of the solar system.


What are the factors that make life on earth possible and what is the origin and history of life on earth? Being in the middle of the habitable zone is one of the factor, and like all planets orbits the sun which for earth takes a total of 365 days to make one complete orbit around the sun. Also the earth is tilted on an axis that is 23.5 degrees and because of the tilt, one half of the planet, called the northern hemisphere receives more sunlight than the lower half or the southern hemisphere which receives less sunlight and also in another part of the orbit, the southern hemisphere can receive more sunlight while the northern hemisphere receives less.


Earth rotates giving the daily cycle of day and night and earth has an atmosphere that is composed of nitrogen which makes up 78% and oxygen 21% which is the result of photosynthesis. The combination of being in the middle of the habitable zone, rotations that are 24 hours long, and with plate tectonics which help in the recycling of elements such as carbon dioxide together with keeping carbon dioxide away from the atmosphere, and locked up in the form of carbonate rock which sooner or later is heated to release carbon dioxide to the atmosphere, all of this keeps water as a liquid, making life possible.


Also, the constant flow of sunlight to the earth’s surface where given the distance and the size of the planet which intercepts only a tiny amount of sunlight is enough to warm up the earth’s surface releasing infrared radiation, the type of radiation that is felt as heat, which tends to escape into outer space but carbon dioxide, as well as water vapor and methane hold back the infrared resulting in a warm surface, and that is one of the factors why water is so abundant in that the average temperature is enough to keep at as a liquid. The pressure of the lower atmosphere also plays a critical role in acting as a lid from preventing water from escaping into space.


There is uneven heating of the earth’s surface and that is responsible for the weather and also there is also heat flow in the oceans and with uneven heating, this results in areas in both land and sea that are neither too hot and too cold (with the exception of the poles) but warm enough for an ecosystem to occur.


The earth’s surface is also not static but dynamic, on scales of millions of years, in the form of plate tectonics where the earth’s crust, the uppermost part of the earth, is divided into many plates which contains the continents and ocean and hundreds of millions of years, powered mainly by heat flow from the core, results in movements of the continents and wherever continental plates collide slowly the surface is slowly but inevitably altered.


Since earth is a dynamic planet, cycles are present and one such cycle that is important and vital to the evolution of life on earth is the cycle of water or the hydrological cycle.


The surface temperature of earth allows water to be in one of three states solid (as in ice which is found at the poles), liquid (as indicated by the oceans that are the most dominant features on earth) and gas ( as in water vapor which makes up earth’s clouds).


Because earth receives sunlight, the energy of sunlight powers the water where the energy is used in heating up the surface of the earth by striking on the oceans, since about 2/3 of the earth’s surface is covered in water, and when heat the water turns into vapor which condenses into clouds and clouds can release water in the form of rain back into the oceans but clouds can release rain as well as hail or snow on land. Water can flows as rivers towards the sea completing the cycle but also water can be found in lakes or underground and but eventually the water in lakes will evaporate to the atmosphere and even some water from underground will eventually reach surface , in the form of springs or in the case of heating from nearby volcanoes , as geysers or as hot steam from volcanoes themselves. Water cycles around between ocean and atmosphere, which as a gas, traps infrared heat , further warming the surface or the water can go to the land as precipitation and with the availability of water, life has made such use of it as has been for 3.8 billions of years.


Another cycle that is as important as water is the element carbon and like water, carbon is never stuck in one place but moves from one place to another. Carbon is one of the most important elements for terrestrial life, for without carbon’s ability to form a variety of bonds, much less the ability to cycle between earth, water, and atmosphere, then it is not likely that life could have originated and evolve.


Starting as a gas, in the form of carbon dioxide, which is present in the atmosphere in small but alarmingly in increasing amounts due to human activity, this gas along with water is utilized by all green plants through photosynthesis to produce carbohydrates from glucose up to cellulose, a complex carbohydrate, while releasing oxygen and indeed the bulk of the atmospheric oxygen has a biological origin that it is likely possible that if oxygen is detected on a planet that is orbiting a distant star, then it is likely that the oxygen is the result of life on that planet. The oxygen can support animal life which use oxygen in combination with glucose, proteins, and fats and together through respiration or “burning” with the oxygen, carbon dioxide is release from all animal life and the carbon dioxide can once again enter into photosynthesis.


In addition, carbon dioxide reacts with water to form carbonic acid and with carbonic acid can react to form certain kinds of rocks called carbonates and with carbonate formation, these rocks act to remove any excess carbon dioxide and these can form the sediments which eventually, through plate tectonics is recycled into metamorphic then igneous rocks and through volcanoes, the carbon dioxide is then release into the atmosphere, and it is through plate tectonics that the atmospheric composition of carbon dioxide has been maintained along with green plants within certain limits for millions of years.


The earth also has a magnetic field that is generated from the inner core of the earth and this magnetic field acts to prevent charged particles from bombarding the earth’s surface. The magnetic field is believed to be generated by the slow continuous motion of a liquid outer core of nickel and iron.


The surface of the earth that can support the diversity of all known life forms, or the biosphere, is one of the most distinctive feature of earth. There are over a million varieties of animals, plants, and fungi, and even more varieties of microbes and all life, despite their diversity, is based on carbon and water, uses a genetic code where sequences of three bases of nucleic acid code for one or more of twenty amino acids, the building blocks of proteins, which are the molecules that function as enzymes, or catalysts of biochemical reactions, use chemical energy in the form of ATP, which is both a product of photosynthesis and respiration, and are based on cells which use a phospholipid bilayer for keeping all vital biochemical reactions inside and function as a semi permeable membrane where certain substances are taken but not other and wastes are released.


All these properties that life has in common suggests that all of life is the descendant of some simple life form that existed long ago, in the earth’s ancient past and fossil evidence that dates around 3.8 billion years do show that the first life forms were simpler in structure but through evolution by natural selection, many life forms but not all of them become more and more complex, one species giving rise to several different kinds of species, each adapted to the earth’s ever changing variety of ecosystems.


Since we are confident that the first life form was simple in structure and already had the properties of life, then the question now becomes; just what were the conditions that allowed non life to become life? Or in other words, what is the origin of the first life forms?


Like the origin of life, the origin and evolution of life depends on the origin and evolution of earth since the biosphere depends on  the geosphere ( chemical elements such as carbon, nitrogen, and phosphorous from the earth’s crust), the hydrosphere ( water in liquid form) and the atmosphere (oxygen for respiration and carbon dioxide for photosynthesis), and since chemical elements cycle, this depends on energy flow which comes from the sun.


How was it possible for chemical elements to come together in order for the molecules of life to be form and how exactly can these complex molecules arrange themselves in a way that display the seven characterstics of life?


In the past 60 years, it can be showed that the chemical elements of carbon, hydrogen, oxygen, nitrogen, and phosphorous when exposed to forms of energy can rearrange themselves into the complex chemical compounds, including those that are found in life and this suggests that given an energy source that is provided by a star or energy flowing from a planet and together with heavy elements that were released from supernova explosions, then in a suitable platform , which can be a planet in orbit around a star or moon around a large planet, and as long as there is energy flow, then molecules of increasing complexity can arise and even network to form a crude kind of metabolism where molecules are synthesized and some are released as waste.


Indeed it is easier to show that the molecules of life can be synthesized in the laboratory but it is far more difficult to create a system even resembling that of life. The first , in a series of abiotic experiments or experiment that involves making the molecules of life, began in 1952 at the University of Chicago where a graduate student Stanly Miller along with his mentor Harold Urey set out to conduct an experiment where if it was possible to create carbon based molecules in what are called reducing conditions or conditions where hydrogen is present.


This was based on the assumption that in the early formation of the planets, the starting material was hydrogen and a few other heavy elements and if earth condensed out of the nebula that spawned the sun, then the early earth would have had an atmosphere that only had hydrogen and hydrogen based compounds, so starting with a chemical apparatus where a sample of water was boiled into steam and with methane, hydrogen, and ammonia flowing into the steam, the mixture was zapped by electrical discharges and the experiment ran for about two weeks. The results? After bombardment with electricity, the mixture formed a dark brown substance and analysis of the substance revealed that there were amino acids which included glycine and alanine.


Since then various versions of the Stanley-Miller experiment where various hydrocarbons and different energy sources such as ultraviolet, x-rays, heat, and shock waves were used and they point to the same conclusions, it is easy to form the molecules of life and so if earth had an atmosphere of methane and ammonia but no oxygen, then through various forms of energy , it is possible that organic molecules including those with the capacity to replicate and act as a source of energy for metabolism would have been available.


Although we are still fully ignorant in exactly how biochemical, can end resulting in that complex systems of reproduction, molecular replication, metabolism, and membrane enclosure that we would recognize of life, since there is hardly anything in the fossil record that would indicate how inorganic chemicals are transformed into biochemical which would then transformed into the simplest life forms, what is more certain is that the process would be gradually and as the earth in it’s early formation, once it surface cooled to allow liquid water to form, there had to be someway for the biochemical to come together under the influence of molecular forces to form the first cell membranes and also in order to maintain cell membranes there had to be a metabolism, for the synthesis of building blocks such as proteins and a source of information for coordinating all of this such as nucleic acids. Whatever and however the process occurred, it could not have been a truly random process for if it was, then there would have been no life at all. In fact, an argument involves the many possibilities  there can be for a protein molecule such as an enzyme and that is given an enzyme with a specific function and if that enzyme is composed of 20 different amino acids that is 120 building blocks long  or in other words, the enzyme is composed of 120 of all 20 amino acids, what are the number of possibilities for the arrangements of these 20 amino acids? It turns out it would be way more than 120^20 which turns out to be a very huge number that is astronomical! Even more so, finding the right enzyme with the correct configuration of amino acids expressed as a probability is also the reciprocal of that huge number and there lies the problem.


The problem is that mere chance, by itself, is not sufficient enough for explaining how complex polymers can just come together to form life, even as something as complex as a bacterium. Chance does play a role but not the dominant role. Rather, necessity such as the fact that certain combinations of chemical elements are more favorable than others, reactions can occur in some input of energy but not others, and also the fact that systems that do reproduce will leave descendants that are somewhat different than their parents and if one of these descendants can cope with new environmental changes better than those that do not will be favored.


If the first systems of life had the ability to adapt, then according to natural selection, those that can will be allowed to survive to the next generation and if there was an ability to transfer the information to their offspring , that too will be favored.


A mutation in the early genetic system, which is a chance event, that would end up ruining the life form’s ability to reproduce would of course be selected against but if a mutation allowed better efficiency in metabolism or for the ability to produce changes in the genetic information to reproduce then it will survive.


A combination of chance and necessity would then not only favor a system that can reproduce, carry metabolism, and mutate, but gradually the first life forms would then evolve and indeed this is likely how life on earth had its start and from the fossil record, there is evidence that the first life forms had a simples structure that would be recognizable as bacteria , which is dated around 3.8-4.1 billions of years.


From the earliest life forms, most life forms were microbes but other life forms evolved into eukaryotes and the accepted theory of the origin of eukaryotes was that two separate life forms, an early eukaryote cells and bacterium that could do perform respiration and through the union, that eukaryote that absorbed the bacterium , and through a process called endosymbiosis or “living together from within”, the bacteria that was not digested, provided valuable service such as providing an efficient way of making ATP for the large cell, and with this union this would have been the ancestor of all eukaryotic organisms such as fungi, plants, and animals. Also another ancient eukaryote cell that absorbed a bacteria capable of photosynthesis, would have been the ancestor of all plants.


From the evolution of eukaryotes came the evolution of a method for generating genetic diversity, meiosis, which allowed various ways of genes on chromosomes to be arranged, resulting in progeny with novel genetic combinations and hence somewhat different phenotypes such as enzymes for digesting foods, new behavior such as avoiding predators, and so on.


The result was the gradual evolution of life forms that in various species of animals and plants that lived on earth , only to go extinct, and through the long history of life on earth, there was a total five extinction events, where each extinction event was followed by diversification of species that survived each extinction, and the largest extinction , interestingly enough was followed by the largest diversification of multicellular organisms that was known as the Cambrian explosion , and for reasons that are still unclear, this one event in the natural history of earth, involved a massive speciation of many life forms, most of which likely to belong to phyla, that are no longer there , although the ancestors of all extant species also began in that period of rapid diversification, only to have been followed by a massive extinction called the Permian Triassic extinction where about 90% of all life went extinct, allowing surviving species to diversify, one of which was the ancestors of today’s mammals including humans.


Earth is the only planet, in this solar system, that is home to an intelligent species, Homo sapiens, and there are now 6 billion all over the planet with the exception at the poles, and from the perspective of evolution, the appearance of a species with a large brain, a fully developed language, and the ability to manipulate objects, or tool making, humans would be something of an anomaly in that unlike all the other life forms which inherit genetic information, not only do we inherit genetic information but also we inherit information in the form of language, set of beliefs, the stories of those who came before, in short we are a species with culture. Also we are a species that use technology ranging from making fire to making computers.







Mars (Kevin Gill)


Of the four terrestrial planets and other than earth, Mars is the other planet that shows a strong possibility of life, at least microbial life, and indeed there was one attempt by sending a pair of space probes which were on the surface for the purpose of determining whether there was life but failed ( although some scientists think that the results were neither negative nor positive which I will explain later) and later it was discovered that a meteorite of martian origin showed evidence and even a possible life form but even that has yet to be conclusively proven.


There is plenty of evidence that Mars once supported life and for one thing there is plenty of evidence that Mars had flowing water for there is plenty of ancient river beds and sedimentary rocks some of which can only be formed in water, and there is even evidence of methane in the atmosphere which can either have a biological and/or abiological origin, and it was possible that early in its history the surface was warm enough not in having running water but there could have been an abundance of organic compounds needed for a prebiotic chemistry and also with an atmosphere that was thick enough for flowing water but eventually that did not last since the atmosphere became thinner and thinner and most of the water begin to evaporate and now the surface of Mars is cold at least during the winter months , and even though it can get warm on Mars , even as high as 95 degrees Fahrenheit (36 degrees Celsius), it can get extremely cold during the winter and even colder during the winter months.


Nonetheless, despite the fact that the atmospheric pressure of Mars is 6/1000 making it much less dense than earth’s atmosphere and with no ozone layer to shield the surface from harmful UV rays and no global magnetic field to shield the surface from even harmful cosmic rays, the possibility of extant/and or extinct life, albeit at the microbial level, still has yet to be ruled out, even though finding evidence for life on Mars has been even so tricky.


Two attempts were made to determine whether life , at least microbial life, were present. One involved the use of a space probe, the Viking landers which landed on two spots near to one another, and the other in the form of a martian meteorite, and the results from these studies? It turned out that finding life on Mars was rather ambiguous, in that the findings were neither supportive for evidence of life, nor against.


Despite the  disappointment of not  finding martian life, recent findings in terms of the geology of the surface raises the possibility of past habitability. One of the evidence that Mars may have supported life (by this, I mean microbial life) is that there is plenty of evidence that liquid water, one of the solvents that makes life possible, and one of the criteria that I have listed, once existed and the evidence ranges in the form of ancient river beds, certain types of rocks that form in the presence of flowing water, and areas on the martian surface where there may have been evidence of an ancient ocean, but to what extent on the size of the ocean still remains controversial.


In addition, the properties of the soil has been found to contain various minerals some of which can even support plant life if plants from earth could be grown in martian soil which they would likely do.


Another unsuspected bit of evidence of possible past life involves finding an unexpected gas in the Martian atmosphere. What is the composition of the Martian atmosphere? The composition of the Martian atmosphere is mainly carbon dioxide, about 96% together with tiny traces of nitrogen and oxygen. Even though, carbon dioxide is a greenhouse gas, the atmosphere cannot hold any heat from the surface simply because of the atmospheric pressure which is much thinner than even the atmosphere above Mt. Everest!


The surprise that came when studying the Martian atmosphere and this was confirmed by two independent observations was that there was a gas that appeared in the atmosphere which shouldn’t be there but it was and it lasted for about four years before disappearing. What is this gas and what does it have to say about the possibility of life on Mars? It turns out that the gas is none other than methane.


At first, it may not seem extraordinary to learn about methane generating much excitement but it has and it turns out finding methane on Mars could be the key to finding life on Mars. On earth, methane that is generated tends to be biological in origins and the source includes a type of extremophilic bacteria called methanogens which as the name suggests produce methane gas as a waste product.


Could the fact that methane found on mars be a sign that there is still living things, notably live microbes on mars? That is one possibility and it has been shown in a model that if underground deep away from the radiation that bombards the surface and also underground with pressure for water to remain as liquid along with some chemicals, there could be populations of methanogens that release methane gas as a waste product but that is just one possible scenario. Another scenario, which doesn’t involve life, involves water and carbon dioxide reacting in the presence of a mineral called olivine which could also release methane gas. Future missions to mars must be made in order to resolve the issue.


All of this is in favor for supporting hardy extremophilic microbes but if there are still microbes living on Mars, what would they actually be like? This is still a difficult question to answer and for one thing, we still do not know, if life on Mars would be based on a carbon and water biochemistry or it may be based on something other than carbon and water and if the latter is true, it may be one of the reasons why we haven’t find actual life is simply because of the sheer unfamiliarity of life based on alternative biochemistry. We really don’t how a life form based on alternative biochemistry would function and if there were on Mars or on other planets, we would not know what to look for, other than the patterns that life would share, which could be the only important criteria for proving that it is life.


Despite this, we must continue the search for life on Mars and every time we do make the effort for studying Mars, time again we find unexpected surprises and indeed there are plans once again to search for life on mars with some upcoming future Mars mission, and who knows what interesting and surprising discoveries await on the red planet?




Jupiter (Steve Hill)


The largest planet in the solar system, Jupiter is unlike the other planets mentioned so far. Jupiter, like Saturn, Uranus, and Neptune are the jovian planets and these are large planets composed of two gases, hydrogen and helium, and unlike the terrestrial composed of a dense metal core with layers of silicate rock, jovian planets like Jupiter are mainly hydrogen and helium throughout the center where it is possible that there is a dense core at the center but other than that, the bulk of these planets are just hydrogen gas.


With planets that are much more massive, these planets tend to have large gravitational fields which allows them to hold to many natural satellites, and Jupiter, being the largest has up to 16 satellites.


Given the size and composition of Jupiter, and although there are some organic compounds , such as methane, in the upper atmosphere, it is not likely, if at all, that there may be Jovian life forms.


It may or may not be possible that there could be life in the atmospheres of Jupiter, however one of the satellites of Jupiter may actually be another abode of life, and that satellite are one of the four moons or Galilean satellites, which has been studied and will continue to be the future subject of exploration and that is the satellite, Europa




Europa (Marissa Fessenden)



Aside from the two terrestrial planets that could possibly harbor life, Europa is a moon of Jupiter that could be a possible abode for life, especially life as we know it. What is about that moon that makes scientist believe that it could be harbor life? Ever since the Voyager flyby of 1979 followed by the Galileo spacecraft mission, it turns out that Europa may be the one world, other than earth to actually have liquid water!


How do we know that Europa may have an ocean of liquid water? From the surface of the moon, it is pretty bright and reflects a lot of light, which is unusual for a moon which tend to be dark mainly because of silicate rocks. In addition, closer examinations reveals no craters or any impact craters which tends to be erased quickly, unlike a rocky moon where an impact crater is a permanent feature. Also, there are plenty of ridges on the surface that criss cross one another and also the ridges tend to be red, possibly indicative of inorganic materials.


From these observations, what can we conclude? The only logical conclusion to draw is that the surface of Europa is covered by a shell of ice and in order to explain the ridges and lack of crater, the shell of ice has a layer of liquid water and if true, then this a world with liquid water other than Earth.


Also, since Europa is the second of the Galilean satellites, from Jupiter, close to the innermost moon Io along with the other two moons Ganymede and Callisto, and because of the gravitational pull between Jupiter and the other three moons, Europa is expected to experience tidal stretching and because of tidal stretching, enough heat is generated within Europa’s interior, which what may be responsible for keeping a layer of water warm.


Another intriguing possibility is that because of the tidal stretching, there could be also be underwater geothermal vents, not unlike those found at mid oceanic ridges on earth and with a source of chemical energy that is constantly flowing, a thriving ecosystem could also be present, which is not an unrealistic possibility provided that in addition to water, in order for their to be life as we know it, there must be carbon as well as nitrogen, phosphorous, sulfur, for the building blocks of biological molecules such as proteins, and a source of energy for building and maintaining biological cells. If true, then Europa may be the one habitat with life other than earth.


In the 2020’s, and if all goes well, NASA will send a space probe called Clipper which will orbit around Europa about 45 times and it will detect using sophisticated instruments, the presence of both inorganic and organic compounds. Clipper will even use radar to determine the thickness of the ice crust, measure salinity or the amount of salt present in the water, as well map the surface of the ice.


To determine whether or not, there is life would require sending a probe below the ice but for the moment we still do not know the thickness of the crust to warrant sending of such a probe but eventually, if all goes well with Clipper then eventually a specialized probe that could function underwater will someday be sent and the answer of whether or not there could be life outside the habitable zone of earth will be answered.




Saturn (Steve Hill)


Saturn, the second jovian planet, is somewhat smaller than Jupiter and like Jupiter is composed of hydrogen and helium, and some organic compounds and inorganic compounds in the upper atmosphere. Although given the conditions on Saturn, it is very unlikely that it’s surface is habitable but like Jupiter, it has a large number of moons in orbit and one of the moons of Saturn may or may not be habitable but it does have some interesting surface features where it has a large amount of methane and nitrogen and a dense atmosphere, which is something unusual even for a moon and it has been the subject of a probe flyby called the Cassini Huygen probe which even successfully deployed a probe on it’s surface in 2004, and like Europa is also of interest as astrobiology concerned and may actually hold the key for understanding the origin of life, and that moon is called Titan.




Titan (Justin Cowart)


Titan, a moon of Saturn, is very unusual that it is the only moon with a dense atmosphere.


Just what is the atmosphere of Titan is composed of? From spectroscopic measurements ( I will discuss spectroscopy in detail later) taken from earth and from the Voyager and Cassini Huygen probes, the atmosphere is composed of both nitrogen and methane.


Interestingly enough the composition of Titan’s atmosphere is similar to what is assumed to be the atmosphere of the early earth, except that the surface temperature of Titan is estimated to be so cold and frigid that methane can exists as a liquid.


Titan is also exposed to ultraviolet radiation and it is though that the interaction of ultraviolet and methane results in the synthesis of complex organic compounds.


Indeed after measurements were taken by the Voyager flyby of Titan in 1980, and using data of the chemical composition of Titan’s atmosphere, an experiment where methane and nitrogen, was subjected to ultraviolet did reveal the presence of a class or organic compounds called tholins and if tholins are put into water, tholins will turn into amino acids.


However, because the surface of Titan is cold, there can be no chemical reaction where water would react to tholins or other similar organic compounds since any water that is present would be ice.


On earth, water flows between oceans, atmosphere , and land but on Titan, thanks to a dense atmosphere, methane can easily flow on it’s surface and there is something like a methane cycle where methane can exist as liquid and can change state existing as liquid and as a gas, much like water can be liquid and vapor on earth.


What are the chances of life on Titan? Although there are plenty of hydrocarbons and other organic compounds, given that the temperature is cold for methane to be in a liquid state and other than having radiation synthesize complex organic compounds, it seems unlikely that there could be life yet Titan does display the characterstics for an abiotic chemistry where in addition to radiation, if the surface is warm enough for water to be a liquid, complex molecules like proteins and nucleic acids could easily be synthesized and it is likely that life forms could arise and evolve but since the temperature is cold enough for liquid methane, and any water that is present is present as ice, then life as we know it is very unlikely but still the possibility of life that could live on liquid methane has not yet been excluded. Only future probes to Titan will resolve the issue.



Enceladus(Kevin Gill)


Another moon of Saturn, Enceladus, has recently been showed to be another environment with liquid water, in addition to Europa and thus could also be a possible candidate for life.


Around 2005, the Cassini spacecraft detected plumes of water vapor emanating from the south pole of Enceladus and also measured heat radiating from the surface.


Given that most of the surface is smooth and few craters , along with plumes of water emanating. Even Cassini managed to analyze the chemical content of the water spray and indicated that it happens to be salty water.


These lines of evidence indicate that , like Europa, Enceladus is another moon with a likely ocean and being in orbit around a massive planet, together with the gravitational influence of other moons, Enceladus is expected to experience tidal flexing, generating enough heat to keep an ocean of liquid warm.


Aside from salty water, further analysis revealed the presence of nitrogen, carbon, even methane and a few organic compounds even formaldehyde.


Finding compounds like water on a distant body is one thing but detecting organic compounds is another. In regards to methane, the question now is, how is methane being generated in Enceladus?


First, we must consider an abiotic source since there are processes that can generate methane without life and one such process is what is called serpentization and this reaction with water with certain minerals which include the presence of minerals such as olivine and pyroxene. This requires these minerals to be under strong pressure and heat and it is these factors that allows methane to be produced as a product. This is the same process that is suspected to be the source of methane in Mars.


What evidence do we have for serpentization in Enceladus? Being a satellite of Saturn, Enceladus is expected to experience a degree of tidal flexing and a current model of Enceladus is that there should be a rocky interior making up the bulk of the moon and together with an icy crust there should be layers of water.




Extra Solar Systems


Recent findings indicate that planets are indeed a common occurrence throughout the galaxy, proving that our solar system is definitely not a unique phenomenon , as far as the principle of mediocrity goes, but wherever there are regions of gas and dust in our galaxy and in other galaxies, as these regions collapse and form stars then wherever there is star formation, planets will eventually condense and orbit stars and so solar systems will be common as a result. Indeed, planets are a side effect of star formation!


With many solar systems, and since each planet has an unique environment just like the planets and their moons in our solar system, then the chances of the origin and evolution of life in these solar system are very high and even though we are just finding out that there are plenty of extra solar systems, we still have yet to know if these planets as well as their moons can support life but that will still not stop us from making reasonable assumptions about life on these distant worlds and so first I will talk about some of the methods used in finding which stars harbor planets and how it is possible to estimate which planets can and cannot support life and how in the future we may be able to determine which extra solar planet may have the conditions not only to support life but to determine, even from way afar, which planets shows signs of life.


Detecting Planetary Wobbles from a Great Distance


How is it even possible to detect the presence of planets that are in orbit in stars that are so far away? One such method to find extrasolar planets is through detecting the tiny changes in the orbits of stars relative to us and if we could find such tiny wobbles in the paths of stars, that likely indicates the presence of a possible planet. This method, which is still used is known as the radial velocity method and it was through this method that many planets outside our solar system have been discovered and here is how it works.


Relative to the solar system,  all the stars that we observe move relative to the center of the galaxy. Some stars that do move relative to us, show small but noticeable shifts in the form of wobbles which manifest as a wavelike motion. How is this done?


When two bodies that are in orbit, call them body A and body B and since they are in orbit because of the gravitational attraction between them and if body A is much more massive than B, then B will be much closer to A but suppose there was a line between A and B. That line would be closer to A much more than if A and B were of the same mass, and the line would define the what is called the center of attraction.


If we could see the orbit of A and B from afar, not only would we see B orbiting A but for the A and B system, there would be a wobble in the path, because the center of attraction would be in A , and since smaller objects orbit massive objects anyway, by measuring the wobble, we could infer the existence of B.


Indeed this is one method of finding planets. Since planets orbit their stars, and if some of the stars do show a noticeable wobble, then we can infer that these stars are likely to have planets.


It was the detection of such stellar wobbles that led to the discovery of the first extra solar system 51 Pegasi b in 1995 and from what we know since then, this is a Jovian planet since Jovian planet are much more easy to detect than smaller planets.


Most extra solar systems have been observed this way and not surprisingly seeing something as a planet orbiting star and resulting in a tiny shift in the star’s path through space, is not only a time consuming task but it also a very delicate procedure to conduct. Nonetheless, most planets that have been observed tend to be massive planets that are of the Jovian type since massive planets are more likely to produce pronounced wobbles than terrestrial planets.


What this seems to indicate that, if this interpretation is correct, Jovian planets are far more common than terrestrial planets and with many jovian planets in orbit, it seems likely that the gravitational influence of each jovian planet could end up disturbing any terrestrial planet. Some extra solar systems even have jovian planets that orbit too close to their parent star, making terrestrial planets orbiting near their stars impossible due to the large gravitational influence of a bigger planet.


Recall that a planet , in order to be able to support life, must be in orbit within the habitable zone of a star, but also recall that the habitable zone of a star, doesn’t necessarily require the constant energy input of a star. The internal energy source of a moon caused by tidal flexing from a jovian planet would do just fine.


Also because of their large mass, jovian planets tend to hold on to more moons and if our solar system is to be of a useful guide (but which shouldn’t be taken as the sole guide in that even though the planets and their moons are unique in our solar system, our solar system with the terrestrial planets near the sun and with the jovian planets on the outside may actually be the exception rather than the rule since every extra solar system is much different from one another) and since the moons of the jovian planets tend to be different from one another, in terms of environment, then it is possible that there could be moons that are like Europa with a surface covered with water or even possible, a moon with a dense atmosphere that could either hold on to methane just like Titan or possibly a moon that could have an atmosphere of water, oxygen, and maybe a surface of water, the possibilities are just endless in terms of habitability. In fact, life does not necessarily have to have a planet in orbit around a star, and if we broaden our definition of habitable zones, then a moon in orbit around a jovian planet could do just nicely, provided there are the right chemical elements, a solvent for biochemical reactions, and a constant energy source.


Are there other methods that can be used to detect the presence of extra solar systems? Indeed there are, and I’ll talk about some of them.


Spinning Spectra


If we were to observe the spectra of stars and notice that one of the spectral lines show either a broadening or a narrowing, it will likely indicate that planets could be in orbit.


This involves an indispensable technique in astronomy called spectroscopy and with this technique this can be used to determine the chemical composition of stars, the motion of galaxies and even prove the existence of life on other worlds.


What is spectroscopy and how is it used in astronomy? Spectroscopy involves taking the light from a source such as the sun and using a prism to break up sunlight into many colors. This was accomplished by Isaac Newton where he used a glass prism to break up a thin beam of sunlight in a dark room and he proved that white light is a mixture of colors.


How can colors be used to determine the various properties of stars? Every chemical element such as hydrogen and oxygen has its own particular spectra or colors when heated in that different elements give of light of different colors or rather different frequencies , and assuming that light is a wave, each element emits colors of differing wavelengths.


Hydrogen, when heated, gives off certain frequencies and when examined using a device called a spectroscope where light of various frequencies are analyzed and if a substance is heated and if a spectra or a set of frequencies are observed, then we can conclude this is hydrogen for example.


You can then see that once all chemical elements were studied from their different kinds of spectra, then spectroscopy becomes a very indispensable tool for astronomy and if we were to use a telescope in combination with a spectroscope in studying the light from a star, and if we observe a spectrum that is identical to spectrum emitted by hydrogen, then we can conclude that hydrogen is present.


Indeed it is with spectroscopy that gives us the most powerful confirmation of that principle of science that makes astrobiology valid, the principle of mediocrity, and recall that this principles assert that everything that we observe on earth is universal and it is with spectroscopy that all the chemical elements are the same throughout the universe, including the elements carbon and oxygen, for example for they are also universal.


Back to finding planets. Aside from determining composition of distant stars, light spectra can also be used to determine the spin rate of stars and different stars have different rates of rotation and to see how this is done, we must understand the Doppler effect.


The Doppler effect relates the wavelength, or the distance between crests and crest, the highest point of a wave to the speed source. The wavelength of a moving source will compress the wavelength towards an observer if it moves towards the observe and if away from the observer, the wavelengths get stretched.


A most familiar example is a police’s car siren. The sound waves from a siren moves in all direction and has a definite wavelength. If the police car moves towards you, the sound increases in pitch. That is because wavelength and frequency are inversely related. Frequency, is the number of waves that move at a point in one second, so longer wavelengths correspond to low frequencies while shorter wavelengths correspond to higher frequency.


For sound waves, shorter wavelengths means higher pitch and vice versa, so when the police car with its siren moves towards you, the wavelength is compressed, resulting in high frequencies and a higher pitch and when the car moves away from you, the opposite will occur.


What is true for sound is true for light. Visible light is composed of different colors which corresponds to differing wavelengths and frequencies. Red light has longer wavelength and hence a low frequency while blue light has a shorter wavelength and high frequency.


For a source of light such as star, if  it is moving towards us the light waves will get compressed resulting in what is called a blue shift and if away, a red shift since the waves moving away from us will be lengthened.


Stellar spectra can not only be measured to indicate how fast a star is moving relative to us but also the speed of its rotation.


Observing a particular wavelength from starlight, if a star rotates, its wavelength will not remain the same but will change. If starlight from the edge of a star moves towards us, there will be shifts towards the blue part of the spectrum and if away from us then it is vice versa.


Using this method, various stars do show rotation and from measuring the change in the width of the spectra, we can find the rate of rotation. From this, there are stars that rotate fast and some slow. What could this indicate?


Since it is the rule rather than exception that stars , after formation will either be accompanied by other stars as well as planets, stars that show fast rotation likely indicate that it is a single star while some that show a faster rotation indicates orbiting bodies such as planets.


Why would the spectra of stars indicate a faster rotations because of orbiting objects? In addition to gravity, there is the conservation of angular momentum and this applies to spinning objects as well as bodies in orbits. This states that objects that are in orbit may have different velocities when in orbit but the total angular momentum for a system of orbiting bodies remains constant.


A planet orbiting a star has an angular momentum that depends on it mass, orbital velocity, and distance form the star and with the planet orbiting the star, the planet as well as planets will exert some gravitational influence of the star, which will slow the star’s rotation by a tiny amount which would be different if there were no orbiting planets.


What we would observe in the line spectra of a rotating star is a narrow spectra indicating a slow rotation and with this method, we can conclude that , the star is being orbited by planets, as well as other small stars, which could also result in a narrow spectra.


In both finding spectra broadening in spinning stars, spectroscopy can also be used also to determine the radial velocity in that during the path of star near earth, there would be a red shift, followed by blue shift, and again red shift, and so on. This indicates, indirectly, the presence of a planet.


Radial velocity, along with spinning stars can both indicate planets in orbit, but radial velocity can also determine the mass of the planet, and from careful analysis of the wobbles of stars, various masses of planets have been estimated, many of which are within the range of jovian planets but even this method has it drawbacks in that some of the planets, may actually be low mass stars mainly dwarf stars in orbit around other stars.


Starlight Dimming


Another method of finding planets, transit photometry, involves measuring light dimming as a result of an orbiting planet blocking part of the light from the star.


Some stars show regular periods of starlight dimming and brightening at regular intervals. This can be interpreted as when a planet orbits a star in our line of sight, and if there is a tiny amount of dimming at a regular interval, this indicates that the size of a planet is small, such as a terrestrial planet since from our perspective, a tiny planet will block a tiny amount of starlight. Likewise, a large regular change in starlight will indicate a jovian planet.


Transit photometry can indicate the size or rather the volume or how much space an object such as planet occupies but is unable to determine the mass of a planet which is only accomplished by radial velocity. Transit photometry can even do more and since this method involves how starlight interacts with a planet, this method has been used to determine whether a planet has an atmosphere or not. If a planet has an atmosphere, then starlight undergoes absorption where gases such as water vapor and oxygen absorb the spectra at different wavelength, it is possible to determine not only the presence of an atmosphere but also what the atmosphere is composed of.


Transmit photometry can even determine the temperature of the planet, in addition to composition, so with radial velocity, along with rotating spectra which together collectively is called Doppler spectroscopy , both of which can determine the presence of planets while radial velocity can estimate mass, along with transit photometry which can determine volume along with temperature and atmospheric composition, we can then estimate which planets are likely to be habitable or not.


Many planets have been found , in addition to radial velocity, but with transmit photometry, and in March 2009, NASA launched the Kepler satellite with the goal of finding distant planets and with the ability to observe an area of the sky where each area surveyed , from Kepler, that contains up to 145,000 stars, at each point in earth’s orbit, the chances of finding planets increases dramatically, and so far more than 2700 planets have been detected


Another similar satellite, CoROT, which was launched by a joint expedition of the European Space Ageny and the French Space Agency, also uses transit photometry, and with this satellite, the presence of a terrestrial planet , within the size range of earth has also been detected.




In both Doppler spectroscopy and Transit photometry, these methods can only detect extra solar solar systems that are tens of light years away. Even though, we are confident that there are solar systems that are nearby ( in terms of light years, of course,) are there solar system that are hundreds or thousands of light years away? If it was difficult enough to find solar systems that are tens of light years away then the difficulty gets larger when finding planets in orbit around stars that are hundreds of light years away!


Is there a method where it is even possible to find distant planets? Indeed there is and there is one technique that does just that and it is called microlensing or rather gravitational microlensing.


This techniques uses general relativity which describes gravity as a manifestation of the curving of space time by massive objects , as was first predicted by Albert Einstein and then confirmed by many observations.


In general relativity, objects move in a straight line through space time and if a large object is nearby, that large object will warp the surrounding space time and if a small object is nearby , it will curve and from this curve, this object will change position and this is what is manifested as gravity.


Not just material objects will be curve but a beam light will also be curved and if a light beam follows a straight line path in space, which it usually does, and if it is near a massive object such as a planet or the sun, then from an observer, the light beam will follow a curved path, and this is predicted from general relativity and it was confirmed by studying stars near the sun during a total eclipse.


Gravity bending light is used also to infer the presence of planets and this is how microlensing works.


Suppose a planet is orbiting a star, and behind the planet and star is another star. That distant, background star will emit light, and if the light beam is near the planet, the planet will bend that light path, and from an observer on earth, as the planet moves near the light from the background star, an effect known as microlensing will occur where, instead of single a single image of the background star, there will be two images of that star. That is because gravity from the planet bends starlight much like a glass lense bends light, and the amount of bending caused by the planet can be inferred to indicate that a planet was responsible.


From careful study of the light bending, not only the mass of the planet but also the orbit and period can even be deduced, and this can be done for solar systems that are thousands of light years away!


One such planet was discovered using this technique, the planet called OGLE-2005-BLG-390 was determined at a distance of 22,000 light years away ( a light year is the distance light travels in one year and is the unit used in astronomy and cosmology). Even the mass and size was estimated and it was found to be within the range of a terrestrial planet.


Dusty Discs


Recall that planets form from the condensation of the gas and dust around nascent stars. Some of the dust may end up forming planetisimals which will likely combine to form terrestrial planets while gas will condense to form jovian planets. The rest of the dust may end up forming asteroids and together with frozen volatiles such as water, methane, and ammonia, into comets.


An indirect method for finding planets is to find the dust surrounding stars.


This method was actually used for finding extra solar systems even before the discovery of the first exo solar planet. In 1983, the Infrared Astronomical Satellite or IRAS for short, actually detected a disk of debris near the star Vega and in 1984, another debris disk was detected around another star called Beta Pictoris and although these were the two stars ever found to have debris disks where planets are likely condensing, it was then far from clear that they were planets forming since the resolution for observing planets was then too crude and hence was pretty unreliable.


To detect dusty disks, we would have to observe within the infrared since dust particles absorb and emit in the infrared and with ever improvement in resolution for detecting infrared emissions from dust, it was not only possible to detect dust around stars , but to observe small scale features within the disks indicating the presence of planets.


As newly formed planets orbit through the disks, their gravitational fields will sweep up dust and gas resulting in circular gaps and indeed some disks do reveal gaps, betraying planets of the jovian type and also measurements of infrared in beta pictoris indicate that there is a huge infall of comets, which can only be caused by a massive planet.


One such satellite the Spitzer Space Telescope, can detect dust around stars with a sensitivity that is 1000 times that of IRAS and one such star that has been observed with a well defined disk called Formalhaut and from observations of the structure of the dusty disk likely indicates planets as well as planetesimals in orbit.



Direct Imaging


These methods for finding planets are not without their difficulties and detecting a planet far, far away, is indeed a monumental feat in itself, would it not be possible to actually find a planet by actually observing one in orbit around a star?


Direct imaging, as the name suggests, is actually finding an orbiting planet around a star by simply observing a planetary orbit, and as you can imagine this is not the only the most difficult and challenging method of finding planets but it rarely works.


Aside from vast distance, a planet since it can only reflect starlight will easily be lost in the glare of the star and observing a planet would be as challenging as trying to find a firefly flying near a spotlight at a distance of 260 miles away using only binoculars!


How is it even possible to actually see planets? One method would be to use a special device in a telescope called a coronograph and this functions by blocking the incoming light from a star, allowing anything in orbit to be seen.


Using a coronograph, is what made possible the finding of dusty disks and in rare cases, actual planets and in 2008, such a method reveal not just the fine structure of the disk around the star Formalhaut but an image of a planet, possibly of the Jovian type.


Estimating which Extra Solar Planet is Habitable


From all these methods that astronomers have at their disposable and with the growing confidence that planets are common, the next step would to find out which planets are likely to be habitable or to have environments conducive for the origin and evolution of life.


How can these methods be used to determine which planets are likely to support life?


I have explained that for life to evolve, it must need a platform which would have to include a planet, as well as moon in orbit around a jovian planet but for the moment let’s that life is present on a planet of the terrestrial kind. Just what is it that distinguishes a terrestrial from a jovian planet and how could our search methods find out those terrestrial planets which could likely harbor life?


One of the properties of terrestrial planets involves the density and density is really mass divided by volume. This is expressed as grams per cubic centimeter or g/m^3. What are the densities of terrestrial planet. There is not one single density but a range of densities and for the terrestrial planets of our solar system, the average densities are  within the ranges 5.4 to 3.9 g/cm^3. Why these ranges? For comparison the density of water is 1.00 g/cm^3, and as a rule of thumb, any density higher than water will sink and the densities of terrestrial planets are much higher than water? Why is that?


That is because terrestrial planets are composed of both silicates and metal, mainly iron and nickel, the amounts of which vary for different terrestrial planets, hence the range of densities that are above the density of water, for comparison.


How can the methods for finding planets be used to estimate the density of extra solar planets?


Recall that density is just mass divided by volume so to estimate density that are within the ranges expected for terrestrial planets. If we could use two or methods of observation to find mass and volume, not only can we get the density but from the density we can make reasonable guesses about the geology and even say with some high degree of confidence whether that planet may be habitable.


In Doppler spectroscopy, this is the method used to estimate mass of an orbiting planet and transit photometry can determine the size or volume of a planet so using these two  techniques for observing the same extra solar planet, the density can be found.


If the density is higher than water, then it is likely that the planet may have a silicate crust and iron core. Planets with iron cores and silicates crusts tend to display a degree of plate tectonics and that is because a planet with an iron core generates enough heat, which passes through a thick semi solid mantle and it is this flow of heat that drives plate tectonics and the gradual flow of various plates can recycle chemical elements which can end up releasing greenhouse gases for keeping the surface warm enough for there to be liquid water, together with a flow of chemicals at mid oceanic ridges which is liable to support a viable ecosystem.


So far, a combination of these methods has indeed revealed the presence of terrestrial planets but with sizes that are within ranges from Earth to Neptune and since these planets are five times more massive than earth, they have been given the name “super earths”.


Planets come in a spectrum of sizes from the small planet Mercury to planets that are massive as Jupiter but there are limits to the size of what astronomical body is defined as planets for as the size and mass increases there will come a point where a planet ends up emitting more thermal energy and beyond a critical point where the object emits heat from thermonuclear fusion, then it is no longer a planet but a star.


For one of these planets to meet the criteria for habitability, such as a planet with an environment that supports life, for one thing various stars have different sizes of habitable zones and stars with larger sizes tend to have wider habitable zones but have a short lifetime on the main sequence, which is around a million to 10 million years, and so with that short amount of time, even a terrestrial planet in orbit of such a star, is less likely to have a long lasting biosphere. Stars that are within the size range of the sun have habitable zones that are within the range of 1 Astronomical Unit, or the mean distance from earth to sun, and since these stars last from 5 to 10 billions of years, then terrestrial planets around these kinds of stars are more likely to have biospheres.


As for the super earths, using a combination of Doppler spectroscopy and transit photometry, together with theoretical modeling and computer simulations, we can estimate which geophysical properties of these super earths are likely to be similar as well as different and whether or not they are likely to be habitable.


If they are plate tectonics and if plate tectonics are a key requisite for an evolving biosphere, what are the physical characterstics needed for plate tectonics for a super earth?


Like earth, a super earth, due to their large density, is expected to have a core, mantle, and crust and if the temperature is hot enough then there will be heat flow which will result in plate tectonics but unlike earth, a super earth, with its large size, will likely result in a much bigger mantle along with a much larger core and hence with a larger amount of heat flow. What of the crusts on top? It turns according to theoretical models, the crusts would end up being much thinner than the continentinal crusts on earth and with much heat flow, the rate of plate movement would up being much faster.


Recall that because of plate tectonics, there is a slow recycling of greenhouse gases because of the carbonate and cycle and it is thanks to plate motion, that carbon dioxide levels have been more or less constant ( excessive greenhouse release due to human activity notwithstanding) and without plate motion, there would probably would have been no greenhouse gases to keep the earth’s surface warm and any water would have likely been frozen.


The carbonate cycle on a super earth would likely be much more rapid, thanks to a rapid plate tectonics and since a super earth is more massive than earth, it would have a larger gravitational pull and thus would be able to hold on to a atmosphere and if the atmosphere has a water cycle , with water vapor, also a greenhouse gas like carbon dioxide, then with a much active carbonate cycle more carbon dioxide would be removed from the atmosphere but also there would have to be volcanoes to release all the carbon dioxide, so in the long run, the atmosphere would have enough greenhouse gases to keep the super earth’s surface warm enough for there to be liquid water.


From these theoretical modelings, then we are confident a super earth that has the observed parameters, mass and volume, which makes up density, then it is likely that a super earth would have an atmosphere with cycling caused by stellar radiation, and if its orbit places within the star’s habitable zone, and with rapid plate tectonics maintaining a level of greenhouses that is enough to allow water to be liquid along with a dense atmosphere then , a super earth is more likely to be habitable and hence could support an independent biosphere.




With so many planets detected from afar, how can we be sure that as long as they  are in orbit in habitable zones, that these planets do show evidence of life?


A feature of life is that it can alter its environment revealing its existence. For example, on earth, and for 2 billion years, the evolution of photosynthesis resulted in the production of molecular oxygen. There is no other abiotic process that can produce oxygen at the observed amount of 21% and the conclusion is that the bulk of the terrestrial oxygen is produced by life.


In addition, the oxygen can easily be detected because of its spectral signature since it is interacting with the frequencies of visible light. If oxygen can be detected from earth’s orbit, then is it not impossible to find the presence of oxygen in other extrasolar planets?


In principle, if we could analyze the chemical composition of stars and surface temperature of stars using spectroscopy then there is no  reason to think that we can do the same for planetary atmospheres if the goal is to determine that one important biosignature, oxygen.


There are now developments in the technique of spectroscopy for detecting radiation from molecules that are produced from life as well as non life and these new techniques must be sensitive enough to distuingish between molecules made by life and those that are not.


Oxygen is not the only biosignature gas. Other examples include molecular nitrogen, which is also used by life forms for making proteins and nucleic acids, and methane which is produced by a class of bacteria called methanogens which produce methane as a waste product. Of course, not all of these gases are biological in origin, so we must be careful in the interpretation of the data.


Dwarfs and Binaries


I have made the assumption that any planet or moon in orbit around another planet is in orbit around a single star. We must face up to the fact that there may be complication in regards for detecting habitable planets mainly because many stars are what are called “dwarfs” or stars that are within and less than the size of our sun.


It turns out that many stars in the galaxy are not only dwarfs but many of them are even 1/5 the size of the sun.


If planets orbits dwarfs, and there is no doubt, that they do since NASA’s Kepler telescope has detected planets around dwarf stars. If a planet with liquid water and an atmosphere where to orbit, around a small star, how would this be different in terms of habitability?


Recall that the habitable zone depends on the size of the star. Larger stars tend to have larger habitable zones while smaller stars tend to have smaller zones.


For a star such as a dwarf or more specifically a red dwarf, a habitable planet would have to orbit within a narrow habitable zone that happens to lie close to the surface, and in fact its orbit would much closer to the star than Mercury.


At this close range, the gravitational field will cause the planet to be tidally locked in, or when the planet faces the same side as it makes on orbit, which is similar to earth’s moon. The moon always faces the same side as it completes it orbit since the moon is much closer to the earth.


It is hypothesized, that due to tidal locking, a planet near a red dwarf would still be habitable but one side of the planet would be hot and the other side, cold. As long as there is water and an atmosphere, it would still be habitable but the range of a biosphere may be limited.


Such hypothetical biospheres are called “eyeball earths” since the temperature would be warm enough for liquid water but away from the center or to use the analogy, the warm spot on the surface is like the center of the eye, where water is liquid but away from the warm region, the region gets colder and water turns to ice.


Life could still thrive , provided that the planet has an atmosphere and active plate tectonics along with a magnetic field to protect life from harmful radiation.


Also, many stars in the galaxy exists in not one but are bound gravitationally to two or more stars and these stars are called binary since two stars orbit within each other’s gravitational influence.


If most stars are in binaries, will this have any effect on a planetary orbit and if it does can there be a planet, much less a habitable planet?


At first, with two stars in orbit, it would seem there can be no planet mainly because of the orbit of the two stars but there is evidence to suggests that a planet can orbit two stars.


In a computer simulation, Sagan and Shlovskii  (1966) argued that for two stars with the same mass, there can be a stable orbit but within certain constraints in regards to orbital parameters. Of course, as long there was no observational evidence, it would seem unlikely that there could be solar systems with two stars until astronomers Doyle and Welsh provided observational evidence for a planet observed in a binary star and according to them, this planet does have a stable orbit.


Extraterrestial Life and Alternative Biochemistries


With recent findings in astronomy with the characterization of extra solar planets, I believe we are now in a good position to make good reasonable judgement of whether life is something that is common or rare but nonzero, but also we must keep open to the possibility of life forms that are based other than a carbon and water biochemistry.


With the principle of mediocrity, let’s assume that a carbon and water based biochemistry is universal. What compelling reasons do we have that a carbon and water biochemistry should be universal? Recall my brief discussion of spectroscopy. It was with spectroscopy that we could determine the composition of a star just by studying the light the star emits and in addition to determining the abundance of chemical elements, molecules have also been detected and surprisingly, even complex molecules have been found in regions of space where there are stars and these molecules , believe it or not, are the amino acid glycine and the molecule aldehyde, which is a simple carbohydrate, and even chemical precursors of adenine, one of the building blocks of nucleic acids.


Aside from spectroscopic observations, there have been samples of meteorites that are know to contain amino acids and in September 1969, a rare type of meteorite, a carbonaceous chondrite, so called because it is a meteorite that is high in carbon fell in a town in Australia and it was recovered after it fell. Chemical analysis revealed the presence of amino acids.


What this indicates is that with carbon, one of the elements along with oxygen and nitrogen, the byproduct of supernova explosion, there is no reason to think otherwise that wherever in our galaxy there are supernovas that release these biogenic elements and in the ashes of giant stars, there will be stars with planets and as long the conditions are not too extreme, there will be planets that have all the characteristics of habitability ( knowing that thanks to mediocrity, there is nothing at all special about earth), biospheres with carbon, water, and a flow of energy will be common.


But since we now know that planets are commons and in additions for making estimates for which planets could be habitable, is it even possible to detect a planet with life? Or to put this question differently; can we use the technique of spectroscopy to determine whether the atmosphere of an extrasolar planet shows obvious signs of life?


Before we get into the question on how it may actually be possible to find signs of life using spectroscopy, first compare the atmosphere of earth with Venus and Mars. Both Venus and Mars have atmosphere composed mainly of carbon dioxide, roughly about 90% while the atmosphere of earth, on the other hand, is composed of about 21% oxygen.


From the perspective of chemistry, especially the study of chemical reactions that go to equilibrium where in equilibrium nothing else happens, there shouldn’t be that much oxygen and that is because oxygen is a reactive gas. Any  reaction with oxygen, such as oxygen burning in the presence of an organic compound will produce carbon dioxide and so with carbon dioxide, a stable waste gas, there should be plenty of carbon dioxide on earth, like venus and mars. In addition, oxygen has been present on earth for about 2 billion years so how is it even possible to have that large amount of oxygen for so long?


There is no known geochemical process that can produce oxygen in that amount and so the conclusion; the oxygen is the result of biological activity and indeed it is the result of photosynthesis, a complex biochemical process.


In addition with oxygen, ultraviolet radiation can convert oxygen, a two atom molecule, into another form of oxygen, ozone, a three atom molecule. With a steady input of oxygen, some of that oxygen , in the upper atmosphere, will convert into ozone and with a layer of ozone, less ultraviolet radiation and together with oxygen, which has stimulated the evolution of multicellular organisms while an ozone layer protects any landbased life form from ultraviolet, an atmosphere with oxygen is a good guarantee that any extrasolar planet will harbor life.


In principle, detecting both oxygen and ozone in an extrasolar planet will likely betray the presence of a extraterrestrial biosphere. Of course detecting spectral emissions of oxygen would be even more difficult in practice and if you think finding a planet around a star 40 light years away is hard, just imagine trying to make sense of the spectra from light from a reflected planet lost in the glow of its star!


That would be an even more formidable task, but with improvement in astronomical methods, the chances of finding an atmosphere with signs of life will no doubt increase.


What would life forms in an alien biosphere be like? It is a hard question but not an impossible one, and I have talked about the inevitables biases that cloud even the best of judgement, so if we assume biochemistry of the kind that we are familiar, along with a universal definition of life, we should then make some reasonable hypothesis of what life forms could inhabit alien planets.


Lets assume that with the origin of a planet, a planet with organic compounds, water, and energy will likely produce life and through natural selection and other evolutionary mechanisms, life forms speciate into many forms and often times the evolution of life usually depends on the physical characteristics of the planet.


Super earths have been found and from careful modeling, these planets are likely to have a large ocean with maybe some continents and with an ocean for life to begin and with continents then it is inevitable that some complex metazoan or animal life could evolve from water to land. However, a super earth is much more massive than earth and so with have a stronger gravitational field. On such a massive planet, animal life would likely to be large, slow, and sluggish, and any evolutionary transition would be even more challenging and so there could be less animal life on earth but going about would be quite challenging and so extraterrestrial animal species could be rare while marine life could be more dominant on a super earth with water.


Likewise, a planet that is less massive than earth, and I have no doubt that there will be “mini earths” orbiting stars in habitable zones, would likely have animal life that is tall and spindly and perhaps with a graceful build. Imagine how animal life could move on their planets! Their motion could be something like a ballet that would require more control and precesion that would rival even the most skillful ballet dancer.


I have assume that the origin and evolution of life on other worlds depends on carbon and water as the starting materials but there is also the possibility of extraterrestrial life that uses a different biochemistry and now it is time to discuss how that may be possible.


Carbon, like oxygen and nitrogen are examples of biogenic elements and basically these are the elements that form biological molecules and this is made possible for the fact that previously , because of the atomic structure of carbon, carbon is the reason why it can so many kinds of compounds such as proteins and nucleic acids, but if life could start with carbon, is it conceivable then that elements other than carbon can play the same role?


One such alternative form of biochemistry involves the chemical element silicon and why silicon? Like carbon, silicon can form four valent bonds or four bonds to other atoms, and that is because silicon is in the same column in the periodic table as in carbon. As a rule of thumb, chemical elements in the periodic table that are arranged in columns share the same chemical properties so it would seem that silicon can form as many compounds as carbon. If silicon is in the same column as carbon, then like carbon silicon is expected to form molecules includings those with a viable biochemistry, right?


Although, silicon does forms polymers it turns out that the variety of molecules based on silicon is limited compared to the polymers with carbon. For one thing, silicon is a much bigger atom than carbon and because of its size, molecules based on silicon tend to be limited in the kinds of number of elements and also there is not much variety of atoms that silicon can form.


Carbon, can not only covalently bond to atoms like oxygen, hydrogen, and nitrogen but other elements like magnesium such as the magnesium that is bound to chlorophyll , which is an organic polymer responsible for photosynthesis, iron present in hemoglobin which binds to molecular oxygen in animal blood, and with phosphorous and nitrogen, the sequences of nucleotides that form DNA for coding the information for a variety of proteins, all of which are carbon based.


Compared to organic compounds, the variety of silicon polymers are pretty limited. Not to mention, that silicon is less abundant compared to carbon when present in the universe as astronomical observations of the interstellar medium reveals.


What this may indicate is that carbon biochemistry may be the rule rather than the exception. Of course this is not to deny that all life forms on other planets are just carbon based and it may be possible that there could be life forms that use a silicon biochemistry but chances are, these kind of life forms could form in very specialized conditions on their home planets or moons and are probably not as common as life forms based on carbon.


Even on earth, some life forms do use silicon and there are microorganisms called diatoms that use silicon for forming shells , even though the cells are still carbon based.


Recent experiments suggest that it is possible, but by no means certain, that there can be a viable biochemistry where silicon can play an important role.


Last year, in 2016, a group of  scientist published a report in Science magazine where a bacterial species Rhodothermus marinus was engineered to catalyze the formation of carbon and silicon between two chemical compounds, one of which is silicon based.


This suggests, but does not conclusively prove, that a silicon biochemistry is viable. Why would that not be conclusive?


This kind of experiment was done in a controlled environment and the bacteria Rhodothermus was engineered to catalyze silicon carbon bonds, which in its native environment ,  is something that it never does and also, like diatoms, Rhodothermus still uses a carbon biochemistry.


Perhaps maybe, a silicon biochemistry may be rare but a combination of carbon and silicon could be a viable possibility, but until we have evidence of life forms that use both carbon and silicon or for that matter a silicon biochemistry, we must assume that extraterrestrial life is carbon based.


In addition to having chemical elements other than carbon as building blocks for polymers, chemical reactions that are vital to life usually takes place in a liquid state since the atoms and molecules are moving within one another , colliding with one another and reacting or in others words reactions need a solvent.


The one such solvent is of course water, for water is a liquid within temperatures around 37 degrees Celsius ( around 98 degrees Fahrenheit) and not only does water provide a liquid matrix for biochemical reactions but water is also a metabolite , in that water molecules are released when monomers such as amino acids combining to form proteins, release water and also respiration which involves molecular oxygen combining with glucose also releases water as a waste product.


Water, when in exists as ice, can easily float on top of water, and this also help to maintain the temperature of the water from going completely solid. For there to be carbon biochemistry, I have previously argued that water must be liquid over a large range of temperatures, and aside from earth, there are places in our solar system where there may be water still present, such as the moon of Europa, and possibly there could still be liquid water on Mars, although most of the water is Mars is frozen in the north polar ice caps, and Mars does show evidence of past liquid water.


Can other chemical solvents beside water play a role in extraterrestrial life? One such solvent happens to ammonia, and how can that chemical play a role in an alien biosphere?


Like water, a solvent that is important for an alternative biochemistry needs to have these properties, which includes being a liquid over a range of temperature, ranging from hot to cold, a high heat capacity or the amount of heat added to a substance, a high heat vaporization or the amount of heat needed to turn a liquid into a gas, and to dissolve a variety of compounds.


I will compare these properties of ammonia to water and see what we can then conclude for ammonia being a viable solvent for an extraterrestrial biosphere.


Biochemical reactions must occur in a liquid, so the solid and gas phases are then ruled out. Can there be chemical reactions in liquid ammonia just as there are chemical reactions in water?


For water, chemical reactions are possible and that is because water is a polar molecule, with the oxygen being negatively charged while the two hydrogen atoms are both positively charged. This is one of the reason why water can dissolve most substances with polar atoms. Can ammonia do that? It turns out that yes ammonia, like water is a polar molecule.


That is because ammonia consists of one nitrogen covalently bonded to three hydrogen atoms and nitrogen, like oxygen does have a negative charge, while the other three hydrogen atoms, are, as you guessed it, positively charged.


Ammonia can form compounds thanks to its polarity, just like water and in certain reactions with ammonia, one of the compounds can include amines which are NH_2 molecules which are also the precursors to amino acids.


Chemical reactions are important for biochemistry but what of the physical characterstics that are also important such as heat of vaporization ?


For water, the heat of vaporization is the amount of heat needed for liquid water to be a gas, and water has a high heat of vaporization thanks to the attraction every water molecule has for one another.


For ammonia the force of attraction is less than that of water so its ability to go from liquid to gas is much less than water, and here we begin to see the problems with ammonia as a chemical solvent. Also ammonia easily reacts with oxygen and if life on another planet evolved that could use oxygen for metabolism, then any life form that used ammonia as a solvent would likely be selected against.


Like silicon as a building block, ammonia as a solvent will likely be rare except under such narrow specific conditions and ammonia is present in jovian planets such as Jupiter but that would not mean that there could a biosphere of ammonia based life forms, although in abiotic experiments, ammonia is the source of nitrogen and hydrogen for amino acids and nucleotides.


There are also other biochemistries involving methane and boron but it would seem very unlikely that there could life forms based on these compounds. Even problematics is the question of even more exotic biochemistries such as beings of energy based on plasma. For the moment we can ignore such speculations since it may not be possible how such life forms could even survive in such environments such as the surface of stars as well as the vacuum of space, since any life form that somehow evolved in such places are likely to face malnutrition.


So it would seem that our only chance is to find life based on water and carbon and on terrestrial planets and it would seem more likely that these would be more common in our galaxy and if there is life, then there is the chance of intelligent life and this will be the last of the three questions of astrobiology that we will turn to.



Is there Intelligent Life in the Universe?


I have shown how is it likely that life can arise in whatever planetary system but of the three questions of astrobiology, the last as well as the biggest question is the one question that, I’ll admit, has far more questions than answers, but still resolved within science, and that is whether or not earth is the only planet that not only harbors life but only one species of animals with an advanced intelligence or that earth is not only typical in a universe where life can exist but intelligent life or more simply is intelligent life common or rare in the universe?


What is Intelligence, Really?


Before defining intelligence, we must remind ourselves that if the goal of the last of the three questions of astrobiology is to determine whether intelligent life is common or rare, a working definition of intelligence, like a working definition of life, is a useful starting point while being mindful of the biases that are associated in our search and unless there is actual evidence of life forms that display intelligence , perhaps of a form that we are not fully aware of, then we must begin our definition.


Intelligence is defined as the ability of an organism to solve problems in a way that aids it survival. Such ability that allows survival will be selected for , allowing survival for that species that are intelligent. What exactly are the core properties of intelligence that allow survival and how does it relate to the third question of astrobiology?


An organism must possese a degree of awareness of its surrounding environment, and using the textbook definition of life, one of the characterstics of life is to sense its surroundings and react accordingly which could be finding a mate or avoiding a predator. Those abilities a complex biological system of processing sensory input and relaying which are relevant to the parts of the organisms if it is to reproduce or to move and avoid being eaten.


From this, all organism display a degree of intelligence, according to this definition, and even unicellular organism such as bacteria can sense their surroundings and move away from obstacles, the details of which are still being studied.


The same is true for animals with a nervous system, fashioned by the slow process of natural selection, for taking in sensory input together with a muscular system to allow movement and through evolution, many species of animals evolve a variety of behavioral responses that have permitted survival along with adaptation.


Behavior is one thing , communication is also vitally important and it seems that intelligence and communication go hand in hand.


If we free ourselves from the bias that humans are the only animals with a complex vocal communications, with our uncanny ability to string together various sounds into words which form the basis of spoken language along with our opposable thumbs which is important for written communication, communication, like behavior, is universal in animals.


Animals have ways of communicating with members of their own species as well as members of other species. Take the tropical tree frogs, for example. These small frogs are colorful and this indicates to would be predators that they are poisonous and not good to eat so bright colors are a form of communication saying ” I am dangerous because I’m poisonous so leave me alone”


For members within species, communication is vital for letting members of the opposite sex, knowing when to mate. For bird species, this is indicated through the use of  song and different bird species have different kinds of songs, only recognizable within each bird species. Mating displays not only include song but also dance and different birds, have different mating displays.


The most interesting form of communication used by animals is used in one species of insect, the honeybee, and since honeybees form an organized society consisting of a queen who does nothing but produce bees and those bees are divided into workers , which care for the beehive and soldiers who defend the hive from predators.


Bees, like insects, usually communicate through both smell and touch, and even sight. The worker bees fly outside the hive in search of pollen and nectar and if one bee find a flower, she will fly back to the hive and let her fellow workers know where the source of nectar use by doing a special dance, called the “waggle dance” or “figure 8 dance” and this consists of moving in a figure eight pattern and in the middle of the eight, the worker waggles side to side. This tells the workers the direction of the source in relation to the sun and this is effective even if the sky is overcast.


What we can see from the example above, that with the evolution of behavior, communication will evolve so any life form evolving on other planets with environments that are more or less similar will likely have animals with various degrees of intelligence.


From the description, should the definition of intelligence include those organisms with an advanced ability to manipulate objects, capable of complex reasoning, and even wonder of other life forms?


There is nothing in the definition that says that there should be no species with the ability to communicate with other beings, just like in a definition life, it wouldn’t matter if it is life based on carbon or something else, then there is no reason to think otherwise that this definition of intelligence is applicable to both terrestrial and extraterrestrial life forms.


The question now is just how common the intelligence of humans is? Are humans the only species capable of interstellar communication. Are there other different species of animals that even have the same cognitive or thinking abilities as we do, and can they apply their thinking in solving problems?


It turns out humans are not the only species with that ability. There are other animal species whose intelligence rival our own yet are unlikely to advance even to our level and these two group of animals are the cetaceans which includes the dolphins and whales, and the primates which includes our cousins the chimpanzee.


It seems likely that extraterrestrial life forms with the human body plan which includes bipedalism or walking on two feet (which is a good but not sufficient condition for advanced intelligence, since there are animals that walk on two feet such as ostriches which we know are non human animals), a large brain with specialized areas for processing information, two eyes placed forward, resulting in stereoscopic vision or three dimensional vision, and an upright posture with a body symmetry that is also common in the animal kingdom, bilateral symmetry.


Why would terrestrial and extraterrestrial life forms with this kind of symmetry be universal? There are various evolutionary reasons why this may be  a universal body plan.


Standing upright or possessing bipedalism is one characteristic but it is not the only truly defining factor in the evolution of intelligent life, so what other factors to consider? There is our central nervous system with a large brain and perhaps maybe a large brain would be a good indicator and hence a good chance of detecting extraterrestrial intelligences.



A large brain does not necessarily mean intelligence, at least our anthropomorphic version of intelligence. There is an upper limit to the size of our brain and even though the fossil record does show an increase in the average size of our brain, it does not seem that our brains will get any larger and the main reason has to do with childbirth.


Female pelvises are wide mainly to accommodate childbirth and there would be a selective pressure against even larger brains since that would likely be fatal to the mother in delivering the child and like brains, there is a limit to the size of the female pelvis.


Also, natural selection likely favored our distant primate ancestors with both hands for manipulation along with a brain for processing incoming information and if natural selection favored the body plan with a brain in the head and hands with a wider grasping capability then can natural selection favor organisms with several arms and eyes?


Throughout the animal kingdom there are animals that do have several arms but with two eyes and show a degree of intelligent behavior and they are the octopuses.


Octopuses have a nervous system which has to coordinate the movements of all its eight arms and what is interesting about the octopus is its degree of intelligent behavior. Laboratory studies on the behavior of octopuses show their ability to solve problems and such experiments reveal that they even have a short and long term memory!


The ability to solve problems if it involves surviving would have been favored by natural selection since behavior as well as body is an inheritable and favored trait but why octopuses have not dominated the world in terms of their intelligence?


It would take a lot of effort to coordinate many arms and with such coordination, there would not many species of multi armed animals like the octopus and second, living in an marine environment, it is pretty difficult to build objects underwater and so there is not much chance of developing the equivalent of an underwater civilization considering that the octopus only has tentacles but no hands with opposable thumbs. Add to the fact that unlike our vertebrate ancestors that crawled out to land and on land there were many things to do that is not possible underwater, an octopus will have a difficult time staying indefinitely on lands because of a lack of an internal skeleton and virtually no lungs to take in air.


Since octopuses do have intelligence but unlike ours that transcends just surviving. the species of all octopuses are likely to be inhabitants of marine environments for the near and distant future.


Intelligence such as problem solving is not just something that humans have but is present in other animals. Recall that there is a link between intelligence and communication and are there other animals where they have something such as a language for communication? There is and these are the cetaceans such as dolphins.


Dolphins are one species of animals that use sound for communications and through the use of sounds, it has been determined that this is one reason why dolphins are social animals because of their ability to communicate.


It has been discovered that dolphins,  like humans, do a display a form of culture in that some dolphins teach the use of tools for gathering food and in one species of dolphins, a mother dolphin was teaching her daughter to use a chunk of coral for protecting the snout when foraging.


With language and a culture where behavior is passed on, it would seem to be a defining characteristic for an intelligence to communicate to another different species and in a way, this is correct and a strategy of SETI is to understand an alien intelligence and indeed we are making progress in understanding a different animal species such as dolphin yet it is not likely that even with these traits, that dolphins are going to communicate to the stars for the same regarding the octopus.


Unlike the octopus, dolphins do have an internal skeleton and being mammals, they also have lungs but since dolphins are adapted to life in water, there is no need to have an arm with fingers and an opposable thumb simply because of the surrounding water , so having a streamlined shaped along with fins is useful for swimming but not so much for grasping and fashioning more complex tools such as a radio telescope.


Is Intelligence Universal?


At this point, it is well worth questioning whether not just life but intelligent life and that is what we are interested and that is the rational behind the third question of astrobiology is if the process that leads to life is universal then since life undergoes evolution is it inevitable that intelligent life will even appear and will it even have the capability of communication?



For what reasons do we have in believing that with the many habitable planets out there in the galaxy and with life on these worlds, would a number of them have intelligence capable of interstellar communications?


To understand intelligence, we must approach this question from an evolutionary perspective. How was it then that there could be intelligence of the sort that humans posses, with our ability and willingness to communicate with others out in space?


The reasoning is this; for every habitable planet, wherever there is life, evolution will result in each successive adapted life form, and through reproduction along with mutations that preceed each generation, along with the environment, each species will adapt if it has novel traits, while those that lack the traits will be selected against , and the result is that there will many species, each different, and possibly more complex, so inevitably , there will be species that have a degree of intelligence, and the chances that something similar in planets that are as habitable as earth, will occur frequently.


Thus, there should be an obvious presence of extraterrestrial intelligence but so far, there is no sign of intelligence. Why is that? Perhaps maybe our assumptions may be wrong or it is possible that there is extraterrestrial intelligence but it is in a form that we may not recognize.

Let’s consider the possibility that complex , intelligent life may be rare if non zero and there are reasons why this may be a valid reason.



Paleontological evidence suggests that the ancestors of humans originated in southeast Africa around 8 million years ago. Why this particular region? According to Heidmann (1992), the evolution from our tree dwelling ancestors to Austraulopitecus and the transition to Homo, very likely was an environmental pressure, and at southeast Africa, is the Great Rift Valley, formed as the result of tectonic spreading.


It is possible that the spreading may have affected climate in a way that caused a decrease in the range of tropical forests and with a shrinking forest, along with increase in size of grassland, this would put selective pressure for a species of primate more suited to walking and less for grasping tree branches compared to those primates that are only adapted to life in trees, and this may be the reason why the transition from living in trees to living on forest occurred for the evolution to Austraulopithecus.


Fossil evidence of our ancestors do reveal that they were more adapted to living in the plains and with an adaptation came the ability to fashion simple tools then later along with an increase in brain size and with opposable thumbs, more refined tools and together with the possibility of spoken language until eventually with the species Homo habilis, that species begin to move out of Africa and became a more flexible and adaptable species.


What this suggests is that to steps to becoming human may not be inevitable and what would say for our chances of finding extraterrestrial intelligence?


Evolution, in general, is not about  a steady progression from simple to complex, and with paleontological studies of all life forms, along with molecular sequencing, every species of life living today represent the end points of a vast tree of life, and that any species alive is really an adaptation to current conditions in its environment. Natural selection cannot see ahead in the future but rather occurs in the present.


In another blog, that I’ve written where I talk about extinction, many species that were present at one time in the earth’s history are now extinct, and for all animal life as well as plant life that are alive today, they are the descendants of life forms that survived various natural catastrophes and in addition to adaptation, chance also plays a role of what species lives and what dies.


Why is it then that complex intelligence should even be rare?  Considering that the human species is just one member of a vast branching tree or phylogenetic tree of life, then it is expected that the chance of intelligent life with a human body plan is rare.


One such viewpoint for the uniqueness of the human species, as a result of many successes in every evolutionary experiment has been advocated by Harvard paleontologist Stephen Jay Gould. According to Gould, he considers a thought experiment where if were to rewind the history of life back to the origin of life and let it play again, would there be any of the life that we see around us?


The chances of finding exactly the same kind of life, would be negligible in Gould’s view.  As legitimate as this sounds, there is one aspect of evolution that needs to be considered and this is convergent evolution.


What is convergent evolution? This is evolution where different species that do not share a common ancestor are exposed to the same environmental condition and develop similar phenotypes as a result.


Examples are the streamlined body shapes of fish and dolphins. Both are different species but both live in the same environment and it is expected that these two animals have the same shape for moving rapidly through water?


What does convergent evolution have to say about the prospects of finding intelligent life?


First consider the human brain and the nervous system. If cells are the building blocks of life, then the building blocks of the nervous system  is the neuron or cell that functions by receiving signals in the form of ions and how a neuron works is by pumping out ions or sodium ions mainly because of a molecular pump that functions in creating an electric potential that becomes a propagating wave that traverse throughout the neuron.


Neurons form connections with other neurons at thin junctions called synapses where electric signals are converted into chemical signals and it is this rapid movement of electric signals along with chemical transmission that makes it possible for multicellular organisms, the animals, to sense and respond to their environment.


Just how common is the molecular pump? Is it only confined to just one species of animal or is the molecular pump a universal feature in all life?


It turns out that the molecular pump is universal throughout most of the animal kingdom not just in the human species. As molecular  sequencing of DNA and proteins,  have revealed that all of life shares a common ancestry by studying how similar the DNA as well as protein sequences throughout all of life.


The sodium potassium pump , with it functions for creating the necessary gradients for neurons, is actually universal in all animal life forms, but more so in the animal kingdom. This suggests that the ability to sense and respond stimuli is needed for animals whether in the aquatic or terrestrial environment and no matter which animal species are present, there will be selection for molecular pumps like sodium potassium and once each species has these kind of pumps then the evolution of nervous systems including a centralized and organized mass of neurons for processing incoming sensory information or a brain is likely to be inevitable.


From the perspective of the mediocrity principle and convergent evolution, various degree of intelligent life is inevitable although not all of them will have the capability of interstellar communication but only a small number of species will so if we receive a radio signal that is determined to be intelligent in origin, then this will not just confirm the existence of intelligent life but that convergent evolution is universal as well.


The Drake Equation and its Components


To answer this question, it is reasonable to start with what we know so far about all the conditions we know for the origin and evolution of life and form hypothesis about how and where life and intelligent life could arise or in short to assese the likelihood of the number of extraterrestrial civilizations, that is those civilizations that have the ability to communicate to with other civilizations, the latter part is sheer guesswork and this involves estimating the number of civilizations and this depends on several variables and this can written in equation form which is called the Drake equation, and named after Frank Drake, who was one of early pioneers in the search for extraterrestrial intelligence and using this equation, it depends on seven variables , where I will discuss each of these variables in detail separately, but first , and without hesitation, I will present the equation here below and it is



N= NsfpnefLfIfc(L/Ls)


N= the number of advanced civilizations capable of interstellar communications


The number of civilizations with the technology to communicate not just within members of their own species but with other civilizations is dependent on these seven factor one of which can be split into an astronomical component together with a biological component. The reasoning behind this is that organisms that evolve with a brain for processing neural information together with the ability to communicate such as language will end up with the ability to understand science and use science for technology such as using radio waves for communication and this depends on many of the seven variables that are on the right hand side of this equation.


Ns =The number of stars in the galaxy that can support life.


The number Ns are those stars that are on the main sequence that have life spans on average of 5-10 billions of years and this is one variable that can be estimated from findings in astronomy with a high degree of confidence. This would rule out stars that are likely to go supernova as well as those stars that are much smaller in mass, since they would emit radiation with less energy to initiate abiotic chemistry or to sustain a biosphere.


fp= Fraction of the total number of stars with planetary systems.


Out of the number of stars that are on the main sequence, this number fp is the fraction of these main sequence stars that have planets and like the previous number we have a pretty good estimate of stars with planets but for the sake of the argument , lets only focus on terrestrial planets while ignoring the fact that many stars tend to have jovian planets, and to also to be fair, to rule out the possibility of moons in orbit around jovian planets.


ne  =   fraction of planets that are in habitable zones and can support life.


Unlike the previous two numbers were we are now confident, this variable and with the other variables have a higher degree of uncertainty but giving the assumption of mediocrity, and with the possibility of the universality of carbon and water based life, evolving on other worlds, the number n

are those planets that are in orbit within the habitable zone and have all factors such as plate tectonics, atmosphere, and magnetic field, in short all the physical and chemical factors needed for life to form and to evolve.


fL = The fraction of planets with life.


Assuming that the planet has an atmosphere, has a liquid solvent, biogenic elements, and a constant source of energy from the star, as well as energy flow from the planet, it is very likely but probably not inevitable that there will be abiotic chemistry and with an abiotic chemistry, all the various kinds of molecules combining together with a system bounded by a membrane, along with a metabolism and a genetic code and with a population of such systems that are indicative of life, evolution of life forms and a biosphere so fL

are those planets that have life.


fI =The number of planets that have intelligent life.


If there are planets with life and if natural selection is a universal mechanism wherever there is life with it’s ability to reproduce and use energy for reproduction and if the environments are variable but not too much as to only allow simple life forms, then there will be evolution of species, some of which may evolve with intelligence or the ability to solve problems. One of these species may have the capability to communicate, manipulate, and understand problems and this requires evolution of a brain to not only process incoming information but to use this information for language, to communicate, and even to create civilizations. Of course every life form on earth, for example, is intelligent to some degree but humans are the only species to create an advanced technology and likewise the same would be true for an extraterrestrial intelligence so fI

are those fraction of planets with intelligent life.


fc =  The fraction of planets with intelligent beings capable of interstellar communication.


Like the previous three variables, the amount of uncertainty becomes more greater simply because of our ignorance of which planets are likely to harbor life, which that do have life, and which that have complex, intelligent life and so fis the fraction of planets with extraterrestrial with the ability and willingness to communicate with other intelligent beings.


L  =The lifetime of a communicating civilization.


This variable is one of the most uncertain factor when estimating the number of extraterrestrial civilizations and this involves the mean lifetime L of civilizations that can exist before being destroyed or in other words how long a civilization can exist when it is communicating with other civilizations.


Ls = Average lifetime of planet and star.


The lifetime of a planet with biosphere and intelligent life ultimately depends on the parent star on the main sequence and it is easy to estimate those planets that are in orbit of stars which are on the main sequence for 5 billion years even if it is difficult to know which planets are habitable, nonetheless, I will even attempt to make some estimates which will be in an appendix in this blog. What we can say for sure is that N depends on both the number of stars, fraction of stars with environments suitable for life, intelligent life, and civilizations.


Also I will talk about the methods extraterrestials with an advanced intelligence can use to communicate through interstellar distances and discuss their advantages as well as disadvantages together with some exotic possibilities that are also being considered.


Method of Extraterrestial Communication


Just what methods would extraterrestials use to make their presence known? If there are civilizations that are sending messages, how would we detect them and can we make any headway in determining their messages? If extraterrestials are communication within some vast interstellar Internet , should we respond by going online their network of communications even if we can understand their language or languages? What would be the consequences of such a discovery?


There are many ways but it we would have to consider narrowing down the possibilities before considering even exotic alternatives and two such methods of communications would 1. To send a space probe, and 2. To send messages using electromagnetic radiation, and I will even discuss 3. Other methods of communication.


Space Probes


Humans have sent robotic space probes to study the planets and stars so if humans can send probes to vast distances, why not extraterrestials?


When sending a probe to a far away star, and if the intent is to announce the presence of life from its home planet, the message that a probe would carry must be durable and must be able to survive the vacuum of space. Second, as far as the message goes, it must be written in a language that both humans and aliens can understand and that is the language of science and mathematics.


Two sets of probes were launched which did carry messages and the first pair of interplanetary probes, Pioneer 10 and 11, carried a metallic plaque which contain a language based on binary, and it indicated the number of planets around the sun, a diagram of the probe itself, the path of which the probe took from earth, and the position of the sun relative to a cluster of stars called pulsars which are dense, rotating stars that give off periodic radio signals, and two images of a naked man and woman.


Another set, the Voyager probes also carried a pair a messages , one for each probe which included images as well as sounds all of which are contained in a phonograph, and images include planet earth, DNA, cells, humans, and human societies while the sounds includes music, greetings in many languages, and even the sounds of whales.


Both of these probes are now outside the solar system and in a way they are rightly so the artififacts of a civilization that are from a third planet orbiting a planet and every civilization here on earth  has always left its mark in the form of artifacts whether in old temples, coins, books, and old digital computers to name just a few, and also the same for the Pioneer and Voyager probes.


Of course, since the Voyager probes are now moving along the direction to the nearest star, Alpha Centauri, it will take more than 50,000 years to reach the star, and given the vast distance of space, it would seem unlikely if not impossible to be intercepted by emissaries of an advance civilization.


If it is hard to intercept a Voyager probe, then the argument can be extended for extraterrestrial artifacts and so far, there is no hard evidence for their existence but it is still an open possibility that an extraterrestrial probe may be in our direction but then the question would be , what would an extraterrestrial probe look like and if it was carrying a message, could we understand the message?


In terms of understanding messages, since both humans and aliens inhabit the same universe, the only language we would have in common is mathematics and indeed both the messages in the Pioneer and Voyager probes were written in a binary form and in the case of the Pioneer messages, any extraterrestrial with an intelligence that is at or even beyond our own, is less likely to have difficulty in understanding where the Pioneer probes came from.


From the Pioneer message, the units of measurement for space and time, were expressed based on the length of two hydrogen atoms and the time between a transition between two energy levels of hydrogen. Why use hydrogen as a unit of measurement and why is it so relevant? We know that the universe is made up mostly of hydrogen and we know this because of spectroscopy, and wherever we analyze radiation from the sun, the stars, and galaxies, the majority of all the celestial bodies as well as the vacuum of space is composed of hydrogen and hydrogen is a simple chemical element composed only of one electron and a proton so using a binary system where a basic unit of information is represented as “1” or “0”, the simplest way to convey information together with using hydrogen with the distance between two atoms bonded together and the time it takes from one energy level to another, is a logical way to express information and from there, information that is relevant to where the probe came from along with where the sun is relative to 14 pulsars together with the fact that nine planets orbit the sun and one of these planets is the source of a probe.


There is only two images on the probe that the extraterrestials may find a hard time understanding, and that is the two images of a man and woman standing next to one another all nude.


Unless the extraterrestrial share the same body plan, because of convergent evolution and recall  that this is when two or more unrelated species develop similar characteristics in the same environments, the presence of the man and woman standing upright with the man holding his right hand, which is meant to be a symbol of good will , which could be interpreted that way although there may be a chance that an extraterrestrial with a radically different body plan will probably not see it a such, much less trying to understand the upright postures of two separate beings next to each other.


Of course, the chances of intercepting a probe like Pioneer and Voyager are extremely slim since outer space is so vast and traveling at a speed that is still a tiny fraction of the speed of light, that it would take more than 50,000 years to reach the nearest star, Alpha Centauri and more likely that for the probes to reach halfway around the galaxy, the trip will take  more than 100 million years!


Using probes for extraterrestrial contact is one method that may be heroic but a two way communications would be extremely daunting to say in the least.


If it would take a 100, 000 years or more to send a message via space probe and suppose an extraterrestrial civilization, within our galaxy, was within interception of such a probe where the probe took that 100,000 year journey. If the civilization choose the respond for sending a probe that was within our direction, then it is likely to be another 100,000 years for a probe. Quite a long delay! This may be the reason why we have not heard any response simply because of the fact that there may be probes on their way in our direction but , whether this was done in response to the interstellar messages that we send other than our probes (I’ll talk about the other method of communication later) or they are send probes simply out of curiosity, we have not heard from since by virtue of the fact that they are too far away, and we may not hear from them anytime soon, maybe not in our lifetime, but maybe possibly  in the lifetime of our civilization, assuming of course we are wise enough not to end up destroying ourselves.


A Word about UFOs


At this point, even though I am discussing the possibility of  the use of space probes for how extraterrestials could announce their presence, you are probably wondering if this argument that I’m presenting is valid for those who claim that extraterrestials are already here on earth and that is the claim that UFOs or Unidentified Flying Objects represent the presence of extraterrestials.


Scientifically the possibility that UFOs could be the work of extraterrestials has not yet been ruled out but in reality there has never been any convincing evidence that this has been the case and the reason is that observations of the sky are often made by people who are not trained in that field of critical analysis, which is science, not only report what they see but sometimes they are more likely to come up with an interpretation that is not all realistic and the reasons are that what we remember may not be what really happen since human memory is prone to be fallible, and this makes eyewitness testimony all the more problematic.


Of course, I do not deny the fact that people throughout the ages and up to this day have been seeing things in the sky for which they had no explanations and as science begin to progress, what at first becomes hard to explain now becomes understandable .


An example are comets. At first, when they were observed, it was believed that comets were a symbol of danger and destruction and they were once feared partly because sometimes they would appear in the sky without warning since everyone at the time was accustomed to seeing the sun rise in the east and set in the west and that a universe with a setting and rising sun along with all the stars doing the same thing was something that was orderly and hence peaceful but with something that appeared in the sky without warning, would have been understandably frightening who were accustomed to such order.


Eventually, fear about comets gave way to understanding  of what comets are and through science , mainly in astronomy and astrophysics, it was discovered that comets are nothing but dirty snowballs that come from a vast reservoir of icy bodies that lay beyond the orbit of Pluto and that once in a while, mainly because of gravitational perturbation from a nearby star, one of these icy bodies would end up speeding towards the sun and heat and radiation from the sun sublimes the ices making the icy body speed up even further and from observers on earth will see it as a comet.


Since then, many phenomena observed such as comets, meteors, auroras, are those class of phenomena explainable by astronomy and meterology are just one of these classes of phenomena that are likely to generate UFO reports along with seeing aircraft from a different angle or even light reflected from a lamp and onto a window pane which could also act as a mirror can be also be mistaken for UFOs.


Why the belief, if not the hard evidence, that UFOs as extraterrestrial emissaries? I believe that this is likely to both a psychological as well as a sociological phenomenon. Ever since 1947, in the US, of the first reported UFO sighting (prior to 1947 what constitutes as UFO sightings today were not new but the circumstances behind this was completely different), there has not only been a continued increase in the sightings of UFOs but the majority of people believing that they are from other worlds, stems partly from the fact that in countries like the US for example, there has been and continues to be , an increase  in science fiction stories, as well as TV and movies about aliens from other worlds making contact with human societies.


That and the fact that since 1957 up to the present, nations like the former Soviet Union and the US and increasingly the European Union and China are now sending satellites and manned vehicles up in earth orbit , which I thinks adds to the belief that for any planet with advanced intelligence, space flight is an inevitability.


There is nothing wrong in believing that any extraterrestrial intelligence should have the capability of spaceflight and not just the lay public but scientists who specialize in the search for finding extraterrestrial intelligence,  but belief is one thing, hard facts are another.


Indeed, the claim that space is not only full of life, but of intelligent willing to communicate is in itself an extraordinary claim and as the astronomer Carl Sagan in 1980 has stated clearly and briefly “extraordinary claims require extraordinary evidence”.


Here is where the argument for UFOs as extraterrestrial emissaries collapses. People who claim to have seen UFOs have yet to present any extraordinary evidence backing up their extraordinary claim.


You would think that if anyone had an actual artifact from an UFO, that person would be known in history books in schools worldwide but there has not been anyone with a single shred of evidence of an extraterrestrial object.


Also, there is no evidence that extraterrestials have visited the earth in the past, and there are silly arguments that primitive peoples were just too primitive to construct buildings as enduring and even complex as the pyramids, Stonehenge, and the Nazca lines and had to involve help from outside.


Arguments like these only reinforce ignorance about not only how people in pre industrial times constructed buildings and works of art, but thinking that aliens helped them only demonstrates gross errors in logic and reasoning.


Although science is open to the possibility that extraterrestials could contact any other society including ours using space probes, but until there is hard evidence, the belief of UFOs will continue to persist, often by those who really want to believe in such things (indeed there is something emotionally satisfying in such beliefs ) and people, with such ignorance of how science really works, will continue to believe and offer UFO sightings as such “proof”.


The truth is, and time has shown this again and again, that logic has prevailed over human emotions and logic is the underlying force behind science and what was once seen as supernatural eventually becomes understandable and natural.


Electromagnetic Radiation


If sending probes has its obvious drawbacks, then is there an alternative for other methods of communicating? Indeed there is, and this method is easily used to communicate within members of our own species and it has been used for over 100 years and this method involves sending certain frequencies of electromagnetic radiation.


There is one such frequency for sending messages and that is using radio waves for communication. It is relatively easy for generating, encoding messages in the form of sound for radio and also for encoding both images and sound which is the basis of television.


Radio is and is still the best method for interstellar communications and this is the basis behind the communication strategy used by SETI or the Search for Extraterrestial Intelligence.


If radio is to be used for communication for both sending as well as receiving, and if we do receive a message from elsewhere, just what characterstics would a signal have that would indicate that it is intelligent in origin and how would it stand out from other radio signals and frequencies? In short, what would an alien message be like?


Radio waves come in various frequencies and wavelengths, most of which are used for communication in both for the transmission of sound and images. Out of all possible frequencies of radio and microwaves, also in use for communication, just which frequencies would be suitable for communication with extraterrestrials?


One such possible communication involves a type of radio emission with a wavelength around 21 cm and this wavelength happens to be present in outer space but why this particular wavelength?


It turns out that the wavelength of 21 cm is emitted by hydrogen and this happens because in a hydrogen atom, both the electron and proton, in addition to electric charge and mass, also have something called spin, where both electron and proton spin like little tops. Both proton and electron can be both spin up and spin down. Spin up is one quantum level and spin down is a different quantum level and there can be a transition from spin up to down and in the process, radiation is emitted and this radiation is within 21 centimeters.


Hydrogen is the most abundant element in the universe and with a hydrogen atom releasing the 21 centimeter radiation, there is nothing in principle wrong of generating 21 centimeter wavelengths in an artificial and if a signal was detected in our direction that was around 21 centimeters, that would indicate that this is the result of an intelligence attempting to make contact but finding a signal and determining whether or not it is artificial or not, is in itself problematic for the signal is natural since of all the hydrogen atoms giving off that kind of radiation. Add to the fact that the signal is obscured by radio waves and microwaves of various wavelengths so perhaps maybe we should try other wavelengths and indeed one such wavelength happens to be hydroxyl.


Why hydroxyl ? Hydroxyl is just oxygen and hydrogen bonded together and the molecule has its own distinct wavelength which is 17 centimeters and add to the fact that when atomic hydrogen binds to hydroxyl, you get water and indeed since water played a huge role in the evolution of life as well as human societies, the frequencies of emissions from 17 centimeters to 21 centimeters, which corresponds to microwave emissions, is also called the “waterhole frequencies”.


There has been efforts to determine if a signal of extraterrestrial could be detected through these narrow range of frequencies and indeed one such project , called project OZMA, the precursor to today’s SETI, which was then under the direction of astronomer Frank Drake in 1960 carried out the first such search within that frequency band. The results? Nothing.


Despite the initial failure, since given the receiving technology at the time which operated at a very narrow bandwith along with a single channel, the chances of missing an extraterrestrial signal would have been enormous. Ever since then, techniques for analyzing space signals have increased in terms of bandwidths and channels where as of today , there are more way more bandwidths and channels so now with ever, the chance of missing an extraterrestrial signal will become lower and lower.


Also, a single radio receiver would no longer be used since even with a radio receiver with more bandwidth, there is still a chance of missing a signal of potential interest so the solution is to combine many radio receiver through a technique called interferometry where many radio telescopes are working together simultaneously and with so many recievers, the chances of finding a signal of a frequency that could carry information from an extraterrestrial intelligence vastly improves.


To date, there is one such array of radio recievers and this is the Allen Telescope Array which is also supported by SETI. This array of radio telescopes has all the capabilities for detecting any intelligent signals as well as astronomical sources of radio waves such as nebulas and galaxies that emit radio radiation. With such a refine ability to pick out any signal of interest, it is likely that as long as it is functional, then it is possible that a contact between our civilization and another civilization light years away is likely within the near future.


Of course there are drawbacks in sending and receiving radio messages through interstellar space and one of the obvious problem is the time it takes for any message to between solar systems and this has to do with the universal speed of light.


According to relativity, mainly the special theory of relativity, light has the  fastest but not infinite speed in the universe and only light has the largest speed of 300,000 km/sec (186,000 miles/sec) and only at that speed can light travel in a vacuum. Nothing can travel faster than light and it not just visible light but also radio waves, microwaves, x-rays and gamma rays, since these are all forms of electromagnetic radiation and all travel at 300,000 km/sec in space.


The fact that light travels at the speed is the reason why interstellar distances are measured in light years which is the distance light travels in a year and since the speed of light is finite, information about the properties stars and galaxies travel at light speed and as we see the stars whether through visible light or through radio waves, we are seeing the cosmos not as they are today but at some time in the past.


What would this have to do with finding extraterrestrial intelligence? Suppose our radio telescopes do pick up a signal that is not natural but artificial and if we knew were that artificial signal came from, and suppose it was found to be 50 light years away. A solar system that is 50 light years means that we are seeing it , as it was 50 years ago, and if we do receive the message, we may make the decision to respond by including information about our planet such as the dominant intelligent species, what the life forms are based on, where our planet is located and so on, which can also be transmitted to the aliens on the planet 50 light years.


Because of the speed of light, it will not be possible to receive an instant reply and in our example, it would take 50 years for the signal that we sent to the aliens to be received and if the aliens understand our message and decide to reply, it would take another 50 years for a response, making a total of 100 years.


If that is long, suppose we received a message from a solar system that is 500 light years. Imagine how long a two way communication would occur. The total time would be 10,000 years since it would take 500 years total for a signal to reach us and if we responded, another 500 years for the reply to reach them (whoever or whatever “them” may be).



There has only been two attempts in sending messages where any extraterrestrial intelligence in the messages path could easily intercept. In 1974, at Arecibo, Puerto Rico, home of the largest non steerable radio telescope, a message carrying basic information about earth, it location relative to the sun, the kinds of chemical elements that make up life, the DNA double helix, a human being standing upright, and the radio telescope that was used to send the message.


A near replica of the Arecibo message that was sent in the direction of the globular cluster M13 that is 25000 light years away. From the top to bottom the first row are the numbers 1 to 10, in the second row, the atomic numbers of carbon, hydrogen, oxygen, and nitrogen, to indicate the chemical elements that make up all earth life, the third row are the formulas of the bases that make up DNA and in the fourth row, a DNA double helix, the sixth row , a human being, the seventh row, the nine planets orbiting the sun, and the last row on the bottom, the antennae that was used to send the message. Should any aliens intercept this message and should they respond, it will take another 1000 years to send a reply. (Plakboek)


In fact, the message was meant only as a demonstration of the capability of sending such a message , not as an actual attempt. Nonetheless, the message is an artifact of human intelligence and it was even aimed at a particular area in space which is in the direction of the globular cluster M13 which is 25,000 light years away so the message would take 25000 years to reach its destination and if there is a civilization in its path that could intercept radio messages and could understand the meaning of the message, then if they interpret as an attempt to communicate, then a reply that the extraterrestials could send would take another 25000 years.




Radio signals can be one method for communication and there is no reason why it cannot be done, after all we humans are using radio every day for communication so aside from the fact that it is “easy” to generate radio signals for carrying sound and images, is it not possible that there are other ways of sending messages other than radio?


It turns out there is another possibility of distant contact and this involves the use of light as a form of communication.


There is nothing illogical in the use of light for communication. After all, we evolved to sense visible light and we are visual organisms because of our sophisticated imaging system that is responsive to all wavelengths of visible light. We see each other in light and we use light to find our way home as well as to see who our friends or even our enemies are.


It is one thing to use light when there are people and other things nearby, but it is quite another if the goal is to use light for interstellar communication.


A beam of light, if allowed to shine through space, will end up dispersing and become less intense, since light, as waves, will also disperse and become less and intense. This is because the source of light is what is called incoherent light and that is light that is composed of various wavelengths and as the light moves away, it is this incoherence that results in wider dispersion.


Can there be coherent light that can be narrow and suffer less dispersion? Indeed there is, and the coherent form of light using only one wavelength and this kind of light is known as light amplified stimulation and radiation or laser for short.


All the wavelengths making up incoherent light are generated by energy level transitions in every atom but in lasers only one energy level can generate one wavelength and this stimulates nearby atoms to emit radiation with the same wavelength and this continues until a laser beam consisting of only one wavelength is generated.


With a laser beam, the beam is not only coherent but it is less likely to disperse even at larger distances so in principle with a coherent beam, even messages can be encoded and also a powerful laser beam could even be picked out even from all wavelengths emitted from a star, so if we could encode a message, such as in the form of pulses that would indicate a sequence of numbers, where one pulse would represent “1”, two pulses “2”, three pulses “3”, and so on, and if there was a civilization that was 50 light years away, anyone there with sophisticated observing technology could easily pick out an intense beam of radiation that would be different from the steady output of radiation and if that civilization happen to focus their attention on a particular yellow star, and if they found out, that the star emits visible light in the yellow part of the spectrum but that is orbited by eight planets, then if they somehow noticed the sequence of numbers, what could they conclude?


Since there are no natural processes that could produce such a regular output and even the star does not vary in this manner, then the extraterrestials could conclude that it is not the sun but rather one of the planets is the source of that intense but regular variation of light, and the only conclusion that would force upon the observers that there is intelligent life of some sort, perhaps as a way of saying “We are Here”.


Other Forms of Communications


Radio and light are just two forms of communications but there is also the possibility of different forms of communications and we must consider what other means extraterrestials can communicate to other intelligent life forms


One way an extraterrestrial society could betray its presence is by analyzing the light of the star where a planet with an extaterrestial intelligence exists. How would analyzing starlight determine whether or not there is extraterrestrial intelligence?


Suppose our search could focuse on stars that are within the main sequence and it stars with the same surface temperature as the sun. We know that the stellar atmosphere is composed of hydrogen and helium but also sodium since sunlike stars are third generation stars that have heavy elements.


If we were to analyze stars similar to the sun but we find spectral lines of certain elements that should not be there and these are elements that are radioactive such as uranium and although when on the main sequence, hydrogen is converted into helium and there is carbon which acts as a catalyst speeding up hydrogen fusion, stars that are within the sun’s mass and radius do not have enough energy to fuse helium into uranium.


If there are radioactive elements present in the atmospheres of sun like stars, what could this indicate? It could indicate that the presence of radioactive elements was done as a result of an intelligence which either added these kinds of elements but another possibility is that it was done by an intelligence that likely destroyed itself in a massive nuclear holocaust, which is a rather poor way of announcing one’s presence in space.


Suppose an extraterrestrial civilization were to advance beyond its home planet and that is it chooses to expand into the neighborhood around its star. In order to support its civilization would require energy and one way it could do that is to use all the energy emitted from the star in order to power its civilization and to capture all the energy would require building a large shell around it to capture the outflow of radiation which can be used as a power source.


Such massive hypothetical structures have been termed Dyson spheres after the physicist Freeman Dyson who proposed the concept. Despite the name however, a sphere that large would be mechanically impossible so it would consist of a single ring or several rings which would be enough to capture energy.


Is there evidence for any Dyson spheres around stars that could be visible within our observation capability? It seems that a particular star may show signs of possibly having a Dyson sphere and if it is what would it look like?


To detect a possible Dyson sphere, first we must understand how it may function. If the purpose of a Dyson sphere is to generate energy for a civilization and if a Dyson sphere encloses the area around a star, then in order to generate useful energy, waste energy would need to dumped into the surrounding space outside the sphere. How would we know if the energy emitted was from a star or from an artificial structure such as a Dyson sphere.


If the goal is to observe Dyson spheres, then the radiation emitted from a sphere would be different than the radiation from the star. That is because the emission spectra of an artificial structure will likely indicate the presence of heavy metals which are not likely to be found in the stellar atmosphere (recall my previous argument regarding the presence of radioactive elements in stellar atmospheres).


There is one star observed, called by its long name KIC 8462852, in the past two years that could indicate the possible existence of a Dyson sphere, and observations of the star are still being made before anything else is ruled out and it was its variation of light that indicate something unusual about the star.


The observation of light variation which is aperiodic which means there is no regular pattern of variation but an irregular pattern and by the amount of dimming it is rather unusual, considering that this star is similar to the sun in terms of spectra and surface temperature and like the sun is still on the main sequence.


Could this indicate the presence of a jovian planet ?Jovian planets are much more common so it may be logical to assume the presence of a jovian planet blocking the light (recall the technique of finding planets by dimming the star planet)


Based on the amount of dimming, it turns out it is unlikely that the amount of dimming is caused by a jovian planet and according to the study, a jovian planet should dim the star light by about 1% given the size of a jovian planet but the observed dimming indicates the dimming is by about 15%, which is a rather large drop in brightness.


What could account for this huge drop in brightness? Perhaps this could be the result of dust blocking the star and it could be a solar system caught in the act of coalescing into a planets and dusty discs do block a considerable amount of light. However a careful study for the presence of dust orbiting the star revealed nothing.


What other scenarios are plausible? From analysis of the variable light data, perhaps if there is no dusty disc around the star, then maybe the star itself is responsible. In fact, there are stars that do vary in brightness and are called variable stars.


Variable stars, as the name suggests, are those kinds of stars that change brightness in a regular way that is there a pattern of dimming and brightening. Is KIC 8462852 a variable star? From observations, it is very unlikely that it is a variable star. That is because of the irregular pattern of dimming while a variable star shows a regular pattern of dimming and brightening. Also, it is a young star on the main sequence and many stars that are variable are outside of the main sequence.


Other hypothesis proposed are that there is a narrow field of comets colliding into the star, which could explain the large drop of brightness but even this scenario has its drawbacks and is still being determined whether it is plausible or not.


Considering that the drop of brightness can only be explained by something that is narrow and in orbit, we are left with one possible alternative. The unusual drop in brightness could indicate that this may the result of an artificial structure built by an extraterrestrial intelligence, perhaps as part of a construction project for a Dyson sphere.


How could we tell that this is a Dyson sphere. If a network of communication is needed to establish the project via radio, then perhaps it is possible to eavesdrop on their communication signals, and indeed SETI was involved in determing if the star does have an extraterrestrial civilization so by listening to signals that are narrow in frequency, this should indicate the presence of extraterrestrial intelligence.


The Allan Telescope Array  was used to intercept signals from KIC 8462852, signals that are characterstic for communications but also broadband signals were searched or more specifically around 1 Hz or 1 cycle per seconds and signals within 1-10 GHZ where a GHz is a billion cycles per seconds. The result? Nothing even remotely within these bands was ever detected.


Despite this, investigations are continuing and here this is an example of investigating extraterrestrial intelligence using a combination of radio technology for eavesdropping on signal along with optical and infrared astronomy and it is likely that all approaches will succeed in finding evidence for intelligent life instead of relying on one single method alone.


Of course, in the field of optical astronomy, in addition to observing dimming, spectral emissions must also be used to determine what is blocking the starlight and that could  either  be dust which should give of infrared emissions or emissions of silicon and carbon indicating dust or if the emissions are heavy metals then it could likely be part of an artificial structure, maybe a Dyson sphere, indicating the evidence of extraterrestrial life. The search must continue for if we do not, then we will never know if we are alone or not.




Life in other worlds is indeed one of the most profound questions ever asked and we do have a need to know whether or not we are alone. At first what was then speculation is now a provable fact and the science of astrobiology with its three questions will eventually be answered in our lifetimes. There has never been a more fascinating and more profound question as to whether or not life on earth is one out of many possible biospheres in the universe.


Appendix A: Estimating the Number of Communicating Extraterrestial Civilizations in the Galaxy


The Drake equation that was presented illustrates the many factors for assessing the number of civilizations capable of interstellar communication in our Galaxy. For the sake of calculation I will present a different version of this equation that is due to Sagan and Schlovskii (1966) as




where N is the number of civilizations that have the capability and willingness for interstellar communications, R*   is the mean rate of star formation, averaged over the whole lifetime of the galaxy, fp

is the fraction of stars with planets in orbit, ne is the number of planets with environments that could support the origin and evolution of life,fthe fraction of planets with life and a biosphere,fi   the fraction of planets where in the long course of biological evolution, there arises a species with advanced intelligence, fplanets with intelligent beings that have formed a civilization, and L the lifetime of each civilization.


The equation that I will use for the calculations, was the original form written by F. Drake in November 1961, and this depends on knowledge of astronomy, astrophysics, geology, biochemistry, biology, evolution, neurobiology, psychology, and sociology when it comes to estimating the number of communicating civilizations. According to Ulmscheider (2003), this equation can be compacted into a much simpler where the first three variables will form the astronomical part of the equation or


NHP= R*fpne


while the last four variables is expressed as


fIC= flfifeL


where  NHP


is the number of habitable planets while  fIC  is the fraction of planets with intelligent life. The number N is now the product of these two numbers.


N= NHP fip


In order to do the calculations , I will first do the calculations for NHP,  the astronomical part , and then fip,  the number of planets with intelligent life. The first part will be relatively easy to do now that we sufficient observational evidence of extrasolar planets but we are still far from certain the second part since this involves estimating which of these planets have environments that are not only habitable, but with life. Despite our uncertainty, I can only make reasonable assumptions based on knowledge about the origin of life as well as the evolution of life and technological societies. With this mind, I will proceed with the first part.


First, estimate NHP.Begin with the first variable R* and that is the total number of stars that are forming in the galaxy. Our galaxy is estimated to contain around 100 billion stars, all of which are converting hydrogen into helium, a prime factor for all of them to be on the main sequence. There are still regions in the Galaxy where stars are forming such as the nebula in the Orion constellation. On average, how many stars are forming now? To find the rate of star formation that has occurred in the whole history of the Galaxy, divide the estimated number of stars, which is expressed as 1011 stars.  The total age of the galaxy is 1010  years. Divide the estimated number by the age, and we get


1011/ 1010  = 10 stars/year


and R* =10 stars/year. The first variable R*

is a non zero variable and it has to be for if the rate of formation is 0, then N is 0 so we will not take into consideration. During the early stages of the galaxy, the rate of star formation was much higher than it is today but we will take R*

as 10 stars/year as the starting point. Proceed to the second variable fand with recent advances in planet detection methods, we are now confident in the number of extrasolar systems. Many planets detected so far are of the Jovian type, but there is no doubt of also terrestrial planets in orbit in other stars, and it is reasonable to make an estimation, in regards to the probabilities of the presence of terrestrial planets. In making the estimations, let’s not consider moons but terrestrial planets within the habitable zones of their parent stars where the lifetime of the habitable zone depends on the lifetime of the star on the main sequence. From probabilities that are 0 which is impossible to 1, a certainty, and thanks to observations of extrasolar planets along with recent models of planet formation, let   fp ~ 1 or that  fp    is approximately equal to the higher probability of 1 or that there is a high probability of terrestrial planets in orbit within their habitable zones.


The third variable is the fraction of planets with environments where abiotic processes are synthesizing all the subset of all the total number of polymers which will lead to the evolution of life forms capable of Darwinian evolution and are moved far away from thermal equilibrium. Any terrestrial planet in orbit around a star with a continuous source of electromagnetic energy as well as geothermal energy will have a range of conditions that will allow the origin of any life form. This not only includes the G type stars are stars that have the same size as our sun as well as surface temperature, but this does not rule out the K and M stars that are much smaller , have lower surface temperature which allows radiation in the red part of the visible spectrum. Like the second variable, the third variable will be approximately equal to the high probability of 1 or ne ~1.


We estimate for NHP by multiplying the first three variables R*fpne and we get


NHP = R* fpne = (10)(1)(1)=10


This is only the first part of the equation. We must proceed to the second part, and it is in the second part that we will make some reasonable assumptions before determining N in total.


The last part fIL  depends on four variable with the last L being the most uncertain variable to estimate. Nonetheless, I will make reasonable estimates based on probabilities of occurrences for each variable


There are good reasons to suspect that for any planet around stars of type G, K, and M, and as long as there planets with plate tectonic activity and situated within the center of a habitable zone, then the surface of each planet will likely consists of self replicating systems that once crosses a critical threshold where if a non equilibrium abiotic systems is endowed with self replication, metabolism, and mutagenicity then life is likely to arise on a given planet. It has happen on earth and there is no reason to think otherwise for other extrasolar planets around dwarf stars. The first factor of fIL, which is  fl


and since we also know that electromagnetic energy of the type that can increase the rate of abiotic synthesis along with carbon based compounds which has been found to be ubiquitous, the probability of planets where there is life is  fl` ~ 1.  With the second variable  fi or the number of planets where there is intelligent life, we are forced to make guess as to how probable that variable will be.


We know that Homo sapiens are not the only primate species with intelligence for understanding concepts, applying concepts to build functioning objects, and solving problems for other primate and non primate species do show degrees of intelligence so in the evolution of life, degrees of intelligence is not only inevitable, since this is related to surviving in changing environment, but for any planet with evolving life, it is likely that intelligence should be as common as life arising on planets but if we are considering evolution of life forms with complex brains for storing short and long term memory, planning ahead, and communication, the probability of intelligent species will likely be small but non zero.


This is because in the evolution of life, each species is the result of many previous generations of surviving species while there are other species that went extinct. Human intelligence is the result of not only of a long gradual evolution but a series of unlikely events which may be rare on other worlds but not zero for if fi


is zero then N will be zero which is not tenable since out of all planets in the galaxy, there is one planet that harbor intelligent life and that is earth so this factor must include other planets with intelligent species. The factor fi

is ~ 10-1 =0.1, and this probability is close to zero but not zero.


For comparison, earth and the solar system is 4.6 billions of years old and life is estimated to be about 3.8 billion years while the human species has existed on this planet for a tiny amount of geological time and human civilizations lasted for an even tinier amount and now for most societies, we are entering a communication phase, not just with communications with other nations via Internet but also an ability to listen for any signals of extraterrestrial origin and here we reach the third variable, fc the fraction of planets with intelligent life that has formed a civilization and with the ability to communicate, and like the previous variable, this depends on a number of unlikely events throughout the long history of a planet so this will have the same value as the previous variable or


fc ~ 10-1 =0.1


we will now multiply all six variables with one unknown variable and this is


N=  NHPfIL = R*fpneflfifcL =(10)(1)(1)(1)(.1)(.1)L


The number N ultimately depends on L, the longevity factor or L factor for the lifetime of a civilization. Technological capability only increased exponentially in the middle to the later pater of the 20th century and is continuing to do so such as an improved ability to make contact with other worlds. However there is also the capability of a civilization to produce weapons of mass destruction and with such weapons together with hostility between nations, there is the possibility of  complete self destruction resulting in a planet with no civilization to make contact so there are two possibilities for L.


One is that there can be the use of technology for purely destructive means and together with the possibility of a major natural catastrophe such as an asteroid collision which could also destroy civilization and so L would be around 100 to a 1000 years or take L to be 103


as the upper limit for the first scenario. In the second scenario, hostilities are lessened even decrease between nations resulting in mutual cooperation along with the understanding for no need for weapons of mass destruction and together also with the ability to defend ourselves against natural catastrophes so in the second scenario, the average lifetime is much greater than a 1000 years and maybe it could be around a million years so I will take 106

as the upper limit for the second scenario


First Scenario


In the first scenario, with L=1000 years, we get this number of civilizations based on the pessimistic assumption of civilizations with self destructive tendencies or


N= (10)(1)(1)(1)(.1)(.1)(1000)=100


In this scenario, it is the fate of civilizations to destroy themselves so with N=100 civilizations in our galaxy, it is a small but nonzero numbers. This scenario may be the reason why we have not heard from other civilizations. Self destruction may not be the only factor with such a small number, other possibilities for low L is of course natural catastrophes in that the inhabitants may not have the ability to defend themselves from such catastrophes. These two factors then set N=100 of civilizations that have the capability of interstellar communications


Second Scenario


In the second scenario with L=1000000 years, the amount of civilizations in our galaxy increases to




If, it is conceivable for civilizations to avoid self destruction, then it is possible that each civilization can live long during the geological history on its planet and it is expected a total of 100,000 civilizations. There is no reason to think otherwise that there are no scenario if all civilizations must avoid self destruction but even with this optimistic scenario, this raises more questions such as why have we not heard from them? Plausible but varied answers range from the vast distances involved in communication to a willingness not to communicate. I have presented the many factors for estimating the number of civilizations using the Drake equation to see how this works.




Allen Telescope Array (n.d). Retrieved April 20 2017, from


Archaea (n.d). Retrieved December, 9, 2016, from


Arecibo Message (n.d). Retrieved April 21, 2017, from


Ardila, D.R ( 2004, April). The Hidden Members of Planetary Systems. Scientific American , 65-69


Arnold, H.F, Kai Chen, L, Kan Jennifer, S.B . (2016).  Directed evolution of cytochrome c for carbon- silicon bond formation: Bringing silicon to life. Science, 354, 1048-1051. doi: 10.1006


Ammonia (n.d). Retrieved April 20, 2017, from


Asimov, I (Winter 1981). Not as we know it-The Chemistry of Life. Cosmic Search.(9)3. Retrieved from


Asimov, I Extraterrestial Civilizations (1979) New York, NY: Crown Publishers Inc.


Asimov,I The Collapsing Universe: The Story of Black Holes (1977) New York, NY: Walker and Company


Astrobiology. (n.d). Retrieved April, 20, 2017, from


Bain, W, Petowsky, J.J, Seager. S, (2016). Toward a list of molecules as potential biosignature gases for the search for life on exoplanets and applications to terrestrial biochemistry, 16(6), Astrobiology 465-485, doi: 10.1089/ast 2015.1404


Biology. (n.d). Retrieved November, 25, 2016, from


Biosignatures. (n.d). Retrieved May 5, 2017, from


Callahan, M.P, Cleaves, II, H.J, Dworkin, J.P,  Fernandez, F, M, Neish, C, Parker, M, (2014). Amino acids generated from hydrated Titan tholins: Comparisons with Miller Urey electric discharge products, Icarus 237, 182-189,


Cassini samples the icy spray of Enceladus icy plumes (2011, June 22). ESA. Retrieved from


Chaisson, E Epic of Evolution: Seven Ages of the Cosmos (2006) New York, NY: Columbia University Press


Chela-Flores,J The New Science of Astrobiology: From Genesis of the Living Cell to Evolution of Intelligent Behavior in the Universe (2001) ,  the Netherlands Dordrecht : Kluwer Academic Publishers


Christensen, P,R (2005, July). The Many Faces of Mars. Scientific American , 293(1), 32-40


Choi, C.D, (2013). Eyeball Earths. Astrobiology Magazine. Retrieved from


Convergent Evolution (n.d). Retrieved April 25 2017


Darling, D, Schulze-Makuch, D ; We are Not Alone: Why We have already found Extraterrestial Life (2010) Oxford, England: OneWorld Publications


Darwin, C.  (1859) On the Origin of Species By Means of Natural Selection or the Preservation of Favoured Races in the Struggle for Life.  London, England: Murray London


Dolphin (n.d). Retrieved April 20, 2017 from


Doyle, L.R, Welsh, W.E (2013, November). Worlds with Two Suns, Scientific American, 309(5), 40-48


Dyson spheres (n.d). Retrieved April 22, 2017 from


Earth (n.d). Retrieved April 10, 2017 from


Einstein, A, Infeld, L The Evolution of Physics: From Early Concepts to Relativity and Quanta (1938) New York, NY, Simon and Schuster


Enceladus(n.d). Retrieved April 23, 2017 from


Eukaryote (n.d) . Retrieved December, 9, 2016, from


Europa (moon) (n.d). Retrieved April,5, 2017, from


Folsomme, C The Origin of Life: A Warm Little Pond (1979) San Francisco, CA: Freeman H. and Company


Gould, J,S Wonderful Life: The Burgess Shale and the Nature of History (1989) New York, NY : W.W Norton and Company


Hawking, S A Brief History of Time (1988) New York, NY, Bantam Books


Heidmann, J Extraterrestial Intelligence (1995) (S. Dunlop, Trans.), Cambridge, Great Britain: Cambridge University Press (Original work published, 1992)


Holldobler, B, Wilson, E.O The Superorganism: The Beauty, Strangeness, and Elegance of Insect Societies (2009) New York, N, W.W Norton & Company


Hypothetical types of biochemistry (n.d). Retrieved April 20 2017, from


Jupiter (n.d). Retrieved April, 5, 2017, from


KIC 8462852 (n.d). Retrieved April 22, 2017, from


Lemonick, M.D. (2014, July). Is Anybody Out There?. National Geographic ,226 (1), 26-46


Looking for Deliberate Radio Signals from KIC 8462852. (2015, November 5). SETI Institute. Retrieved March 22, 2015


Lozano, E, Saez, A.G, Zaldivar-Riveron (2009). Evolutionary History of Na,K-ATPases, and their Osmoregulatory Role, Genetica 136(3), 479-490, doi: 10.1007/s10709-009-9356-0


Mars (n.d). Retrieved April 4, 2017, from


Martinez, A. (2015, Aug 19). What is Life, Really? [Weblog post]. Retrieved from


Martinez, A. (2015, Nov 24). Energy, Life, and the Second Law [Weblog post]. Retrieved from


Mediocrity principle (n.d). Retrieved April 5, 2017, from


Mercury (n.d). Retrieved February 17, 2017, from


Mix, J,L Life in Space: Astrobiology for Everyone (2009), Cambridge, MA : Harvard University Press


Octopus (n.d). Retrieved April 20 2017, from


Oparin, I.A Origin of Life (1953), (Morgulis, S ,Trans), New York, NY: Dover


Pauling, L General Chemistry (1970) San Francisco, CA: W.H Freeman and Company


Peacocke, A.R The Physical Chemistry of Biological Organization (1983) Oxford, England: University of Oxford Press


Prokaryotes. (n.d). Retrieved December 9, 2016, from


Rhodothermus marinus (n.d). Retrieved April 2017, from


Sagan, C; Shklovskii, I Intelligent Life in the Universe (1966) San Francisco, CA: Holden Day


Sagan,C; The Cosmic Connection: An Extraterrestial Perspective (1973) New York, NY: Doubleday


Sagan,C; Cosmos (1980) New York, NY: Random House


Sasselov, D,D, Valencia, D (2010 August) Planets We Could Call Home. Scientific American


Schopf, W. (Eds.) Life’s Origins: The Beginnings of Biological Evolution (2002), Los Angeles, CA: University of California Press


Sodium-potassium adenosine triphosphatase (n.d). Retrieved May 16, 2017, from


Sun. (n.d). Retrieved February 16, 2017, from


Super earth (n.d). Retrieved April 15, 2017, from


Terrestial Planets (n.d). Retrieved February 17, 2017


The Planetary Society (2017). How to Search for Exoplanets. Retrieved from


Tholin (n.d). Retrieved April 25 2017, from


Ulmschneider, P; Intelligent Life in the Universe: From Common Origins to the Future of Humanity (2003) Heidelberg, Germany: Springer-Verlag


Wilson, E.O Consilience: The Unity of Knowledge (1998) New York, NY: Vintage Books


Variable Star (n.d). Retrieved April 22, 2017, from


Venus. (n.d). Retrieved February 21, 2017, from


What is a Dyson Sphere? (2017, March 28). Earth Sky. Retrieved from



Image Credits


NASA Goddard Space Flight  Happy Summer Solstice Northern Hemisphere CC BY 2.0


Shigella CC BY-SA 2.0


Iqbal Osman Anaerobes CC BY 2.0


Bacillus Subtilis felixtsao BY 2.0


Amoeba Tim Menzies CC BY-SA 2.0


Paramecia bursaria Picture pest CC BY 2.0


Volvox Craig Pemberton CC BY-SA 2.0


Euglena Mutabilis Picture Pest CC BY 2.0


Yeast e-Life the Journal CC BY 2.0


Fungal Mycelia CC BY 2.0


The Sun David Warrington CC BY 2.0


Mercury NASA Goddard Space Flight Center CC BY 2.0


Venus CC BY 2.0


Earth Kevin Gill  CC BY-SA 2.0


Mars Kevin Gill  CC BY-SA 2.0


Mold ThomasBresson CC BY 2.0


Mushrooms Karen Neoh CC BY-SA 2.0


Algae Roban Kramer CC BY-SA 2.0


Moss rjp

Life as Information Technology

Deoxyribonucleic acid or DNA is the universal carrier of biological information of the biosphere. With its ability to store information needed for the survival or living organisms not only is DNA the carrier of information but the same information can be expressed as proteins and because of its helical nature, the information can be passed on to future generations. Also the information can change during every generation of reproduction resulting in new species. From the perspective of information technology, DNA can be justifiably considered as a computer with the double helix being the hardware and the genetic code where three bases specify one or more amino acid as the software(Enzymlogic)

Deoxyribonucleic acid or DNA is the universal carrier of biological information of the biosphere. With its ability to store information needed for the survival or living organisms not only is DNA the carrier of information but the same information can be expressed as proteins and because of its helical nature, the information can be passed on to future generations. Also the information can change during every generation of reproduction resulting in new species. From the perspective of information technology, DNA can be justifiably considered as a computer with the double helix being the hardware and the genetic code where three bases specify one or more amino acid as the software(Enzymlogic)


Everywhere and wherever you go, information is available whether you are accessing the information, which is taken to mean digitally, on a computer such as a personal computer, a laptop, or on a smartphone whether for looking up on a website or sending an e-mail. This form of technology has matured in the past 20 years as computer systems worldwide began to interconnect thus transforming every aspect of our lives but manipulating, storing, and accessing information is of course an example of the human use of technology, the use of information really is nothing new and even before computer technology or rather more generally information technology, life has been using information technology even before humans arrived and indeed life happens to be the first example of information technology where information such as how to reproduce and how to make a living is stored in the molecule DNA which functions both as a storehouse of biological information ranging from how the organism will function down to how the information is to passed on to generation to generation.


As the science of molecular biology which studies the molecular aspects of life mainly how cells and whole organisms store their hereditary information, in the form of nucleic acids, and how all the complex activities inside every organism is the result of proteins, began to mature, it was not unnoticed that apparently all of life was using information in the sequences of nucleotides or the building blocks of nucleic acids and that information includes how a sequence of amino acids or the building blocks of proteins are arranged in one particular order which allows that one particular protein to carry out a task such as breaking down food molecules for the organism in question to use. As molecular biology was maturing, so was computer science or the study of how computers use, manipulate, and store information in a sequences of zeros and ones which is how all digital computers of today function.


It was never a coincidence that information , in the sense of computer science, was already being used in life such as protein synthesis which depends on genetic information whereas in computers of today, all must function using software. Some scientists were then beginning to realize that what could be studied in computer science can apply to molecular biology.


                            The Nature of Information and its Relation to Biology


Just what is information exactly? There can be several definitions of the same word and depending on how it used and with so many definitions confusion can arise but for the sake of this argument, one definition of information can be given and that is any property of a system whether it is a physical system or an abstract system that can tell what it is, how it came to be, how it functions, and what it can do or more simply what it does and how it does it would be a good enough definition. As stated clearly by Loewenstein (1998) ” Information, in its connotation  in physics, is a measure of order- a universal measure applicable to any structure, any system (pg. 6).” An example that would be present would be a sequence of 0’s and 1’s that code for a number or a word that could be easily read in a computer, another would be the sequence of amino acids in a protein molecule or any sequence of any material or abstract quantity with a given order. Indeed,  information can be present in the abstract world of mathematics as well as in the world of the physical universe especially that part of the universe that I will specifically mention which is the biosphere and indeed with all species of organisms, large and small and various kinds that make up the biosphere and likely other undiscovered biospheres throughout the cosmos, each organism is related to one another through evolution, and how it makes a living and from the definition of information, any living system has the ability to reproduce which can be found in its cells, the basic building blocks of life and the fact that cells come in two main forms, prokaryote or simple cells and eukaryotic cells or complex cells and that the ability to reproduce comes from the molecular structure of DNA and that the information in DNA contains the instructions for proteins and with proteins a cell can do a lot and because of evolution by natural selection, cells can either remain unicellular or evolve into multicellular beings in the forms of fungi, plants, and animals. Notice that the description in biology nicely fits with the one definition of life that I’m describing and you can then sense the connection between information and biology.


Just like a computer has both hardware and software, the same can be argued for life and in biology there is something equivalent to hardware and software and that is the phenotype and genotype respectively. What do these terms in biology have in common with computer science?


If you have read one of my blogs where I give a definition of genotype and phenotype or if you have decided to look up terms in biology relating to my blogs then you would have no difficulty in making the connections between these two terms in biology to the other terms in computer science but for those of you who are unfamiliar I will give a definition. In biology, genotype are the genetic instructions that all organisms possese while the phenotypes includes the structure as well as function.


In computer science, software are the instructions on how a computer will function while hardware includes the physical components such as screen and mouse. There are similarities between these two set of terms and that is genotype is like software while phenotype is like the hardware.


What is the equivalent of software and hardware in the living world? We can go much further and say, without hesitation, that life has invented information technology in the form of the genetic code and central dogma , the major difference is that in computer science, which a human activity where the development of computer software is done with a goal in mind and that is a form of program that will allow a computer to do a given task but in evolution by natural selection, on the other hand, natural selection can only operate on what is going on the present and what has happen in the past but it cannot see ahead in the future and during the origin or possibly origins of life, the ancestor of all life, whatever it may have been, would have needed a genetic system of replicating molecules and the information to do so would have been the sequence of nucleotides, which according to one hypothesis, it would have been RNA , a form of nucleic acid that like DNA would store genetic information but unlike DNA, which only stores genetic information, RNA is also a molecule with catalytic properties in that it can even replicate itself without the need for protein enzymes so as both catalyst for replicating itself along as the storehouse of genetic information, natural selection, which can acts on the molecular level as well as on the cellular level would favor RNA molecules that can direct the information for creating enzymes for metabolism together with the ability of passing information from one generation to the next, while in each generation, mutations are also inevitable and this becomes the source of variation for natural selection to operate on, favoring those variants with superior abilities of information transmission while rejecting those that are unable to do so and this would have continued until DNA became the molecule that could store information while RNA was relegated to the role of transmitting genetic information for protein synthesis and this became the ancestor of all of life which uses DNA to store the information for all protein molecules including those that will replicate DNA.


So much for comparing genotype and phenotype to software and hardware respectively but there is more to the biological characteristic of information than just a given sequence of nucleotide basis. In the case of biology, a sequence of bases in a DNA molecule codes for a protein molecule and suppose I were to isolate the gene from one of body cells that codes for the blood protein hemoglobin which functions to carry oxygen from my lungs to all my body cells. After isolation, the gene is purified and crystallized and placed in a vial and is allowed to stand on a shelf. What can the DNA molecule really do? Absolutely nothing at all. If, somehow the crystallized DNA was reintroduced into my body, then it will end up expressing the information to make hemoglobin. From this example, the information for creating a functional protein molecule, hemoglobin in this case is present in both my cells as well as your cells but as long as the gene is in the living cells, the genetic information will be translated into protein and after the protein is synthesized it will have the task of delivering oxygen to the body’s cells. This is in contrast to the example of the gene crystallized into a vial which really does nothing at all. The information of the gene can only be relevant in the right context which in this case is the hemoglobin gene in the cell nuclei surrounded by protein synthesizing structures called ribosomes which use messenger RNA to carry the information of base sequences and together with another set of RNA molecules, transfer RNA which actually carry a given amino acid and with the ribosome, a protein molecule is synthesized.


This indicates that information has a broader aspect and in the case of the activities of that fundamental unit of life, the cells, the concept of information must be broadened to include the relation of information to where and when it can be used and there are three levels of information and these go by the names of syntactic, semantic, and pragmatic and using the example of the hemoglobin gene I will carefully explain how each level relates to molecular biology in a general way.


Beginning with the syntactic level it is nothing more than the order of a sequence of numbers, amino acids, nucleotides, or any thing whether physical or abstract arranged in a particular order. A polypeptide or a combination of amino acids, the building blocks of proteins and there are twenty amino acids in nature and any polypeptide can be formed from any given sequence of various amino acids. Likewise, for the polynucleotides or a combination of nucleotides the building blocks of DNA and RNA and since there are four nucleotides and in the case of DNA there are the bases adenine, thymine, guanine, and cytosine while in RNA the base uracil replaces thymine while the other three bases are still present and there can be many different sequences.


You can begin to sense that like in information technology where computers manipulate and store information in binary language which of course is 0’s and 1’s, biological information, in the form of proteins and nucleic acids, are based on various monomers or the building blocks which for proteins are the twenty amino acids and for the nucleic acids, the four nucleotides. Whatever computing device such as computer and laptop cannot function properly without software or the information to make it function, the biological equivalent of software would be the sequence of amino acids in proteins which is determined by the sequence of polynucleotides  and without the correct sequence of polynucleotides or rather genes, there can be no proteins of a definite sequence and likely there can be no vital life processes such as metabolism which depends on protein molecules enzymes which speed up biochemical reactions and have a high degree of specificity for each molecule. Here is the problem since we are still considering the syntactic level of information; to have a protein molecule such as an enzyme that can break down a molecule such as glucose, the sequence of amino acids must be in the correct sequence and that is determined by the corresponding sequence of amino acids but there can be not one but many possible arrangements of nucleotide sequences  and also there can be not one but many different possible sequences of amino acids in a polypeptide.


To make this even clearer, consider once again the hemoglobin molecule. The sequence of the molecule has been determined using sophisticated biochemical, biophysical, and computational methods and the hemoglobin molecule can function as oxygen carrier because of it’s sequence. It is also well known in the hereditary disease, sickle cell anemia, that a single mutant gene can code for a hemoglobin with a different amino acid and there will be a hemoglobin that is unable to carry oxygen resulting in the disease sickle cell anemia. The hemoglobin molecule consists of the 20 different amino acids and there are a total of 146 amino acids. I have mentioned that for a polypeptide such as hemoglobin, that there can not be one but many possible arrangements. How many in the case of hemoglobin or what are the possible alternate arrangements of  polypeptide sequences of hemoglobin.


At the syntactic level since it is a sequence of symbols, whether that is the bases in genes or the 0’s and 1’s in computer memory, is it possible that the information can be quantified in the sense of predicting using a mathematical formula? As a matter fact yes but for only the syntactic level of information and the science that studies information, aptly named information theory was originally developed in then infant field of computer engineering which began in the late 1940’s and in 1948, an American mathematician Claude Shannon devised a formula that was based on the probability of sending a message that was similar to the original message and indeed the meaning of a message can be corrupted by noise which ranging from hearing static on telephone which can interfere with the meaning of the message ( in terms of meaning or the pragmatic level, which we will get to shortly) or rather the formula that Shannon devised measures the probability of the many ways of the message being arranged upon receiving and the formula is


  1.   H=  ΣpI=-Σpln(p)


In words, this formula means that for each message out of N total messages, each one arrangement of the symbols that defines a given message has an equal probability p of being found and the probability ( in the sense of finding any of the numbers 1,2,3,4,5,and 6 on a dice thrown about where all numbers have an equal chance of being presented on the top part of the dice after each toss) is multiplied by I, which is the information content of each message after receiving. In regards to probability of finding each message, any sequence is as probable as a sequence that conveys either information , in the sense that it may tell an observer something meaningful as well as messages that may be just pure nonsense. Once again consider the hemoglobin molecule. The polypeptide sequence is known but since it is composed of  up to 146 amino acids, all 20 naturally occurring amino acids, and from the polypeptide sequence which is coded by a gene for hemoglobin according to the genetic code and from DNA to RNA, carrying the specific polypeptide sequence is then  translated from nucleic acid language to protein language, the hemoglobin molecule is just one sequence out a vast number of possible sequences and out of this number of possible sequences one sequence is as likely as the other sequence and H or what is called the Shannon entropy, measures how probable one message over the other but the formula only deals with probabilities of messages; it says absolutely nothing about what the message means and what it can do? Interestingly enough, as the science of information theory began to mature, the fact that message can only mean something if there is someone or something to observe it or if the message was sent from a source to a receiver and if out of an ensemble of messages, a message with meaning does have an effect.


Out of the ensemble of many alternative polypeptide sequences, the sequence for a normal hemoglobin molecule has been found by natural selection for the survival of the organisms and hence survival of the species of organisms with that organism indicates that for a the sequence to be of any use, it must benefit the organism such as ourselves and according to information theory, information is as meaningful as long as it effects receiver, whatever the source and from this, it has been known that information flows from genes to proteins and in cells such as eukaryotic cells or cells with nuclei, the genetic information is present in the nucleus where it is the source and in the cell where proteins are made requires the information present in RNA so in cells there is already a source and receive which is in the cells themselves so to see how sequences can be meaningful we must go from the syntactic to the semantic level.


Any symbol in a sequence can have any probability but for a sequence to make some sort of difference depends not just on the arrangement of sequences but on the effect a sequence can have between two related levels, call one of these levels the “sender” and the other level “receiver” and it is the relation between these two levels that some sort of meaning can result in relation between the two, and now there is a new dimension of information called the semantic level.


The semantic level depends between two or more levels and this is where information now shows it effects. Starting from the syntactic level, which as you recall is nothing but the sequences of letters, marks on papers, amino acids in a polypeptide, and the probability of arrangements. On a simpler level, there are the microstates or the number and kinds of things, whether physical such as an amino acid in a protein or abstract such as a number or letter and with a large number of arrangements of any sequences, there is a transition from one level to another higher level and this higher level is called macrostate.


In between two levels, the relation between them makes the difference whether a message can have an affect or not and this depends on what the two levels have in common. To make this clear, we will use an example. Suppose you have a “sender” who is a person who speaks and a “receiver” who listens and takes action. Both sender and receiver have something in common and that is the use of spoken language and suppose that the sender sends out a message consisting of this one particular arrangement of symbols, which in this case are the 26 letters of the English alphabet





At the syntactic level, each arrangement in each separate grouping of words is as probable and hence this is just one arrangement out of many possible arrangements and let’s first consider this one particular sequence.  Recall that at the level of microstate, are the fundamental constituents and from looking at this, segments of curves and lines form the letters of the Latin alphabet which of course are the letters, O,G, T, I, K, H, N,C, and E. These letters define the microstate. Letters combine into words which are then the macrostate but this one particular sequence looks at first like gibberish since this is written out and this is what sender says to the receiver. This one particular sequence then has no effect and the receiver does nothing at all. Why?


Both sender and receiver are using the same language and with knowledge of grammer and syntax, and a sentence like that, does not carry any meaning that would influence the future behavior of the sender but suppose that in another possible arrangement using the same letters as before we have




The sender now recognizes two of the words which are “GO” and “THE” and even though these words are recognizable to the sender, the sender still has no idea if the new sentence is some sort of instruction or not, but here comes the other arrangements and something novel happens,




This is a recognizable sentence and the sender now understands this to be a command and assuming that sender and receiver are inside a house, then the receiver carries out the order of going to the kitchen in the house.


Now from this example, there is a meaning that is apparent in that particular sequence out of many possible arrangements consisting only of the letters, G,T,O,H,E,I,C,N, and K for example. Both sender and receiver, those two examples of levels that relate to one another can easily relate to one because both posses a knowledge of grammar and syntax of spoken language but also experience of living in a place that both of them share and so information can aquire meaning and only meaning if information can have an effect on each of these levels.


This is true, not just in linguistics but in the field of molecular biology. We will see how using the two levels of sender and receiver, the problem now is when the sender is now DNA and receiver happens to be a part of the cell that is involved in protein synthesis, which are present in all life forms and that is a subcellular structure called a ribosome.


How does a ribosome, a subcellular structure, that is found in all kinds of cells, use the information of DNA to turn into a string of amino acids which will be the protein? First, the information in DNA must be present in the form of another nucleic acid, ribonucleic acid or RNA and there are several kinds of RNA and these are the kinds of nucleic acids that actually carry out the work whereas DNA is something of  a repository of genetic information. The kind of RNA that carries information from DNA to the ribosome, is aptly named messenger RNA and along the length of the messenger RNA are the four bases, adenine, guanine, cytosine, and in place of thymine, uracil. Like the DNA, there are three bases that correspond to a given amino acid so and with the help of a special enzyme, RNA polymerase which synthesizes a strand of messenger RNA carrying the sequence of three bases for each amino acid. If along the gene there is a sequence of DNA bases that reads




because of base pairing where A pairs with T and G pairs with C in the DNA molecule but for RNA where U for uracil replaces T, the corresponding messenger RNA would be




Recall that the DNA is either located in chromosomes which are found in both eukaryotic cells and in prokaryotic cells. For the eukaryotic cells, DNA is wrapped up in special globular proteins called histones and it is this combination of DNA and protein that forms the chromosomes and that all the chromosomes are surrounded by a double membrane layer, called the nuclear envelope which is the really the cell’s nucleus. Prokaryotic cells, on the other hand, have no nucleus and there is no histones surrounding DNA, only the DNA molecule is tightly coiled inside the tiny volume that defines the prokaryotic cell.


As far as information is concerned, both the prokaryotic and eukaryotic cell has something in common and the DNA in both cells is the sender of genetic information. Messenger RNA carries the information for any polypeptide and the messenger RNA goes to the ribosome both of which are present in eukaryotic and prokaryotic cells. The ribosome, can then translate the information present in the messenger RNA by first using what is called ribosomal RNA which can read messenger RNA codon by codon and the ribosome after reading each codon, utilizes another form of RNA, called transferRNA and this kind of RNA is bonded to a given amino acid so with the ribosome, along with the messenger RNA and a transfer RNA bonded to amino acid is how the ribosome can translate nucleic acid language, composed of just four bases or rather four letters into the language of proteins, composed of 20 amino acids.


Also recall that using the analogy of sender and receiver, where both share a common language and if DNA in cells is the sender then the receiver are the ribosomes. What is the common language between DNA (sender) and ribosomes (receiver) in order for protein synthesis, as it is called, to occur? The answer has to do with the fact that all of life on earth, depends on a code which is appropriately the genetic code.


The genetic code relates the pairing between codons or the three letter bases in nucleic acid, whether DNA or RNA, to either one or several amino acids. Considering that there are four bases, and only three can either code for a single or several amino acids, how many possibilities are there for the coding between nucleotide and amino acids? It is 4^3=64 possibilities and that is there are some codons that can code for one amino acid but there are others that can code for more than one. The code is universal in all life forms ( although in reality there are some exceptions to this but this will not in any affect the main argument here) and at the level of the cell, the genetic code is the language understood between DNA and ribosome.


In addition there is a one way flow of molecular biological information that goes from DNA to RNA to protein, and like the genetic code, this is also universal in all of life and this is known as the central dogma, which simply states that information flow goes from sender (DNA) to receiver (ribosome) and through the ribosome, proteins are synthesized or more simply, DNA to RNA to protein.


To see how the genetic code operates between DNA and ribosomes, consider, for example, the codon TTT or Thymine Thymine Thymine. During transcription, where one section of DNA is read by RNA polymerase which synthesizes messenger RNA and with uracil present only in RNA, this would UUU for Uracil, Uracil, Uracil. According to the genetic codon, this only codes for just one amino acid, which is phenylalanine. As the messenger RNA makes contact with a ribosome, the ribosomal RNA reads the opposite codon as AAA or Adenine,Adenine, Adenine, which also corresponds to phenylalanine. That ribosome with the codon UUU now selects a transfer RNA with the codon UUU which has the amino acid phenylanine. Because of the common language that exists between the DNA and ribosome, a phenylalanine is then inserted into the growing polypeptide, and the ribosome can then add another amino acid via messenger RNA and ribosomal RNA. Thus, we can see , in addition to the syntactic level, which only deals with probabilities of symbols in a sequence such as the arrangements of codons in a polynucleotide such as DNA, there is the semantic level where there is a common language, the genetic code between a molecule which is DNA and along with an information channel, messenger RNA, and a molecular complex, the ribosome, there can be another  property of life which is protein synthesis.


Thanks to the genetic code, any protein molecule can be synthesized, but DNA codes for so many kinds of proteins which allow for survival of the organism and from there is another important level of information and that is how the protein, in the context of the cell, and whether the organism is unicellular such as an amoeba or multicellular like us, allow important biological functions such as growth and adaptation and this would known as the pragmatic aspect.


If sender and receiver have a common language in common and if receiver understood what was sent then this will provoke an action to do something on the the part of the receiver. For the pragmatic aspect, this involves taking action only when both sender and receiver have a language in common. The same is true at the cellular level where the nucleus has the instruction needed for one particular protein and the ribosome, the receiver, uses messenger RNA to synthesize that protein which will end up affecting the cell in some way whether that protein could be an enzyme intended for use if the cell, in question, is an unicellular organism, or in a multicellular organism, such as ourselves, it is a hemoglobin molecule needed for oxygen transport.


In pragmatics, in order for information to carry out an action, there are at least two ways it could do this. The first is what is called confirmation and that is the message must have at least a degree of constancy that is there can be nothing completely novel or else information may not have an effect and the other is novelty where a message may differ to an extent that could elicit an response to the receiver to do something new, that was not present in the previous behavior of the receiver.


Cells reproduce and at the molecular level, DNA has the instructions to carry out it’s own reproduction and so in a step by step sequence of events, one cell becomes two, and so both cells carries the original information that was present in the previous cells. Here is where we can see the confirmation part of the pragmatic aspect of information. All the information needed to construct the cells is present in both cells so the two cells have all that is needed to carry out its normal functions but since replication in DNA is not really an exact process in that mutations in the sequence of genes is inevitable and if the mutations were high enough as to actually disrupt every base sequence in each gene, then it will likely disrupt the entire physiological function of the parent cell and it may not divide, so in order to ensure faithful transmissions of genetic information, cells have evolved a set of special enzymes called “proofreading” enzymes to make sure that in the process of DNA replication , errors caused by mutations are kept at a minimum, so we can then see the confirmation aspects.


Of course, there is really is no perfect transmission of information, and that errors in transmission of information will occur, a fact that was well known to Shannon and also made evident in findings in molecular biology in that no matter how careful the proofreading enzymes are in keeping mutations in checks, there will be some progeny with a slightly different sequence in DNA. Those offspring with a slight difference in genotype may have end up with a novel enzyme that may confirm an advantage that its parents did not have and if it is an enzyme that allows efficient synthesis of protein molecules, then it is likely to be favored by natural selection and so there will be a brand new life form with an ability to survive in its environment. Here, novelty in the sense of a brand new form of information that effects the survival of future organisms. For the evolution of life, there has to be a balance between confirmation and novelty for if there was only confirmation but no novelty, then considering that environments have changed, organisms with perfect replication of genetic information are less likely to survive any changes whereas novelty, in the form of rapid mutations in between generations and any drastic mutations will likely damage biological systems. Evolution of life is then a balance between novelty and information which defines life’s pragmatic aspect.





Although, the computers of today are based on software that have been developed since the late 1940’s when the first electronic computers were developed, life has been using software for about 3.8 billions of years. Life indeed has used a form of information technology and I have written about definitions of life, one of these definitions that the organization of life is made possible by the one software, the genetic code , along with an information channel, the central dogma and together with the three dimensions of information, which life, at the molecular level, uses so well, the information aspects of life is clearly appreciable.






Aldridge, S (1996) The Thread of Life: The Story of Genes and Genetic Engineering Cambridge, England: Cambridge University Press


Computer Science (n.d) Retrieved on June 28 2016:


Frank-Kamenetskii (1996) Unraveling DNA: The Most Important Molecule of Life (Liapin, L, Trans) Reading, MA: Addison Wesely( Original Work Published 1993)


Gribbin, J(1985)  In Search of the Double Helix: Quantum Physics and Life  London, England: Penguin Books Ltd


Hemoglobin (n.d) Retrieved on October 4 2016


Hemoglobin (n.d) Retrieved on October 4 2016


Küppers, O,B (1990) Information and the Origin of Life (Scripta,M, Trans) MIT press


Information (n.d) Retrieved on June 28 2016


Information Technology (n.d) Retrieved on June 28 2016:


Lesk, A. M (2008) Introduction to Bioinformatics  (3rd Edition) Oxford, England: Oxford University Press


Loewenstein, W,A (1998) The Touchstone of Life: Molecular Information, Cell Communication, and the Foundations of Life New York, NY: Oxford University Press


Yockey, P. H (1992) Information Theory and Molecular Biology New York, NY: Cambridge University Press


Image Credit


Enzymlogic DNA  CC BY-SA 2.