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Extrasolar Planets Having Life

Fraction of Extrasolar Planets Having Life

 

This is a tough one, because scientists are not certain what caused life on the earth to appear. The age of the earth is somewhere around 4.7 billion years old. According to present planetary theories, this is when all the flying debris in our part of the solar system coalesced into the planet, more or less the size it is now. Perhaps our moon was also created around this time when a giant asteroid collided with earth and threw up the material that now comprises the moon. Anyway, the planet was too hot to have a stable crust until about 3.9 billion years ago. We now know that life appeared between 3.6 and 3.7 billion years ago. In other words, life did not appear on earth for two or three hundred million years after the crust formed.

 

But what caused it to begin at all? In 1953,  a scientist, Miller, and his graduate student, Urey, performed an experiment that simulated conditions that were thought to have existed on the early Earth. The experiment showed that such basic building blocks of life such as amino acids can be formed spontaneously. This seemed to settle the problem. It seem that life would form automatically whenever conditions were correct.

 

The experiment, however, was problematic. For one thing, the composition of the prebiotic atmosphere of earth is controversial. Other less reducing gases produce a lower yield and variety. It was once thought that appreciable amounts of molecular oxygen were present in the prebiotic atmosphere, which would have essentially prevented the formation of organic molecules; however, the current scientific consensus is that such was not the case.

The other problem is that simple organic molecules are a long way from a fully functional  life form; however, in an environment with no preexisting life, these molecules may have accumulated and provided a rich environment for the spontaneous rise of living things. This is called the "soup theory". Nonetheless, the spontaneous formation of complex forms from abiotically generated monomers under these conditions is not a straightforward process. Besides the necessary basic organic monomers, compounds that would have prohibited the formation of polymers, were formed in high concentration during the experiments. According to Brooks and Shaw (1973), there is no evidence in the geological record that any such soup existed.

 

Another theory postulates the evolution of biochemical pathways as fundamentals of the evolution of life. In this theory, the energy released from redox reactions of metal sulfides is available for the synthesis of organic molecules and for the formation of oligomers and polymers. It is therefore hypothesized that such systems may be able to evolve into  of self-replicating, metabolically active entities that would predate the life forms known today. The experiment produced a relatively small amount of dipeptides (0.4–12.4%) and a smaller yield of tripeptides (0.003%). Another criticism of the result is that the experiment did not include any organomolecules that would most likely cross-react or chain-terminate. (Huber and Wächtershäuser, 1998)

 

The latest modification of the iron-sulfur-hypothesis has been provided by William Martin and Michael Russell in 2002. According to their theory, the first cellular life forms may have evolved inside spreading zones in the deep sea. These structures consist of microscale caverns that are coated by thin membranous metal sulfide walls. This model locates the "last universal common ancestor" inside a black smoker, rather than assuming the existence of a free-living forms. The last evolutionary step would be the synthesis of a lipid membrane that finally allows the organisms to leave the microcavern system of the black smokers and start their independent lives. This postulated late acquisition of lipids is consistent with the presence of completely different types of membrane lipids in archaebacteria and eubacteria (plus eukaryotes) with highly similar cellular physiology of all life forms in most other aspects.

The question "How do simple organic molecules form a protocell?" is largely unanswered. However, there are many different hypotheses regarding the path that might have been taken. Some of these postulate the early appearance of nucleic acids, whereas others postulate the evolution of biochemical reactions and pathways first. Recently, trends are emerging to create hybrid models that combine aspects of both.

 

As you can see, scientists are closing in on the answer, but are not there yet. As mentioned previously, one of the stumbling blocks is that the actual composition of the Earth's early atmosphere is unknown. Since at this point we are not sure what conditions caused life on Earth to come into being, it is difficult to estimate on how many extrasolar planets, even given all the right conditions, life would generate spontaneously. My own guess would be around ninety percent, but I'm an optimist. Many would disagree and put the number much lower, even zero.

 

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