Biological Considerations and Necessary Environmental Conditions
One of the most compelling issue of whether or not life exists elsewhere concerns the factors required for the beginning of life. Since we only know of life on our Earth we do not have anything to compare so our investigation is largely speculative. However, biologists and astrobiologists have attempted to determine the factors necessary for life. The biologists are much more conservative and have suggested that as many as 10 4 5 0 random trials would have been necessary for life to occur. However, experiments such as those done by Miller and Urey have demonstrated that when the compounds that made up the primordial soup (such as carbon dioxide, nitrogen and water vapor which were outgassed by volcanoes) were exposed to an electrical arc, which simulated lightning, amino acids and other compounds necessary for life were formed. Although that was established over sixty years ago, no one has been able to produce even the simplest life form. This is a conund! rum, however, the definitive establishment that life evolved so soon after conditions were possible for life leads astrobiologists to suggest that simple life is, in fact, pervasive although we do not know the mechanism that would have allowed life to come into existence. Let's consider the requirements of life.
For life to be self-replicating there must be a map. This would have to be in the form of DNA or RNA or some similarly complex molecule. This is the most difficult component of life. In addition, it is widely believed that life is likely to be carbon based. This is because carbon is a remarkably flexible element that forms very complex molecules and is abundant throughout the Universe. Another requirement appears to be the existence of liquid water because it is the universal solvent and allows cells to transport the necessary compounds to and from the cell. So in our search for extraterrestrial life we are looking for environments that satisfy these requirements.
There are two methods of searching for places where life could exist. We can attempt to visit sites within our Solar System, which so far has been fruitless, although Mars is still a possible candidate for life in the past and Europa, which is a moon of Jupiter, probably has liquid water below its ice cover, and could support life. The second method is to utilize spectroscopy to analyze the composition of atmospheres of planets once they are detected for relatively high concentrations of methane and oxygen which seem to be indicative of life processes, but this method is rather speculative, and currently beyond our technological abilities.
So as unlikely as it seems, our best prospect currently for detecting life in the Universe may be to specifically search for incoming EM signals that were produced by intelligent life. These signals are currently the only hope that we have of searching the Galaxy beyond our own Solar System. We will now consider the physics of attempting to find other Earth like planets because that drastically reduces the unknown factors regarding the requirements of life, and the methods of attempting to detect intelligent signals.
The Drake Equation
First proposed by Frank Drake, and known as the Drake Equation, although there are many variations of it, there is a probabilistic approach to addressing the likelihood of there being intelligent life in the Galaxy. This method involves determining the factors that would be essential for life to evolve and for that intelligent life to be detectable by us. Some of the factors can be determined but many of them are speculative. The original Drake equation considered the rate at which solar-type stars form in the Galaxy, the fraction of stars that have planets, the number of planets per solar system that are Earthlike or suitable for life, the fraction of those Earthlike planets on which life actually arises, the fraction of those life-forms that evolve into intelligent species, the fraction of those species that develop adequate technology and then choose to send messages out into space, and the lifetime of a technologically advanced civilization. All of these probabilities we! re multiplied to determine N, the number of technologically advanced civilizations in the Galaxy whose messages we might be able to detect. The formula is:
N=R *f pn ef lf if cL (Freedman and Kaufmann, 2005). Based on variations in the predictions, and variations in the basic parameters of the formula, the conclusion varies from 1 (our own) to millions. Future advances will enable us to calculate the relative probabilities of the factors with greater confidence.
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