likelihood of extraterrestrial life
Two arguments that appeal purely to statistics and arrive at opposite conclusions are often brought to bear on this issue. According to the first, life on Earth has come about through such a long and unlikely sequence of events that there is virtually no chance of it having been repeated elsewhere. The opposing statistical claim is that stars are so incredibly numerous (around 400 billion in our Galaxy alone), and planets possibly even more so, that life elsewhere must be commonplace. In fact, although superficially appealing, both these claims are weak. The first is based on the misconception that Darwinian evolution is akin to rolling a die many times and fortuitously achieving one predesignated outcome. The second, in its failure to consider the many factors that might influence the development and survival of life, is too naive to carry much weight. It is characteristic of problems concerning which there is a lack of observational or experimental data that solutions are able to be proposed that are completely contradictory. This is true, for example, of all major problems in philosophy and religion. The fact that no claim concerning the nature or likelihood of extraterrestrial life can yet be refuted indicates that astrobiology is still an embryonic science, in the process of making the transition from a branch of philosophy to an empirical endeavor. No one yet knows if extraterrestrial life exists. Developments and discoveries within the past few years have shifted the consensus of scientific opinion to the view that life, at least in its simplest forms, may be quite common. However, there will be no certainty on that point until the first traces of organisms on other worlds are found.
1. Lineweaver, Charles H. and David, Tamara M. "Does the rapid appearance of life on Earth suggest that life is common in the Universe?," accepted for publication in Astrobiology.