Recent discoveries of planets similar to Earth in size and proximity to the planets' respective suns have sparked scientific and public excitement about the possibility of also finding Earth-like life on those worlds.
But Princeton University researchers have found that the expectation that life—from bacteria to sentient beings—has or will develop on other planets as on Earth might be based more on optimism than scientific evidence.
Princeton astrophysical sciences professor Edwin Turner and David Spiegel, a former Princeton postdoctoral researcher, analyzed what is known about the likelihood of life on other planets in an effort to separate the facts from the mere expectation that life exists outside of Earth. The researchers used a Bayesian analysis—which weighs how much of a scientific conclusion stems from actual data and how much comes from the prior assumptions of the scientist—to determine the probability of extraterrestrial life once the influence of these presumptions is minimized.
Turner and Spiegel, who is now at the Institute for Advanced Study, reported in the Proceedings of the National Academy of Sciences that the idea that life has or could arise in an Earth-like environment has only a small amount of supporting evidence, most of it extrapolated from what is known about abiogenesis, or the emergence of life, on early Earth. Instead, their analysis showed that the expectations of life cropping up on exoplanets—those found outside Earth's solar system—are largely based on the assumption that it would or will happen under the same conditions that allowed life to flourish on this planet.
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