Institute for Advanced Study Astrophysics Seminar

ABSTRACT: Life arose on Earth sometime in the first few hundred million years after the young planet had cooled to the point that it could support water-based organisms on its surface. The early emergence of life on Earth has been taken as evidence that the probability of abiogenesis is high, if starting from young-Earth-like conditions. This argument is revisited quantitatively in a Bayesian statistical framework. Using a simple model of the probability of abiogenesis, a Bayesian estimate of its posterior probability is derived based on the datum that life emerged fairly early in Earth's history and that, billions of years later, sentient creatures noted this fact and considered its implications. Given only this very limited empirical information, the choice of Bayesian prior for the abiogenesis probability parameter has a completely dominant influence on the computed posterior probability. Thus, although life began on the Earth quite soon after it became habitable, that fact is statistically consistent with an arbitrarily low intrinsic probability of abiogenesis for plausible uninformative priors and, therefore, with life being arbitrarily rare in the Universe. The presentation will emphasize generic statistical properties of problems of this general character as well as the particular topic in question.