This week the paper we’re going to be discussing is the long-titled “Life might be rare despite its early emergence on Earth: a Bayesian analysis of the probability of abiogenesis” (Spiegel & Turner 2011). The meeting’s on Thursday 4th August, at 7:10pm UT (8:10pm UK time)
The topic of this paper can be neatly paraphrased by the title of its excellent astrobites summary: How rare is extraterrestrial life? Most of the research in this field is empirical – we send out probes and observe exoplanets in an attempt to answer the question directly. However, the authors of this paper take a theoretical, Bayesian, approach instead. Here’s the 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. We revisit this argument quantitatively in a Bayesian statistical framework. By constructing a simple model of the probability of abiogenesis, we calculate a Bayesian estimate of its posterior probability, given the data that life emerged fairly early in Earth’s history and that, billions of years later, sentient creatures noted this fact and considered its implications. We find that, given only this very limited empirical information, the choice of Bayesian prior for the abiogenesis probability parameter has a dominant influence on the computed posterior probability. Thus, although life began on this planet fairly soon after the Earth became habitable, this fact is consistent with an arbitrarily low intrinsic probability of abiogenesis for plausible uninformative priors, and therefore with life being arbitrarily rare in the Universe.
The paper was suggested by Matt Burleigh. He thought it would be a good choice because it “…has been getting a lot of publicity for an unrefereed paper”! Join the meeting on Thursday and tell us what you think.