Quantifying the origins of life on a planetary scale
Caleb Scharf a,1 and Leroy Cronin b,1
a Columbia Astrobiology Center, Columbia Astrophysics Laboratory, New York, NY 10027;
b School of Chemistry, University of Glasgow, Glasgow, G12 8QQ, United Kingdom
Edited by Neta A. Bahcall, Princeton University, Princeton, NJ, and approved May 17, 2016 (received for review November 23, 2015)
In this paper, we describe an equation to estimate the frequency of planetary “origin of life”-type events that is similar in intent to the Drake Equation but with some key advantages—specifically, our formulation makes an explicit connection between “global” rates for life arising and granular information about a planet. Our approach indicates scenarios where a shared chemical search space with more complex building blocks could be the critical difference between cosmic environments where life is potentially more or less abundant but, more importantly, points to constraints on the search. The possibility of chemical search-space amplification could be a major variance factor in planetary abiogenesis probabilities.
A simple, heuristic formula with parallels to the Drake Equation is introduced to help focus discussion on open questions for the origins of life in a planetary context. This approach indicates a number of areas where quantitative progress can be made on parameter estimation for determining origins of life probabilities, based on constraints from Bayesian approaches. We discuss a variety of “microscale” factors and their role in determining “macroscale” abiogenesis probabilities on suitable planets. We also propose that impact ejecta exchange between planets with parallel chemistries and chemical evolution could in principle amplify the development of molecular complexity and abiogenesis probabilities. This amplification could be very significant, and both bias our conclusions about abiogenesis probabilities based on the Earth and provide a major source of variance in the probability of life arising in planetary systems. We use our heuristic formula to suggest a number of observational routes for improving constraints on origins of life probabilities.
origin of life planetary scale chemical search space exoplanetary science
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Author contributions: C.S. and L.C. performed research, analyzed data, and wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
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