Exoplanet Biosignatures: A Framework for Their Assessment

©NASA

Extrasolar Planet

Finding life on exoplanets from telescopic observations is the ultimate goal of exoplanet science. Life produces gases and other substances, such as pigments, which can have distinct spectral or photometric signatures. Whether or not life is found in future data must be expressed with probabilities, requiring a framework for biosignature assessment.

We present such a framework, which advocates using biogeochemical "Exo-Earth System" models to simulate potentially biogenic spectral or photometric data. Given actual observations, these simulations are then used to find the Bayesian likelihoods of those data occurring for scenarios with and without life. The latter includes "false positives" where abiotic sources mimic biosignatures. Prior knowledge of factors influencing inhabitance, including previous observations, is combined with the likelihoods to give the probability of life existing on a given exoplanet.

Four components of observation and analysis are used. 1) Characterization of stellar (e.g., age and spectrum) and exoplanetary system properties, including "external" exoplanet parameters (e.g., mass and radius) to determine its suitability for life. 2) Characterization of "internal" exoplanet parameters (e.g., climate) to evaluate whether an exoplanet surface can host life. 3) Assessment of potential biosignatures through environmental context (components 1-2) and corroborating evidence. 4) Exclusion of false positives. The resulting Bayesian probabilities of life detection map to five confidence levels, ranging from "very likely" to "very unlikely" inhabited.

David C. Catling, Joshua Krissansen-Totton, Nancy Y. Kiang, David Crisp, Tyler D. Robinson, Shiladitya DasSarma, Andrew Rushby, Anthony Del Genio, William Bains, Shawn Domagal-Goldman
(Submitted on 18 May 2017)

Comments: Part of a NASA Nexus for Exoplanet System Science (NExSS) series of 5 papers, to be submitted to Astrobiology. Comments welcome. 42 pages, 2 figures, 6 tables
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:1705.06381 [astro-ph.EP] (or arXiv:1705.06381v1 [astro-ph.EP] for this version)
Submission history
From: David Catling
[v1] Thu, 18 May 2017 01:04:48 GMT (1953kb)
https://arxiv.org/abs/1705.06381
Astrobiology

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