Exoplanet Biosignatures: A Framework for Their Assessment

David C. Catling, Joshua Krissansen-Totton, Nancy Y. Kiang, David Crisp, Tyler D. Robinson, Shiladitya Dassarma, Andrew J. Rushby, Anthony Del Genio, William Bains, Shawn Domagal-Goldman

Research output: Contribution to journalArticlepeer-review

83 Scopus citations

Abstract

Finding life on exoplanets from telescopic observations is an 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 with future data must be expressed with probabilities, requiring a framework of biosignature assessment. We present a framework in which we advocate using biogeochemical "Exo-Earth System" models to simulate potential biosignatures in spectra or photometry. Given actual observations, simulations are used to find the Bayesian likelihoods of those data occurring for scenarios with and without life. The latter includes "false positives" wherein abiotic sources mimic biosignatures. Prior knowledge of factors influencing planetary inhabitation, including previous observations, is combined with the likelihoods to give the Bayesian posterior probability of life existing on a given exoplanet. Four components of observation and analysis are necessary. (1) Characterization of stellar (e.g., age and spectrum) and exoplanetary system properties, including "external" exoplanet parameters (e.g., mass and radius), to determine an exoplanet's suitability for life. (2) Characterization of "internal" exoplanet parameters (e.g., climate) to evaluate habitability. (3) Assessment of potential biosignatures within the environmental context (components 1-2), including corroborating evidence. (4) Exclusion of false positives. We propose that resulting posterior Bayesian probabilities of life's existence map to five confidence levels, ranging from "very likely" (90-100%) to "very unlikely" (<10%) inhabited.

Original languageEnglish (US)
Pages (from-to)709-738
Number of pages30
JournalAstrobiology
Volume18
Issue number6
DOIs
StatePublished - Jun 2018
Externally publishedYes

Keywords

  • Bayesian statistics
  • Biosignatures
  • Drake equation
  • Exoplanets
  • Habitability
  • Planetary science

ASJC Scopus subject areas

  • Agricultural and Biological Sciences (miscellaneous)
  • Space and Planetary Science

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