Exoplanet Biosignatures: A Framework for Their Assessment

Finding life on exoplanetsfrom telescopic observations is an ultimate goal of exoplanetscience. 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 biosignatureassessment. We present a framework in which we advocate using biogeochemical "Exo- Earth System" models to simulate potential biosignaturesin 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" where abiotic sources mimic biosignatures. Prior knowledge of factors influencing planetary inhabitation, including previous observations, is combined with the likelihoods to give the Bayesian posterior probabilityof 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" exoplanetparameters (e.g., mass and radius) to determine an exoplanet's suitability for life. 2) Characterization of "internal" exoplanetparameters (e.g., climate) to evaluate habitability. 3) Assessment of potential biosignatureswithin the environmental context (components 1-2) and any corroborating evidence. 4) Exclusion of false positives. The resulting posterior Bayesian probabilitiesof life's existence map to five confidence levels, ranging from "very likely" (90-100%) to "very unlikely" ($\le$10%) inhabited.