Abstract
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.
Description
Exoplanet Biosignatures: A Framework for Their Assessment
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