Exoplanet Biosignatures: A Framework for Their Assessment

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. Key Words: Bayesian statistics—Biosignatures—Drake equation—Exoplanets—Habitability—Planetary science. Astrobiology 18, 709–738.

Exoplanetary Atmospheres: Key Insights, Challenges, and Prospects

“…E-ELT). However, whether or not the detected chemical will be a robust signature of life is a subject of much debate (179,49). Nevertheless, the prospects of finding a potential biosignature in the atmosphere of a terrestrial exoplanet are promising given the rapid increase in new discoveries of such planets and the new observational facilities on the horizon.…”

“…Nevertheless, the prospects of finding a potential biosignature in the atmosphere of a terrestrial exoplanet are promising given the rapid increase in new discoveries of such planets and the new observational facilities on the horizon. Recent reviews of various aspects of this area can be found in (179,49,87,228,163)…”

The Surface UV Environment on Planets Orbiting M Dwarfs: Implications for Prebiotic Chemistry and the Need for Experimental Follow-up

“…However, far fewer investigations have been conducted as to the favorability of M-dwarf planets for abiogenesis (the origin of life), i.e., whether life as we know it could emerge on these worlds. Investigations of the ease with which life can originate on different worlds are relevant to prioritization of targets for biosignature search, and for interpretation of putative biosignatures detected from such objects (e.g., Catling et al 2017). In part, the paucity of work on this question is due to limitations in our understanding of the origin of life.…”

When is Chemical Disequilibrium in Earth-like Planetary Atmospheres a Biosignature versus an Anti-biosignature? Disequilibria from Dead to Living Worlds

“…The prebiotic disequilibrium anti-biosignature is, in principle, remotely detectable on exoplanets. Strong spectral signatures of atmospheric CO2, CO and H2O exist, and could be detected with reflectance or transmission spectroscopy (Catling et al 2018). The presence of prebiotic H2 could be inferred with its spectral feature at , or its continuous features in the near-infrared and < .…”