Narrowing the List of Potential Biosignature Gases for the Search for Life on Exoplanets

Thousands of exoplanets orbit nearby stars and the possibility that one of them may contain extraterrestrial life should be explored. Earth’s atmospheric spectrum has a few characteristic gases (O2,CH4,N2O) and others are posited to appear on exo-Earths (e.g., dimethyl sulfide and CH3Cl). Seager et al. (2016) provided a list of potential biosignatures that could be examined to allow for the further narrowing of gases that could be valuable in determining exoplanet habitability and signs of life. Here, we build on this previous work by examining gas stability and volatility in Earth-like temperatures and pressures. This allows us to further refine the number of potential gases that are useful. We draw and expand upon two viable biosignature gases: Methyl chloride (CH3Cl) and dimethyl sulfide (CH3SCH3or DMS). The biggest limitation is this: the biochemistry and ecology of a potential biosignature vary even among Earth-life and so cannot be assumed to be the same on other worlds.