The Feasibility of Detecting Biosignatures in the TRAPPIST-1 Planetary System with JWST

JWST provides the first opportunity to detect gases in the atmospheres of M dwarf terrestrial planets and search for signs of life. Here we present simulations of the detectability of a comprehensive suite of biosignature gases that may have been present at different periods over Earth’s history. We used coupled 1-D climate-photochemical models to generate synthetic inhabited terrestrial planetary environments for TRAPPIST-1 d and e. These include cloudy and/or hazy Archean-Earth-like environments with either a dominant sulfur- or methane-producing biosphere, as well as clear and cloudy modern Earth-like environments with photosynthetic oxygen-producing biospheres. We generate simulated JWST transmission spectra, and use these to assess the likely detectability of different biosignatures with JWST. Our simulations confirm that biogenic oxygen, and its photosynthetic byproduct ozone, will likely be extremely difficult to detect. We explored methyl chloride as an alternative indicator for a photosynthetic biosphere, but find that it will likely require significantly higher global surface fluxes than Earth’s to be detectable. The most promising biosignature is the methane and carbon dioxide disequilibrium pair, which we find to be potentially detectable in 10 transits for both the methanogen-dominated Archean-like environment and the modern photosynthetic-dominated biosphere, even in cloudy atmospheres. This persistent detectability over planetary history, and for both reducing and oxidizing atmospheres, is due in part to a combination of the star’s UV spectrum, and the presence of oxygen and ozone in the modern atmosphere. Both factors slow the production of radicals that would destroy methane. We also considered organic haze and methyl mercaptan as potential biosignatures for the Archean. Given that observing an oxygenic photosynthetic biosphere with JWST is unlikely, we conclude that the methanogenic biosphere revealed by the combination of outgassed carbon dioxide and biologically generated methane may be the most persistently detectable biosphere for an Earth-like planet orbiting an M dwarf. This work was performed by the Virtual Planetary Laboratory Team, under NASA Grant No. 80NSSC19K0829.