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

Abstract

The James Webb Space Telescope (JWST) provides the first opportunity to detect gases in the atmospheres of M-dwarf terrestrial planets and search for signs of life. Here we determine the detectability of a comprehensive suite of biosignature gases that may have been episodically prevalent across Earth’s history. We used coupled 1D climate–photochemical models to generate synthetic inhabited terrestrial planetary environments for TRAPPIST-1 d and e. These encompass 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 transmission spectra and assess the likely detectability of different biosignatures with JWST. Our simulations suggest that biogenically generated O2 and its photosynthetic by-product O3 will likely be extremely difficult to detect. We explored the detectability of methyl chloride (CH3Cl) as an alternative indicator for a photosynthetic biosphere but find that it will likely require significantly higher global surface fluxes than Earth’s. We find that the CH4 and CO2 disequilibrium pair is potentially detectable in ∼10 transits for both the methanogen-dominated Archean-like environment and the modern photosynthetic-dominated biosphere—even in cloudy atmospheres. Organic haze and methyl mercaptan are other potential biosignatures for the Archean. Given the likely difficulties in observing an oxygenic-photosynthetic biosphere with JWST, we conclude that the methanogenic biosphere revealed by the combination of outgassed CO2 in the presence of methanogenically generated CH4 may be the most persistent detectable biosignature for an Earth-like planet.