JWST Constraints on the Nature and Evolution of M Dwarf Terrestrial Planets in the TRAPPIST-1 System

JWST observations of the 7-planet TRAPPIST-1 system provide an excellent opportunity to test outcomes of stellar-driven evolution in terrestrial planetary atmospheres, including atmospheric escape, ocean loss and abiotic oxygen production. M dwarfs like TRAPPIST-1 exhibit prolonged pre-main sequence phases, with XUV output orders of magnitude greater than on the main sequence. This likely leads to extreme water loss through thermal hydrodynamic atmospheric escape on planetary companions - including those that will be in the habitable zone (HZ) once the host is on the main sequence. Through water photolysis and subsequent hydrogen escape, stark abiotic oxygen build-up may occur, suggesting oxygen is a potential false positive biosignature in these environments. Improving on past water loss predictions by considering the full statistical range of host star luminosity evolution, we present probabilistic distributions of the maximum water loss and abiotic oxygen build-up expected through this escape process on the TRAPPIST-1 planets, demonstrating the HZ planets are unlikely to lose > 10 Earth oceans of water before entrance to the HZ. We find that oxygen build-up on the interior planets is highly correlated with initial surface water content, indicating JWST measurements of oxygen on these planets may be able to constrain water loss history in the system. Furthermore, through comparative planetology of the inner two planets, TRAPPIST-1b and c, and considering the most recent JWST observations, we demonstrate the initial water content of the system may be tightly constrained around 10 Earth oceans if oxygen is confirmed in TRAPPIST-1c’s atmosphere. An initial water content of 10 Earth oceans would strongly suggest the HZ planets retained water in excess of an Earth ocean past HZ entrance, increasing the likelihood that they are habitable. We discuss future JWST observations that could characterize post-ocean loss atmospheres, and constrain potential interpretations of the current measurements. Finally, we consider the feasibility and detectability of steam atmospheres, which may be consistent with the JWST data and theoretically justified based on geochemical constraints. This work is supported by NASA’s Virtual Planetary Laboratory.