A roadmap for the atmospheric characterization of terrestrial exoplanets with JWST

Ultracool dwarf stars are abundant, long-lived and uniquely suited to enable the atmospheric study of transiting terrestrial companions with the JWST. Among them, the most prominent is the M8.5V star TRAPPIST-1 and its seven planets. While JWST Cycle 1 observations have started to yield preliminary insights into the planets, they have also revealed that their atmospheric exploration requires a better understanding of their host star. Here we propose a roadmap to characterize the TRAPPIST-1 system — and others like it — in an efficient and robust manner with JWST. We notably recommend that — although more challenging to schedule — multi-transit windows be prioritized to mitigate the effects of stellar activity and gather up to twice more transits per JWST hour spent. We conclude that, for such systems, planets cannot be studied in isolation by small programmes but rather need large-scale, joint space- and ground-based initiatives to fully exploit the capabilities of JWST for the exploration of terrestrial planets.