TRAPPIST-1 d: A Case Study in Atmospheric Loss at the Inner Edge of the Habitable Zone

The TRAPPIST-1 system, comprising seven Earth-sized planets orbiting an ultra-cool M8 dwarf, offers a unique laboratory for studying atmospheric retention on temperate rocky exoplanets. I will present the inaugural 0.6–5.2 microns JWST/NIRSpec PRISM transmission spectrum of TRAPPIST-1 d, a 0.8 Rearth planet situated at the inner edge of the habitable zone (Teq ~ 262 K). Our observations reveal significant spectral slopes (500–1,000 ppm) attributable to unocculted stellar heterogeneities. After correcting for these effects, the resulting transmission spectrum is flat within 100–150 ppm, showing no detectable molecular features.

We can exclude, with high confidence, clear 1-bar atmospheres dominated by CH4 or CO, as well as high mean molecular weight atmospheres analogous to those of a clear Titan, a clear Venus, early Mars, and both Archean and modern Earth. If TRAPPIST-1 d retains an atmosphere, it is likely either extremely tenuous or obscured by high-altitude aerosols, such as nightside water clouds predicted by 3D general circulation models. Alternatively, the planet may be airless, implying that the inner TRAPPIST-1 planets formed with <4 Earth oceans of water.

This study provides empirical constraints on atmospheric loss processes for terrestrial exoplanets orbiting M dwarfs, contributing to our understanding of the cosmic shoreline and informing models of atmospheric evolution and retention in similar systems.