A CO2/CO-rich Atmosphere on the Rocky Exoplanet 55 Cnc e

Characterizing rocky exoplanets is a central endeavor of astronomy because they provide the opportunity to test planet formation and evolution theories in regimes that do not exist in the solar system. The first-order scientific questions about these planets are (1) whether they have an atmosphere, and (2) what their atmospheres are made of. To date, the search for atmospheres on rocky exoplanets has resulted in either tight upper limits on the atmospheric mass (Kreidberg et al. 2019; Greene et al. 2023) or inconclusive results (Tsiaras et al. 2016; Zieba et al. 2023; Lustig-Yeager et al. 2023). Here we report a thermal emission spectrum of the rocky exoplanet 55 Cnc e obtained by JWST’s NIRCam and MIRI instruments from 4 to 12 μm. The measurements rule out a previously proposed scenario that the planet is a lava world shrouded by a tenuous atmosphere made of vaporized rock, and indicate a bona fide volatile atmosphere rich in CO2 or CO. This atmosphere is likely outgassed from and sustained by a magma ocean underneath. These results suggest that highly irradiated exoplanets with Earth-like bulk densities provide a path to study the retention of volatiles and atmosphere-interior exchange on extrasolar rocky worlds.