A secondary atmosphere on the rocky exoplanet 55 Cancri e.

Characterizing rocky exoplanets is a central aim of astronomy, and yet the search for atmospheres on rocky exoplanets has so far resulted in either tight upper limits on the atmospheric mass 1-3 or inconclusive results 4-6 . The 1.95R Earth and 8.8M Earth planet 55 Cancri e (abbreviated 55 Cnc e), with a predominantly rocky composition and an equilibrium temperature of around 2,000 K, may have a volatile envelope (containing molecules made from a combination of C, H, O, N, S and P elements) that accounts for up to a few percent of its radius 7-13 . The planet has been observed extensively with transmission spectroscopy 14-22 and its thermal emission has been measured in broad photometric bands 23-26 . These observations disfavour a primordial H 2 /He-dominated atmosphere but cannot conclusively determine whether the planet has a secondary atmosphere 27,28 . Here we report a thermal emission spectrum of the planet obtained by the NIRCam and MIRI instruments aboard the James Webb Space Telescope (JWST) from 4 to 12 μm. The measurements rule out the scenario in which the planet is a lava world shrouded by a tenuous atmosphere made of vaporized rock 29-32 and indicate a bona fide volatile atmosphere that is probably rich in CO 2 or CO. This atmosphere can be outgassed from and sustained by a magma ocean.