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 H₂/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 CO₂ or CO. This atmosphere can be outgassed from and sustained by a magma ocean.