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 mass1–3 or inconclusive results4–6. The 1.95REarth and 8.8MEarth 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 radius7–13. The planet has been observed extensively with transmission spectroscopy14–22 and its thermal emission has been measured in broad photometric bands23–26. These observations disfavour a primordial H2/He-dominated atmosphere but cannot conclusively determine whether the planet has a secondary atmosphere27,28. 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 rock29–32 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.
