A secondary atmosphere on the rocky Exoplanet 55 Cancri e

Characterizing rocky exoplanets is a central endeavor of astronomy, and yet the search for atmospheres on rocky exoplanets has hitherto resulted in either tight upper limits on the atmospheric mass^1–3 or inconclusive results^4–6. The 1.95-R_Earth and 8.8-M_Earth planet 55 Cnc e, with a predominantly rocky composition and an equilibrium temperature of ~2000 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 disfavor 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 JWST’s NIRCam and MIRI instruments from 4 to 12 μm. The measurements rule out the scenario where the planet is a lava world shrouded by a tenuous atmosphere made of vaporized rock^29–32, and indicate a bona fide volatile atmosphere likely rich in CO₂ or CO. This atmosphere can be outgassed from and sustained by a magma ocean.