  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 or inconclusive results. The
1.95-REarth and 8.8-MEarth 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. The planet has
been observed extensively with transmission spectroscopy, and its thermal
emission has been measured in broad photometric bands. These observations
disfavor a primordial H2/He-dominated atmosphere but cannot conclusively
determine whether the planet has a secondary atmosphere. Here we report a
thermal emission spectrum of the planet obtained by JWST's NIRCam and MIRI
instruments from 4 to 12 {\mu}m. The measurements rule out the scenario where
the planet is a lava world shrouded by a tenuous atmosphere made of vaporized
rock, 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.
