Search

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., (© 2024. The Author(s), under exclusive licence to Springer Nature Limited.)

The recent inference of sulfur dioxide (SO 2 ) in the atmosphere of the hot (approximately 1,100 K), Saturn-mass exoplanet WASP-39b from near-infrared JWST observations 1-3 suggests that photochemistry is a key process in high-temperature exoplanet atmospheres 4 . This is because of the low (<1 ppb) abundance of SO 2 under thermochemical equilibrium compared with that produced from the photochemistry of H 2 O and H 2 S (1-10 ppm) 4-9 . However, the SO 2 inference was made from a single, small molecular feature in the transmission spectrum of WASP-39b at 4.05 μm and, therefore, the detection of other SO 2 absorption bands at different wavelengths is needed to better constrain the SO 2 abundance. Here we report the detection of SO 2 spectral features at 7.7 and 8.5 μm in the 5-12-μm transmission spectrum of WASP-39b measured by the JWST Mid-Infrared Instrument (MIRI) Low Resolution Spectrometer (LRS) 10 . Our observations suggest an abundance of SO 2 of 0.5-25 ppm (1σ range), consistent with previous findings 4 . As well as SO 2 , we find broad water-vapour absorption features, as well as an unexplained decrease in the transit depth at wavelengths longer than 10 μm. Fitting the spectrum with a grid of atmospheric forward models, we derive an atmospheric heavy-element content (metallicity) for WASP-39b of approximately 7.1-8.0 times solar and demonstrate that photochemistry shapes the spectra of WASP-39b across a broad wavelength range., (© 2024. The Author(s).)