Early Release Science of the Exoplanet WASP-39b with JWST NIRSpec G395H.

Measuring the abundances of carbon and oxygen in exoplanet atmospheres is considered a crucial avenue for unlocking the formation and evolution of exoplanetary systems 1,2 . Access to an exoplanet's chemical inventory requires high precision observations, often inferred from individual molecular detections with low-resolution space-based 3-5 and high-resolution ground-based 6-8 facilities. Here we report the medium-resolution (R≈600) transmission spectrum of an exoplanet atmosphere between 3-5 μm covering multiple absorption features for the Saturn-mass exoplanet WASP-39b 9 , obtained with JWST NIRSpec G395H. Our observations achieve 1.46[Formula: see text] photon precision, providing an average transit depth uncertainty of 221 ppm per spectroscopic bin, and present minimal impacts from systematic effects. We detect significant absorption from CO 2 (28.5[Formula: see text]) and H 2 O (21.5[Formula: see text]), and identify SO 2 as the source of absorption at 4.1 [Formula: see text]m (4.8[Formula: see text]). Best-fit atmospheric models range between 3[Formula: see text] and 10[Formula: see text] solar metallicity, with sub-solar to solar C/O ratios. These results, including the detection of SO 2 , underscore the importance of characterising the chemistry in exoplanet atmospheres, and showcase NIRSpec G395H as an excellent mode for time series observations over this critical wavelength range 10 .