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 the chemical inventory of an exoplanet 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 & AP; 600) transmission spectrum of an exoplanet atmosphere between 3 and 5 mu m covering several absorption features for the Saturn-mass exoplanet WASP-39b (ref. (9)), obtained with the Near Infrared Spectrograph (NIRSpec) G395H grating of JWST. Our observations achieve 1.46 times 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 CO2 (28.5 sigma) and H2O (21.5 sigma), and identify SO2 as the source of absorption at 4.1 mu m (4.8 sigma). Best-fit atmospheric models range between 3 and 10 times solar metallicity, with sub-solar to solar C/O ratios. These results, including the detection of SO2, underscore the importance of characterizing the chemistry in exoplanet atmospheres and showcase NIRSpec G395H as an excellent mode for time-series observations over this critical wavelength range(10).

Automated summary

Measuring the abundances of carbon and oxygen in exoplanet atmospheres is crucial for understanding the formation and evolution of exoplanetary systems. In this study, the medium-resolution transmission spectrum of the Saturn-mass exoplanet WASP-39b was obtained using the NIRSpec G395H grating of JWST, revealing significant absorption from CO2 and H2O, and identifying SO2 as the source of absorption at 4.1 μm. These results highlight the importance of characterizing the chemistry in exoplanet atmospheres and demonstrate the capabilities of NIRSpec G395H for time-series observations in this critical wavelength range.