Early Release Science of the exoplanet WASP-39b with JWST NIRSpec PRISM

Abstract

Transmission spectroscopy1,2,3 of exoplanets has revealed signatures of water vapor, aerosols, and alkali metals in a few dozen exoplanet atmospheres4,5. However, these previous inferences with the Hubble and Spitzer Space Telescopes were hindered by the observations’ relatively narrow wavelength range and spectral resolving power, which precluded the unambiguous identification of other chemical species — in particular the primary carbon-bearing molecules6,7. Here we report a broad-wavelength 0.5–5.5 µm atmospheric transmission spectrum of WASP39 b8, a 1200 K, roughly Saturn-mass, Jupiter-radius exoplanet, measured with JWST NIRSpec’s PRISM mode9 as part of the JWST Transiting Exoplanet Community Early Release Science Team program10,11,12. We robustly detect multiple chemical species at high significance, including Na (19σ), H2O (33σ), CO2 (28σ), and CO (7σ). The non-detection of CH4, combined with a strong CO2 feature, favours atmospheric models with a super-solar atmospheric metallicity. An unanticipated absorption feature at 4µm is best explained by SO2 (2.7σ), which could be a tracer of atmospheric photochemistry. These observations demonstrate JWST’s sensitivity to a rich diversity of exoplanet compositions and chemical processes.