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

Feinstein, Adina D; Radica, Michael; Welbanks, Luis and (2023). Early Release Science of the exoplanet WASP-39b with JWST NIRISS. Nature, 614(7949) pp. 670–675.

DOI: https://doi.org/10.1038/s41586-022-05674-1

Abstract

The Saturn-mass exoplanet WASP-39b has been the subject of extensive efforts to determine its atmospheric properties using transmission spectroscopy^1-4. However, these efforts have been hampered by modelling degeneracies between composition and cloud properties that are caused by limited data quality^5-9. Here, we present the transmission spectrum of WASP-39 b obtained using the SOSS mode of the NIRISS instrument on JWST. This spectrum spans 0.6-2.8𝝻m in wavelength and reveals multiple water absorption bands, the potassium resonance doublet, and signatures of clouds. The precision and broad wavelength coverage of NIRISS-SOSS allows us to break model degeneracies between cloud properties and the atmospheric composition of WASP-39b, favouring a heavy element enhancement ("metallicity") of ~10-30x the solar value, a sub-solar carbon-to-oxygen (C/O) ratio, and a solar-to-super-solar potassium-to-oxygen (K/O) ratio. The observations are also best explained by wavelength-dependent, non-gray clouds with inhomogeneous coverage of the planet's terminator.