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

TL;DR

This paper presents the first high-precision transmission spectrum of exoplanet WASP-39b from JWST NIRISS, revealing multiple atmospheric features and cloud properties, and constraining its atmospheric composition and metallicity.

Contribution

It provides the first broad-wavelength, high-precision spectrum of WASP-39b, enabling detailed atmospheric characterization and breaking degeneracies between clouds and composition.

Findings

Detection of multiple water absorption bands

Identification of potassium resonance doublet

Evidence for wavelength-dependent clouds

Abstract

Transmission spectroscopy provides insight into the atmospheric properties and consequently the formation history, physics, and chemistry of transiting exoplanets. However, obtaining precise inferences of atmospheric properties from transmission spectra requires simultaneously measuring the strength and shape of multiple spectral absorption features from a wide range of chemical species. This has been challenging given the precision and wavelength coverage of previous observatories. Here, we present the transmission spectrum of the Saturn-mass exoplanet WASP-39b obtained using the SOSS mode of the NIRISS instrument on the JWST. This spectrum spans $0.6 - 2.8 \mu$m in wavelength and reveals multiple water absorption bands, the potassium resonance doublet, as well as 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, favoring a heavy element enhancement ("metallicity") of $\sim 10 - 30 \times$ 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 best explained by wavelength-dependent, non-gray clouds with inhomogeneous coverage of the planet's terminator.