Sulfur Dioxide and Other Molecular Species in the Atmosphere of the Sub-Neptune GJ 3470 b

TL;DR

This study uses JWST, HST, and Spitzer data to detect water, methane, sulfur dioxide, and carbon dioxide in GJ 3470 b's atmosphere, revealing disequilibrium chemistry and high metallicity in a low-mass exoplanet.

Contribution

First detection of sulfur dioxide in a sub-Neptune's atmosphere, showing disequilibrium chemistry at lower masses and temperatures than previously known.

Findings

Detection of sulfur dioxide at >3-sigma significance

Atmospheric metallicity estimated at ~100 times Solar

Evidence of disequilibrium chemistry through methane abundance

Abstract

We report observations of the atmospheric transmission spectrum of the sub-Neptune exoplanet GJ 3470 b taken using the Near-Infrared Camera (NIRCam) on JWST. Combined with two archival HST/WFC3 transit observations and fifteen archival Spitzer transit observations, we detect water, methane, sulfur dioxide, and carbon dioxide in the atmosphere of GJ 3470 b, each with a significance of >3-sigma. GJ 3470 b is the lowest mass -- and coldest -- exoplanet known to show a substantial sulfur dioxide feature in its spectrum, at $M_{p}$=11.2${\,{\rm M}_{\oplus}}$ and $T_{eq}$=600$\,$K. This indicates disequilibrium photochemistry drives sulfur dioxide production in exoplanet atmospheres over a wider range of masses and temperatures than has been reported or expected. The water, carbon dioxide, and sulfur dioxide abundances we measure indicate an atmospheric metallicity of approximately $100\times$ Solar. We see further evidence for disequilibrium chemistry in our inferred methane abundance, which is significantly lower than expected from equilibrium models consistent with our measured water and carbon dioxide abundances.