Mapping the SO ₂ Shoreline in Gas Giant Exoplanets

Abstract JWST has revealed sulfur chemistry in giant exoplanet atmospheres, where molecules such as sulfur dioxide (SO 2 ) trace photochemistry, metallicity, and formation and migration. To ascertain the conditions that determine whether (or how much) SO 2 , H 2 S, and other sulfur-bearing species are present in exoplanet atmospheres, we present a grid of planetary atmospheres covering metallicities from 0.3 to 1000× solar and temperatures from 250 to 2050 K. These models map out the “SO 2 shoreline,” the region of metallicity and irradiation for which SO 2 may be sufficiently abundant to be detectable. SO 2 is a sensitive indicator of metallicity; expected SO 2 abundances also depend strongly on overall temperature and C/O ratio; the SO 2 abundance depends surprisingly weakly on X-ray and ultraviolet irradiation, also weakly on K zz (for T eq ≳ 600 K), and is essentially independent of internal temperature. Despite its detection in a growing number of giant planets, SO 2 is never the dominant sulfur-bearing molecule: depending on temperature and metallicity, H 2 S, S 2 , NS, SO, SH, and even S 8 or atomic S are frequently as common (or more so) as SO 2 . Nonetheless SO 2 remains the most easily detectable sulfur-bearing species, followed by H 2 S, though perhaps SO and SH could be detectable in some gas giants. Aside from a pressing need for additional observational constraints on sulfur, we also identify the need for future work to account for the effects of clouds and hazes, fully self-consistent atmospheric models, 2D and 3D models, a wider range of planetary masses and radii, and studies to measure and refine reaction rates and molecular opacities of sulfur-bearing species.