JWST light curves & transit depths of HAT-P-26b

We detect water vapor (ln(B)=4.1), carbon dioxide (ln(B)=85.6), and sulfur dioxide (ln(B)=13.5) with high confidence, along with marginal indications for hydrogen sulfide and carbon monoxide (ln(B)<0.5). The detection of SO_2_ in a warm super-Neptune-sized exoplanet (R_P_~6R_{Earth}_) bridges the gap between previous detections in hot Jupiters and sub-Neptunes, highlighting the role of disequilibrium photochemistry across a broad range of exoplanet atmospheres, including those cooler than 1000 K. Our precise measurements of carbon, oxygen, and sulfur indicate an atmospheric metallicity of ~10x solar and a subsolar C/O ratio.

Retrieved molecular abundances are consistent within 2{sigma} with predictions from self-consistent models including photochemistry. The elevated CO2 abundance and possible H2S signal may also reflect sensitivities to the thermal structure, cloud properties, or additional disequilibrium processes such as vertical mixing. We compare the SO_2_ abundance in HAT-P-26 b with that of 10 other JWST-observed giant exoplanets, and find a correlation with atmospheric metallicity.