Detection of CO2, CO, and H2O in the atmosphere of the warm sub-Saturn HAT-P-12b

The chemical composition of warm gas giant exoplanet atmospheres (with Teq < 1000 K) is not well known due to the lack of observational constraints. HAT-P-12 b is a warm, sub-Saturn-mass transiting exoplanet that is ideal for transmission spectroscopy. One transit of HAT-P-12 b was observed with JWST NIRSpec in the 2.87--5.10 μm range with a resolving power of ∼1000. We combined the JWST data with archival observations from HST WFC3 covering the 1.1--1.7 μm range. We analysed the data using two data reduction pipelines and two atmospheric retrieval tools, and performed atmospheric simulations using chemical forward models. CO2, CO, and H2O are detected at 12.2, 4.1, and 6.0 σ confidence, respectively. Their volume mixing ratios are consistent with an atmosphere of ∼10× solar metallicity and production of CO2 by photochemistry. CH4 is not detected and seems to be lacking, which could be due to a high intrinsic temperature with strong vertical mixing or other phenomena. SO2 is also not detected and its production seems limited by low upper atmosphere temperatures (∼500 K at P<10−3 bar derived from one-dimensional retrievals), insufficient to produce it in detectable quantities. This study points towards an atmosphere for HAT-P-12 b that could be enriched in carbon and oxygen with respect to its host star. When including the production of CO2 via photochemistry, an atmospheric metallicity that is close to Saturn's can explain the observations. Metallicities inferred for other gas giant exoplanets based on their CO2 mixing ratios may need to account for its photochemical production pathways. This may impact studies on mass-metallicity trends and links between exoplanet atmospheres, interiors, and formation history.