2025A&A...703A.264C

Context. The chemical composition of warm gas giant exoplanet atmospheres (with T_eq < 1000 K) is not well known due to the lack of observational constraints.
Aims. HAT-P-12 b is a warm, sub-Saturn-mass transiting exoplanet that is ideal for transmission spectroscopy. We aim to characterise its atmosphere and probe the presence of carbonaceous species using near-infrared observations.
Methods. One transit of HAT-P-12 b was observed in spectroscopy with JWST NIRSpec in the 2.87-5.10 µm range with a resolving power of ∼1000. The JWST data are combined with archival observations from HST WFC3 covering the 1.1-1.7 µm range. The data were analysed using two data reduction pipelines and two atmospheric retrieval tools. Atmospheric simulations using chemical forward models were performed to interpret the spectra.
Results. 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 (≳ 800 K required according to photochemical models). H₂S is marginally detected using one data analysis method, but not by the other. Retrievals indicate the presence of clouds between 2 and 11 mbar using one data analysis method, and between 5 and 269 mbar using the other. The derived C/O ratio is below unity, but is not well constrained.
Conclusions. 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, a possibly cold upper atmosphere that may explain the non-detection of SO₂, and a CH₄ depletion that is yet to be fully understood. 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.