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

¹ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
² Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV Groningen, The Netherlands
³ SRON Netherlands Institute for Space Research, Niels Bohrweg 4, 2333 CA Leiden, The Netherlands
⁴ Institute of Astronomy, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
⁵ Max-Planck-Institut für Astronomie (MPIA), Königstuhl 17, 69117 Heidelberg, Germany
⁶ Université Paris-Saclay, Université Paris Cité, CEA, CNRS, AIM, 91191 Gif-sur-Yvette, France
⁷ Department of Astrophysics/IMAPP, Radboud University, PO Box 9010, 6500 GL Nijmegen, The Netherlands
⁸ School of Physics & Astronomy, Space Park Leicester, University of Leicester, 92 Corporation Road, Leicester LE4 5SP, UK
⁹ School of GeoSciences, University of Edinburgh, Edinburgh EH9 3FF, UK
¹⁰ Centre for Exoplanet Science, University of Edinburgh, Edinburgh EH9 3FD, UK
¹¹ Department of Astrophysics, University of Vienna, Türkenschanzstr. 17, 1180 Vienna, Austria
¹² ETH Zürich, Institute for Particle Physics and Astrophysics, Wolfgang-Pauli-Str. 27, 8093 Zürich, Switzerland
¹³ STAR Institute, Université de Liège, Allée du Six Août 19c, 4000 Liège, Belgium
¹⁴ Centro de Astrobiología (CAB), CSIC-INTA, ESAC Campus, Camino Bajo del Castillo s/n, 28692 Villanueva de la Cañada, Madrid, Spain
¹⁵ LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université de Paris Cité, 5 place Jules Janssen, 92195 Meudon, France
¹⁶ Université Paris-Saclay, CEA, IRFU, 91191 Gif-sur-Yvette, France
¹⁷ LERMA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Paris, France
¹⁸ UK Astronomy Technology Centre, Royal Observatory Edinburgh, Blackford Hill, Edinburgh EH9 3HJ, UK
¹⁹ SRON Netherlands Institute for Space Research, PO Box 800, 9700 AV Groningen, The Netherlands
²⁰ Department of Astronomy, Stockholm University, AlbaNova University Center, 106 91 Stockholm, Sweden
²¹ Department of Astrophysics, American Museum of Natural History, New York, NY 10024, USA
²² Centro de Astrobiología (CAB, CSIC-INTA), Carretera de Ajalvir, 8850 Torrejón de Ardoz, Madrid, Spain
²³ Department of Astronomy, Oskar Klein Centre, Stockholm University, 106 91 Stockholm, Sweden
²⁴ School of Cosmic Physics, Dublin Institute for Advanced Studies, 31 Fitzwilliam Place, Dublin D02 XF86, Ireland

^★ Corresponding author: crouzet@strw.leidenuniv.nl

Received: 11 May 2024
Accepted: 13 July 2025

Abstract

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.