Detection of CO <sub>2</sub> , CO, and H <sub>2</sub> O in the atmosphere of the warm sub-Saturn HAT-P-12 b

Context . The chemical composition of warm gas giant exoplanet atmospheres (with T eq &lt; 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 . CO 2 , CO, and H 2 O 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 CO 2 by photochemistry. CH 4 is not detected and seems to be lacking, which could be due to a high intrinsic temperature with strong vertical mixing or other phenomena. SO 2 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). H 2 S 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 SO 2 , and a CH 4 depletion that is yet to be fully understood. When including the production of CO 2 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 CO 2 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.

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Journal
Astronomy and Astrophysics
Published
2025-09-18
DOI
https://doi.org/10.1051/0004-6361/202450690
Citations
1
Primary Topic
Astro and Planetary Science
Type
article
Field-Weighted Citation Impact
1.15

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article

Detection of CO 2 , CO, and H 2 O in the atmosphere of the warm sub-Saturn HAT-P-12 b

F. Lahuis, M. Min, R. Waters, J. Bouwman et al.
1 citations
Astronomy and Astrophysics
Astro and Planetary Science
1.15
article

Detection of CO 2 , CO, and H 2 O in the atmosphere of the warm sub-Saturn HAT-P-12 b

F. Lahuis, M. Min, R. Waters, J. Bouwman, M. Guêdel, R. Gastaud, Adrian M. Glauser, A. Glasse, P. Patapis, L. Decin, B. Edwards, G. Olofsson, Niall Whiteford, D. Barrado, T. Konings, T. P. Ray, L. Colina, Christophe Cossou, N. Crouzet, G. Östlin, J. P. Pye, I. Argyriou, D. Rouan, P. Royer, Th. Henning, B. Vandenbussche, A. Coulais, O. Absil, Lars Heinke, P. O. Lagage, A. Boccaletti, E. F. van Dishoeck
article en
1 citations

Abstract

Context . The chemical composition of warm gas giant exoplanet atmospheres (with T eq &lt; 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 . CO 2 , CO, and H 2 O 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 CO 2 by photochemistry. CH 4 is not detected and seems to be lacking, which could be due to a high intrinsic temperature with strong vertical mixing or other phenomena. SO 2 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). H 2 S 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 SO 2 , and a CH 4 depletion that is yet to be fully understood. When including the production of CO 2 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 CO 2 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.

Astronomy and AstrophysicsVol. 703
Leiden University (NL), Centre National de la Recherche Scientifique (FR), University of Vienna (AT), Dublin Institute For Advanced Studies (IE), American Museum of Natural History (US), Radboud University Nijmegen (NL), University of Leicester (GB), University of Liège (BE), Stockholm University (SE), University of Groningen (NL), Université Paris Cité (FR), Commissariat à l'Énergie Atomique et aux Énergies Alternatives (FR), Université Paris Sciences et Lettres (FR), Université Paris-Saclay (FR), Observatoire de Paris (FR), NASA Exoplanet Science Institute (US), Sorbonne Université (FR), Astrophysique, Instrumentation et Modélisation (FR), Institute of Particle Physics (CA), Laboratoire d’Etudes du Rayonnement et de la Matière en Astrophysique et Atmosphères (FR), Royal Observatory (GB), Max Planck Institute for Astronomy (DE), Laboratoire d’études spatiales et d’instrumentation en astrophysique (FR), Institute of Astronomy (RU), CEA Paris-Saclay (FR), Centro de Astrobiología (ES), AlbaNova (SE), Astronomical Institute of the Slovak Academy of Sciences (SK), Institut de Recherche sur les Lois Fondamentales de l'Univers (FR), Space Research Organisation Netherlands (NL), UK Astronomy Technology Centre (GB), University of Edinburgh (GB), KU Leuven (BE)
National Science Foundation, National Aeronautics and Space Administration, Space Telescope Science Institute, European Space Agency, European Commission, Schweizerischer Nationalfonds zur Förderung der Wissenschaftlichen Forschung, Swedish National Space Agency, Fonds De La Recherche Scientifique - FNRS, Belgian Federal Science Policy Office, Centre National d’Etudes Spatiales, Fonds Wetenschappelijk Onderzoek, KU Leuven, Knut och Alice Wallenbergs Stiftelse, Max-Planck-Gesellschaft, Ministerio de Ciencia e Innovación, Max-Planck-Institut für Astronomie, Horizon 2020 Framework Programme, Science and Technology Facilities Council, Agencia Estatal de Investigación
Openalex Percentile: Top 16%
Astro and Planetary Science
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