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Peer-reviewedCC BYAstronomy & Astrophysics

The atomic C/O ratio of KELT-9b

  • Pelletier S.1
  • Borsato N. W.2
  • Bean J. L.3
  • Seifahrt A.4
  • Kasper D.3
  • Stürmer J.5
  • Costa Silva A. R.6
  • Ehrenreich D.1
  • Fossati L.7
  • Hoeijmakers H. J.8
  • Prinoth B.9
  • Steiner M.1
3 more authors
  • Thorsbro B.10
  • Vaulato V.1
  • Zucker D. B.11
11 affiliations
  1. 1Observatoire astronomique de l’Université de Genève
  2. 2Univ Toulouse, CNES, CNRS, IRAP
  3. 3Department of Astronomy & Astrophysics, University of Chicago
  4. 4Gemini Observatory/NSF NOIRLab
  5. 5Landessternwarte, Zentrum für Astronomie der Universität Heidelberg
  6. 6Instituto de Astrofísica e Ciêccias do Espaço, Universidade do Porto, CAUP
  7. 7Space Research Institute, Austrian Academy of Sciences
  8. 8Lund Observatory, Division of Astrophysics, Department of Physics, Lund University
  9. 9European Southern Observatory
  10. 10Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS
  11. 11School of Mathematical and Physical Sciences, Macquarie University
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Abstract

Context. The carbon-to-oxygen (C/O) ratio of a giant planet’s atmosphere has long been theorised to hold compositional information that can be traced back to its formation history. Typically, the C/O ratio of an exoplanetary atmosphere is inferred from abundance measurements of major C- and O-bearing molecules such as CO, CO2, H2O, and OH. However, some exoplanets have such elevated temperatures that molecules can be nearly completely thermally dissociated at the pressure levels that observations probe, making it difficult to measure their C/O ratio via these traditional tracers. Aims. Here, rather than using molecules, we aim to retrieve the C/O ratio of KELT-9b (Teq = 4000 K) directly from atomic C and O, which are detected in its atmosphere. Methods. We analysed two transits of KELT-9b observed with the MAROON-X high-resolution spectrograph, finding absorption cross-correlation signals from atomic C and O as well as refractory metals. From this, we inferred the vertical temperature structure, the relative proportions of volatile and refractory species, and the C/O ratio of KELT-9b using a 1D local thermodynamic equilibrium atmospheric retrieval framework applied only to spectral regions mostly unaffected by non-local thermodynamic equilibrium effects. Results. We measure the atomic C/O ratio of the atmosphere of KELT-9b to be 0.20−0.07+0.13, which is slightly lower than the stellar value of 0.38 ± 0.15 and significantly below the solar value of 0.59 ± 0.07. We otherwise confirm previous investigations of the terminator region, finding KELT-9b’s atmosphere to be thermally inverted and slightly metal-rich. We measure the volatile-to-refractory ratio, a proxy for the ice-to-rock ratio, to be 1.24−0.78+1.93 × solar ([Mvol/Mref] = 0.09−0.20+0.19), which is consistent with both KELT-9 and the Sun. Conclusions. While there are caveats to interpreting observations of such an extreme planet, our results demonstrate that atomic species can be used to measure C/O ratios of exoplanetary atmospheres in high-temperature regimes where molecules are thermally dissociated.

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Cite

BibTeXRIS
BibTeX
@article{pelletier2026atomic,
  title = {{The atomic C/O ratio of KELT-9b}},
  author = {Pelletier S. and Borsato N. W. and Bean J. L. and Seifahrt A. and Kasper D. and Stürmer J. and Costa Silva A. R. and Ehrenreich D. and Fossati L. and Hoeijmakers H. J. and Prinoth B. and Steiner M. and Thorsbro B. and Vaulato V. and Zucker D. B.},
  journal = {Astronomy \& Astrophysics},
  year = {2026},
  month = oct,
  doi = {10.1051/0004-6361/202555010},
  url = {https://doi.org/10.1051/0004-6361/202555010},
  note = {CC BY},
}
RIS
TY  - JOUR
TI  - The atomic C/O ratio of KELT-9b
AU  - Pelletier S.
AU  - Borsato N. W.
AU  - Bean J. L.
AU  - Seifahrt A.
AU  - Kasper D.
AU  - Stürmer J.
AU  - Costa Silva A. R.
AU  - Ehrenreich D.
AU  - Fossati L.
AU  - Hoeijmakers H. J.
AU  - Prinoth B.
AU  - Steiner M.
AU  - Thorsbro B.
AU  - Vaulato V.
AU  - Zucker D. B.
PY  - 2026
DA  - 2026/10/24
JO  - Astronomy & Astrophysics
DO  - 10.1051/0004-6361/202555010
UR  - https://doi.org/10.1051/0004-6361/202555010
AB  - Context. The carbon-to-oxygen (C/O) ratio of a giant planet’s atmosphere has long been theorised to hold compositional information that can be traced back to its formation history. Typically, the C/O ratio of an exoplanetary atmosphere is inferred from abundance measurements of major C- and O-bearing molecules such as CO, CO2, H2O, and OH. However, some exoplanets have such elevated temperatures that molecules can be nearly completely thermally dissociated at the pressure levels that observations probe, making it difficult to measure their C/O ratio via these traditional tracers. Aims. Here, rather than using molecules, we aim to retrieve the C/O ratio of KELT-9b (Teq = 4000 K) directly from atomic C and O, which are detected in its atmosphere. Methods. We analysed two transits of KELT-9b observed with the MAROON-X high-resolution spectrograph, finding absorption cross-correlation signals from atomic C and O as well as refractory metals. From this, we inferred the vertical temperature structure, the relative proportions of volatile and refractory species, and the C/O ratio of KELT-9b using a 1D local thermodynamic equilibrium atmospheric retrieval framework applied only to spectral regions mostly unaffected by non-local thermodynamic equilibrium effects. Results. We measure the atomic C/O ratio of the atmosphere of KELT-9b to be 0.20−0.07+0.13, which is slightly lower than the stellar value of 0.38 ± 0.15 and significantly below the solar value of 0.59 ± 0.07. We otherwise confirm previous investigations of the terminator region, finding KELT-9b’s atmosphere to be thermally inverted and slightly metal-rich. We measure the volatile-to-refractory ratio, a proxy for the ice-to-rock ratio, to be 1.24−0.78+1.93 × solar ([Mvol/Mref] = 0.09−0.20+0.19), which is consistent with both KELT-9 and the Sun. Conclusions. While there are caveats to interpreting observations of such an extreme planet, our results demonstrate that atomic species can be used to measure C/O ratios of exoplanetary atmospheres in high-temperature regimes where molecules are thermally dissociated.
SN  - 0004-6361
SN  - 1432-0746
ER  -