The atomic C/O ratio of KELT-9b
Year: 2026
Type: article
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, CO 2 , H 2 O, 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 (T eq = 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 ([M vol /M ref ] = 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.
Source: Astronomy and Astrophysics
Institutions: University of Geneva, Centre National de la Recherche Scientifique, Lund University, Centre National d'Études Spatiales, Institut de Recherche en Astrophysique et Planétologie, Université de Toulouse, Macquarie University, University of Chicago, NSF NOIRLab, International Gemini Observatory, Heidelberg University, Universidade do Porto, Institute of Astrophysics and Space Sciences, Centro de Astrofísica da Universidade do Porto, Austrian Academy of Sciences, Space Research Institute, European Southern Observatory, European Southern Observatory, Université Côte d'Azur, Observatoire de la Côte d’Azur
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: gold
Estimated APC paid: $1,865
Funders: National Science Foundation, National Aeronautics and Space Administration, European Commission, Schweizerischer Nationalfonds zur Förderung der Wissenschaftlichen Forschung, Korea Astronomy and Space Science Institute, Université de Genève, Ministério da Ciência, Tecnologia, Inovações e Comunicações, Agencia Nacional de Investigación y Desarrollo, Fundação para a Ciência e a Tecnologia
OpenAlex ID: W7213080412