The atomic C/O ratio of KELT-9b
Abstract
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. Here, rather than using molecules, we aim to retrieve the C/O ratio of KELT-9b (Teq = 4000K) directly from atomic C and O, which are detected in its atmosphere. 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. 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.
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BibTeX
@misc{pelletier2026atomic,
title = {{The atomic C/O ratio of KELT-9b}},
author = {Stefan Pelletier and Nicholas W. Borsato and Jacob L. Bean and Andreas Seifahrt and David Kasper and Julian Stürmer and Ana Rita Costa Silva and David Ehrenreich and Luca Fossati and Jens Hoeijmakers and Bibiana Prinoth and Michal Steiner and Brian Thorsbro and Valentina Vaulato and Daniel B. Zucker},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.13051},
eprint = {2609.13051},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.13051},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - The atomic C/O ratio of KELT-9b
AU - Stefan Pelletier
AU - Nicholas W. Borsato
AU - Jacob L. Bean
AU - Andreas Seifahrt
AU - David Kasper
AU - Julian Stürmer
AU - Ana Rita Costa Silva
AU - David Ehrenreich
AU - Luca Fossati
AU - Jens Hoeijmakers
AU - Bibiana Prinoth
AU - Michal Steiner
AU - Brian Thorsbro
AU - Valentina Vaulato
AU - Daniel B. Zucker
PY - 2026
DA - 2026/09/11
PB - arXiv
DO - 10.48550/arxiv.2609.13051
UR - https://arxiv.org/abs/2609.13051
AB - 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_2O, 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. Here, rather than using molecules, we aim to retrieve the C/O ratio of KELT-9b (T_eq = 4000K) directly from atomic C and O, which are detected in its atmosphere. 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. 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.
ER -