Detection of OH and Fe in the dayside atmosphere of the hottest ultra-hot Jupiter KELT-9b with SPIRou

¹ CAS Key Laboratory of Planetary Sciences, Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210023, China
² School of Astronomy and Space Science, University of Science and Technology of China, Hefei 230026, China
³ CAS Center for Excellence in Comparative Planetology, Hefei 230026, China
⁴ Shanghai Astronomical Observatory, Chinese Academy of Sciences, Shanghai 200030, China
⁵ Department of Astronomy, University of Science and Technology of China, Hefei 230026, China

^★ Corresponding authors: yhyang@pmo.ac.cn; guochen@pmo.ac.cn

Received: 30 April 2025
Accepted: 24 September 2025

Abstract

Simultaneous abundance measurements of volatile and refractory elements are crucial to unravelling the formation and migration history of ultra-hot Jupiters (UHJs). High-resolution infrared emission spectroscopy has recently been employed extensively to investigate the atmospheric components of UHJs, including both molecules and atoms. For the hottest known planet, KELT-9b, whose dayside atmosphere is almost completely thermally dissociated and ionized, no molecular components have been conclusively detected. Here, we present the first detection of the OH molecule in the dayside atmosphere of KELT-9b, based on two thermal emission observations conducted with the SPIRou spectrograph, and confirm the presence of Fe in the dayside hemisphere. We performed a self-consistent retrieval under the assumption of chemical equilibrium, constraining elemental abundances and atmospheric metallicity ([M/H]). We confirm the presence of a significant thermal inversion layer on the dayside. By retrieval, no significant net Doppler shift signals are identified, and the retrieved equatorial rotation speed agrees with the tidally locked rotation speed. The retrieved oxygen abundance is solar to supersolar (0.61_−0.58+1.19 dex). The retrieval suggests a subsolar to solar [C/O] (−0.75_−0.82^+0.64 dex) and a subsolar to solar atmospheric metallicity. The low metallicity may point to a locally well-mixed envelope and interior. The constraints remain broad, and the data are still statistically consistent with supersolar C/O and subsolar oxygen abundances. Taken together, the [C/O] and [O/H] results are compatible with formation beyond the water snowline followed by inward migration, but the present data do not conclusively rule out other scenarios. The volatile-to-refractory ratios, [O/Fe] = 1.25_−0.74^+0.99 dex and [C/Fe] = 0.60_−0.74^+0.62 dex, fall within the solar to supersolar range. However, their large dispersions mean they can only provide tentative indications of volatile enrichment. Overall, the statistical significance of these constraints remains limited, making firm conclusions about the planet’s formation history premature. In the future, the combination of higher-quality high-resolution optical-to-infrared observations and JWST data will enable more precise constraints on elemental abundances, providing more reliable insights into the formation and migration scenarios of UHJs. Finally, we advocate a retrieval-guided cross-correlation strategy to mitigate the risk of overlooking marginal species, exemplified by the tentative inference of CO in this study.