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High-resolution detection of neutral oxygen and non-LTE effects in the atmosphere of KELT-9b


Oxygen is a constituent of many of the most abundant molecules detected in exoplanetary atmospheres and a key ingredient for tracking how and where a planet formed1. In particular, the O i 777.4 nm triplet is used to probe airglow and aurora on the Earth2 and the oxygen abundance in stellar atmospheres3,4,5,6, but has not been detected in an exoplanet atmosphere before. We present a definite ground-based detection of the neutral oxygen 777.4 nm triplet lines in the transmission spectrum of the ultrahot Jupiter KELT-9b7, the hottest known giant planet. The synthetic spectrum computed employing novel non-local thermodynamic equilibrium radiative transfer calculations8 matches the data significantly better than that computed assuming local thermodynamic equilibrium. These non-local thermodynamic equilibrium radiative transfer calculations imply a mass-loss rate of 108–109 kg s−1, which exceeds the lower limit of 107–108 kg s−1 required to facilitate the escape of oxygen and iron from the atmosphere. Assuming a solar oxygen abundance, the non-local thermodynamic equilibrium model points towards the need for microturbulence and macroturbulence broadening of 3.0 ± 0.7 km s−1 and 13 ± 5 km s−1, respectively, indicative of the presence of fast winds in the middle and upper atmosphere. Present and upcoming high-resolution spectrographs will allow the detection in other exoplanets of the 777.4 nm O i triplet, which is a powerful tool to constrain the key characteristics of exoplanetary atmospheres when coupled with forward modelling accounting for non-local thermodynamic equilibrium effects.

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Fig. 1: Transmission spectrum of KELT-9b around the O i triplet.
Fig. 2: Planetary reference frame confirmation.
Fig. 3: Χ2 minimization map of the NLTE synthetic spectrum.
Fig. 4: Atmospheric sound speed and Jeans escape parameter.

Data availability

Data used in this work are publicly available from the Calar Alto archive at

Code availability

The spectral reduction was done with a self-written IDL script. The stellar model spectrum used for the stellar contamination impact was obtained with the Spectroscopy Made Easy tool, which is publicly available from The DE-MCMC routines were taken from EXOFAST, publicly available at


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F.B. acknowledges support from PLATO ASI-INAF agreement 2015-019-R.1-2018. This research has made use of the Spanish Virtual Observatory ( supported by the MINECO/FEDER through grant AyA2017-84089.7. T.K. acknowledges support by the NASA Exoplanet Research Program grant 80NSSC18K0569. D.S. acknowledges financial support from the State Agency for Research of the Spanish MCIU through the Center of Excellence Severo Ochoa award to the Instituto de Astrofísica de Andalucía (SEV-2017-0709). M.E.Y. acknowledges funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme under grant agreement 805445.

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Authors and Affiliations



F.B. carried out the data analysis. L.F. and T.K. performed the theoretical calculations. F.B., L.F. and T.K. contributed to the writing of the manuscript. All authors contributed to the interpretation of the data and the results.

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Correspondence to Francesco Borsa.

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The authors declare no competing interests.

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Peer review information Nature Astronomy thanks Matteo Brogi and Fei Yan for their contribution to the peer review of this work.

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Supplementary Information

Supplementary Figs. 1–6 and Table 1.

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Borsa, F., Fossati, L., Koskinen, T. et al. High-resolution detection of neutral oxygen and non-LTE effects in the atmosphere of KELT-9b. Nat Astron 6, 226–231 (2022).

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