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 atmospheres^3–6, 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.