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^3,4,5,6, 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.

氧气是在系外行星大气中检测到的许多最丰富分子的组成部分，也是追踪行星形成方式和地点的关键成分¹。特别是，O i 777.4 nm 三重态被用于探测地球²上的气辉和极光以及恒星大气中的氧丰度^3,4,5,6，但之前尚未在系外行星大气中检测到。我们对已知最热的巨行星超热木星 KELT-9b ⁷的透射光谱中的中性氧 777.4 nm 三重线进行了明确的地面检测。使用新的非局部热力学平衡辐射传输计算计算的合成光谱⁸与假设局部热力学平衡计算的数据相比，该数据明显更好地匹配。这些非局部热力学平衡辐射传输计算表明质量损失率为 10 ⁸ –10 ⁹  kg s ^-1 ，超过了促进氧和铁逸出所需的 10 ⁷ –10 ⁸  kg s ^-1的下限从大气中。假设太阳氧丰度，非局域热力学平衡模型指向微湍流和宏观湍流加宽 3.0 ± 0.7 km s ^-1和 13 ± 5 km s ^-1，分别表示中层和高层大气中存在快风。目前和即将推出的高分辨率光谱仪将允许在其他系外行星中检测 777.4 nm O i三重态，当与考虑非局部热力学平衡效应的正向建模相结合时，它是限制系外行星大气关键特征的强大工具。