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Abstract

Giant exoplanets orbiting close to their host stars have high temperatures because of the immense amount of stellar irradiation that they receive. The extreme energy input leads to the expansion of the atmosphere and the escape of neutral hydrogen^1-3. An intriguing case among the hot giant planets is KELT-9b, an exoplanet orbiting very close to an early A-type star, with the highest temperature (around 4,600 K on the day-side) of any exoplanet known so far⁴. The atmospheric composition and dynamics of this planet have previously been unknown. Here we report the detection of an extended hot hydrogen atmosphere around KELT-9b. The detection was achieved by measuring the atomic hydrogen absorption during transit by observing the Balmer Hα line, which is unusually strong, mainly owing to the high level of extreme-ultraviolet radiation from the star. We detected a wavelength shift of the Hα absorption that is mostly attributed to the planetary orbital motion⁵. The obtained transmission spectrum has a noticeable line contrast (1.15% extra absorption at the Hα line centre). The observation implies that the effective radius at the Hα line centre is about 1.64 times the size of the planetary radius, indicating that the planet has an extended hydrogen envelope close to the size of the Roche lobe (1.9 1_-0.26^+0.22R_planet) and is probably undergoing substantial escape of its atmosphere.