Giant exoplanets orbiting close to their host stars have high temperatures because of the immense stellar irradiation which they receive .
The extreme energy input leads to the expansion of the atmosphere and the escape of neutral hydrogen [ 1 ] [ 2 ] [ 3 ] .
A particularly 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 ( \sim 4600 K at day-side ) among all the exoplanets known so far [ 4 ] .
The atmospheric composition and dynamic of such a unique planet have been unknown .
Here we report the first detection of an extended hot hydrogen atmosphere around KELT-9b .
The detection was achieved by measuring the atomic hydrogen absorption during transit with the Balmer \mathrm { H _ { \alpha } } line , which is unusually strong mainly due to the high level of extreme-ultraviolet radiation from the star .
We detected a significant wavelength shift of the \mathrm { H _ { \alpha } } absorption which is mostly attributed to the planetary orbital motion [ 5 ] .
The obtained transmission spectrum has a significant line contrast ( 1.15 \% extra absorption at the \mathrm { H _ { \alpha } } line centre ) .
The observation implies that the effective radius at the \mathrm { H _ { \alpha } } line centre is \sim 1.64 times the size of the planetary radius , indicating the planet has a largely extended hydrogen envelope close to the size of the Roche lobe ( 1.91 _ { -0.26 } ^ { +0.22 } ~ { } R _ { \mathrm { planet } } ) and is probably undergoing dramatic atmosphere escape .