An extended hydrogen envelope of the extremely hot giant exoplanet KELT-9b

TL;DR

This study reports the first detection of an extended hydrogen atmosphere around the extremely hot exoplanet KELT-9b, revealing atmospheric escape driven by intense stellar radiation and providing insights into its atmospheric composition and dynamics.

Contribution

The paper presents the first observation of a hot hydrogen envelope around KELT-9b using Hα absorption, highlighting atmospheric escape in an ultra-hot giant exoplanet.

Findings

Detected significant Hα absorption indicating an extended hydrogen atmosphere.

Measured the effective radius at Hα line center to be 1.64 times the planetary radius.

Atmosphere likely undergoing dramatic escape near the Roche lobe.

Abstract

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. 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 (~4600 K at day-side) among all the exoplanets known so far. 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 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 H{\alpha} absorption which is mostly attributed to the planetary orbital motion. The obtained transmission spectrum has a significant line contrast (1.15% extra absorption at the H{\alpha} line centre). The observation implies that the effective radius at the H{\alpha} line centre is ~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 Rplanet) and is probably undergoing dramatic atmosphere escape.