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


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 hydrogen1,2,3. 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 (\(1.9{1}_{-0.26}^{+0.22}{R}_{{\rm{planet}}}\)) and is probably undergoing substantial escape of its atmosphere.

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Fig. 1: The Hα absorption spectra of KELT-9b at different orbital phases.
Fig. 2: The combined Hα absorption spectrum.
Fig. 3: The time-series equivalent width of the Hα absorption.

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This work is based on observations collected at the German–Spanish Astronomical Center, Calar Alto, jointly operated by the Max-Planck-Institut für Astronomie Heidelberg and the Instituto de Astrofísica de Andalucía (CSIC). We thank Calar Alto Observatory for allocation of director’s discretionary time to this programme. We thank L. Nortmann for help in observing preparations and M. Samland for discussions on Monte Carlo analysis. We thank K. Molaverdikhani and C. Huang for discussions on the atmosphere model.

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F.Y. planned the observations, performed the data analysis and wrote the manuscript. T.H. contributed to preparations for observations and to writing the manuscript.

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Correspondence to Fei Yan.

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Yan, F., Henning, T. An extended hydrogen envelope of the extremely hot giant exoplanet KELT-9b. Nat Astron 2, 714–718 (2018).

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