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The signature of hot hydrogen in the atmosphere of the extrasolar planet HD 209458b


About ten per cent of the known extrasolar planets are gas giants that orbit very close to their parent stars. The atmospheres of these ‘hot Jupiters’ are heated by the immense stellar irradiation1,2,3,4,5. In the case of the planet HD 209458b, this energy deposition results in a hydrodynamic state in the upper atmosphere, allowing for sizeable expansion and escape of neutral hydrogen gas2,3,4,5,6. HD 209458b was the first extrasolar planet discovered that transits in front of its parent star7. The size of the planet can be measured using the total optical obscuration of the stellar disk during an observed transit, and the structure and composition of the planetary atmosphere can be studied using additional planetary absorption signatures in the stellar spectrum. Here we report the detection of absorption by hot hydrogen in the atmosphere of HD 209458b. Previously, the lower atmosphere and the full extended upper atmosphere of HD 209458b have been observed2,6,8, whereas here we probe a layer where the escaping gas forms in the upper atmosphere of HD 209458b.

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Figure 1: Photometric transit light curves of HD 209458.
Figure 2: Transit flux ratio spectrum.


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We thank J. Holberg for discussions, as well as R. Yelle for the atmospheric model of HD 209458b. Support for this work was provided by NASA through a grant from the Space Telescope Science Institute which is operated by AURA, under NASA contract.

Author Contributions The work required for this paper was equally shared between D.K.S. and G.E.B. F.H. suggested the Balmer jump spectral identification, and joined discussions.

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Correspondence to Gilda E. Ballester.

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Reprints and permissions information is available at The authors declare no competing financial interests.

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Supplementary Methods

This file contains Supplementary Methods with details about data reduction, derivation of the photometric light curves, correction to systematic instrumental effects, corrections to the transit spectra accounting for the stellar limb-darkening; calculation of the Balmer jump absorption from hot (n=2) hydrogen based on a published model of HD209458b's upper atmosphere, Supplementary Figures 1-3 with Legends. (PDF 262 kb)

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Ballester, G., Sing, D. & Herbert, F. The signature of hot hydrogen in the atmosphere of the extrasolar planet HD 209458b. Nature 445, 511–514 (2007).

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