A method to detect H₂ in the atmosphere of transiting extrasolar planets using the EUV spectrum

¹ Laboratoire de Planétologie de Grenoble, CNRS-UJF, BP 53, 38041 Saint Martin d'Hères Cedex 9, France e-mail: mathieu.barthelemy@ujf-grenoble.fr
² Dept. of Atmospheric, Oceanic, and Space Sciences, University of Michigan, Ann Arbor, MI 48109, USA e-mail: theshire@umich.edu

Received: 23 June 2006
Accepted: 9 July 2007

Abstract

Aims. We present a method to detect the molecular hydrogen in extrasolar planetary atmospheres.

Methods.We model the coupling between H-Lyman lines and H₂ lines due to wavelength overlapping between these lines. We also explore the overlapping between other stellar EUV lines especially the C III line at 977.02 Å and planetary H₂ lines.

Results.If the spectrum of the planet is resolved, we show that H₂ modifies the intensity and shape of the H-Lyman lines emitted by the planet. However, if observed in absorption spectroscopy during a transit, the modification of the stellar lines in the Lyman series by the atmospheric H₂ is too low to be detectable with the current observing facilities. On the contrary, for HD 209458b with a 25 000 km thick H₂ layer, the intensity of the stellar C III line at 977.02 Å decreases by 2.64% during the transit. In the case of the newly discovered planet HD 189733b, the decrease of the intensity of the C III line reaches 3.78%.

Conclusions.Such decreases could be detectable with a FUSE observation of several transits thereby constituting a way to detect H₂ in exoplanetary atmospheres.