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

Mathieu Barthelemy (1) , Jean Lilensten (1) , C. Parkinson (2)

1 LPG - Laboratoire de Planétologie de Grenoble
2 AOSS - Department of Atmospheric, Oceanic, and Space Sciences [Ann Arbor]

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.

Domaines: Planète et Univers [physics]; Astrophysique [astro-ph]

Mathieu Barthelemy, Jean Lilensten, C. Parkinson. A method to detect H2 in the atmosphere of transiting extrasolar planets using the EUV spectrum. Astronomy and Astrophysics - A&A, 2007, 474 (1), pp.301-306. ⟨10.1051/0004-6361:20065891⟩. ⟨insu-00359264⟩