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

Aims. We present a method to detect the molecular hydrogen in extrasolar planetary atmospheres. Methods. We model the coupling between H- Lyman lines and H-2 lines due to wavelength overlapping between these lines. We also explore the overlapping between other stellar EUV lines especially the CIII line at 977.02 angstrom and planetary H-2 lines. Results. If the spectrum of the planet is resolved, we show that H-2 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 H-2 is too low to be detectable with the current observing facilities. On the contrary, for HD209458b with a 25 000 km thick H-2 layer, the intensity of the stellar C III line at 977.02 angstrom decreases by 2.64% during the transit. In the case of the newly discovered planet HD189733b, 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 H-2 in exoplanetary atmospheres.