2007A&A...474..301B

We present a method to detect the molecular hydrogen in extrasolar planetary atmospheres. 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 CIII line at 977.02Å and planetary H₂ lines. 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 HD209458b with a 25000km thick H₂ layer, the intensity of the stellar CIII line at 977.02Å decreases by 2.64% during the transit. In the case of the newly discovered planet HD189733b, the decrease of the intensity of the CIII line reaches 3.78%. Such decreases could be detectable with a FUSE observation of several transits thereby constituting a way to detect H₂ in exoplanetary atmospheres.