Detection of oxygen and carbon in the hydrodynamically escaping atmosphere of the extrasolar planet HD 209458b

Four transits of the planet orbiting the star HD 209458 were observed with the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope. The wavelength domain (1180 - 1710 Angstrom) includes H I as well as C I, C II, C IV, N v, O I, S I, Si II, Si III, and Si IV lines. During the transits, absorptions are detected in H I, O I, and C II (5% +/- 2%, 13% +/- 4.5%, and 7.5% +/- 3.5%, respectively). No absorptions are detected for other lines. The 5% mean absorption over the whole H I Lyalpha line is consistent with the previous detection completed in 2003 at higher resolution (Vidal-Madjar et al.). The absorption depths in O I and C II show that oxygen and carbon are present in the extended upper atmosphere of HD 209458b ( nicknamed "Osiris"). These species must be carried out up to the Roche lobe and beyond, most likely in a state of hydrodynamic escape.