Ultraviolet Studies of Extra-Solar Planets

Transmission spectroscopy is a powerful tool for probing atmospheres of extra-solar planets when transiting their parent star. The first detection of atmospheric species was NaI in HD209458b (nicknamed Osiris), thanks to the Hubble Space Telescope (HST) sensitivity reaching similar to 10(-4) in relative absorption depth. Still with HST, HI, OI and CII were discovered in the far UV. Despite recent controversy (Ben-Jaffel 2007), everyone now recognize that these observations must be interpreted by the evaporation of the so called "hot Jupiters" with an escape rate of the order of 10(10) g.s(-1) (Ben-Jaffel 2008; Vidal-Madjar et al. 2008). More recently H+ and H-2 via Rayleigh scattering have also been detected using UV spectroscopy. This underlines once more the potentialities of the UV-farUV spectral domain where a large number of diagnostic lines is available. The next steps will be the detection and characterization of additional species toward further extra-solar planets, hopefully down to Earth-like worlds which will require sensitivities in the similar to 10(-6) range. Fortunately, wide absorption bands of particular interest do exist in this spectral domain. Key parameters for their detection are the parent star spectral type, the density of the planet and the thickness of the atmosphere. From model simulations one can estimate the number of potential targets for a future UV space mission as a function of its collecting area.