Search for spectroscopical signatures of transiting HD 209458b's exosphere

Following recent attempts to detect the exosphere of the extra-solar planet 51 Pegb in the infrared (Coustenis et al. 1997, 1998; Rauer et al. 2000a), we discuss here a search for optical spectroscopic signatures from a gaseo us extended envelope (called exosphere) surrounding the planet HD 209458b. This planet has a demonstrated photometric transit (Charbonneau et al. 2000 a; Henry et al. 2000), thus offering an increased probability for the spect roscopic detection of such an envelope. Therefore it is the best known cand idate for probing the exospheric composition of a giant planet, orbiting a Sun-like star at a short distance. The observations were performed with UVE S at the VLT and cover most of the 328-669 nm range. We did not detect HD 2 09458b's exosphere at a level of 1%, a value close to the predictions. We d iscuss here the first results obtained and their limitations, as well as fu ture prospective.