2001A&A...371..260M

Following recent attempts to detect the exosphere of the extra-solar planet 51 Pegb in the infrared (Coustenis et al., 1997, ASP Conf. Ser., 119, 101, 1998, ASP Conf. Ser., 134, 296; Rauer et al., 2000A&A...355..573R), we discuss here a search for optical spectroscopic signatures from a gaseous extended envelope (called exosphere) surrounding the planet HD 209458b. This planet has a demonstrated photometric transit (Charbonneau et al., 2000ApJ...529L..45C; Henry et al., 2000ApJ...529L..41H), thus offering an increased probability for the spectroscopic detection of such an envelope. Therefore it is the best known candidate for probing the exospheric composition of a giant planet, orbiting a Sun-like star at a short distance. The observations were performed with UVES at the VLT and cover most of the 328-669nm range. We did not detect HD 209458b's exosphere at a level of 1%, a value close to the predictions. We discuss here the first results obtained and their limitations, as well as future prospective.