Search for an exosphere around 51 Pegasi B with ISO ?

The detection of absorption features in spectra of 51 Peg obtained during transit of its planet would provide a first clue on the physical nature of the planet. We have searched for signatures of an exosphere of the planet orbiting 51 Peg in the wavelength range from 2.39 μm to 3.5 μm and from 4.55 μm to 4.74 μm using SWS on ISO. Careful comparison of spectra taken during the transit of the planet with spectra obtained when the planet was not in the line-of-sight did not reveal any significant differences. We provide the S/N ratio and upper limits on the equivalent width of potential absorption lines in all wavelength bands. The upper limits on the equivalent width can be used for comparisons with predictions of the density of a potential exosphere of 51 Peg in future models.