Searching for helium in the exosphere of HD 209458b

Atmospheric models of the extrasolar, close-in giant planet HD 209458b predict strong absorption features from alkali metals (Seager Sasselov 2000; Brown 2001). This was confirmed by the discovery of NaI by HST observations (Charbonneau et al. 2002). In this study we focus on the search for the helium absorption feature at 10 830 Angstrom, also predicted to be among the strongest ones. Helium is a major component of the planet's exosphere, for which models are yet not as robust as atmosphere models. One full transit was observed with the VLT/ISAAC instrument. We do not report the detection of the HeI feature. The data set is strongly affected by instrumental fringing, at a level up to 5% in the extracted spectra. After filtering, a residual noise of the order of 0.2% remains. An upper limit of the HeI line was derived, which further constrains future models of the HD 209458b planet exosphere. This upper limit, in terms of the feature depth, is 0.5% at 3sigma for a 3 Angstrom bandwidth. Prospects are proposed to lower the detectability limit; the ultimate detectability limit with ISAAC in the absence of electronic fringing and in ideal atmospheric conditions could be as low as a line depth of 0.1% (3 Angstrom width, 3sigma).