An attempt to detect water in the atmosphere of the extrasolar planet HD 209458b using 
transit spectroscopy will be presented.  A radiative transfer model designed and built 
specifically for this project predicts, given a planetary temperature/pressure/composition 
profile, the dependence in wavelength of the stellar spectrum modulation due to a transiting 
planet.  A total of 352 spectra around 1.8 microns were obtained on four nights (three in 
transit) of observations using ISAAC at the Very Large Telescope.

Correlating the modeled modulation with the infrared spectra yields a non-detection of water 
in the atmosphere of HD 209458b.  However, a quantitative model of an improved observing 
strategy for future observations of this kind is presented as well as several original 
data-reduction techniques that were developed during this work.