  Transiting planets, such as HD209458b, offer a unique opportunity to
scrutinize the planetary atmospheric content. Although molecular hydrogen is
expected to be the main atmospheric constituent, H2 remains uncovered because
of the lack of strong transition from near-ultraviolet to near-infrared. Here
we analyse the absorption spectrum of HD209458b obtained by Sing et al. (2008a)
which provides a measurement of the absorption depth in the 3000-6200 AA
wavelength range. We show that the rise in absorption depth at short
wavelengths can be interpreted as Rayleigh scattering within the atmosphere of
HD209458b. Since Rayleigh scattering traces the entire atmosphere, this
detection enables a direct determination of the pressure-altitude relationship,
which is required to determine the absolute fraction of other elements such as
sodium. At the zero altitude defined by the absorption depth of 1.453%, which
corresponds to a planetary radius of 0.1205 times the stellar radius, we find a
pressure of 33+/-5 mbar. Using the variation of the Rayleigh scattering
cross-section as a function of wavelength, we determine the temperature to be
2200+/-260 K at 33 mbar pressure.
