Transiting planets , such as HD 209458b , offer a unique opportunity to scrutinize the planetary atmospheric content .
Although molecular hydrogen is expected to be the main atmospheric constituent , H _ { 2 } remains uncovered because of the lack of strong transition from near-ultraviolet to near-infrared .
Here we analyse the absorption spectrum of HD 209458b obtained by Sing et al .
( 2008a ) which provides a measurement of the absorption depth in the 3000-6200 Ã Â wavelength range .
We show that the rise in absorption depth at short wavelengths can be interpreted as Rayleigh scattering within the atmosphere of HD 209458b .
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 \pm 5 mbar .
Using the variation of the Rayleigh scattering cross-section as a function of wavelength , we determine the temperature to be 2200 \pm 260 K at 33 mbar pressure .