The transit spectrum of the exoplanet HD 189733b has recently been obtained between 0.55 and 1.05 \mu m. Here we present an analysis of this spectrum .
We develop first-order equations to interpret absorption spectra .
In the case of HD 189733b , we show that the observed slope of the absorption as a function of wavelength is characteristic of extinction proportional to the inverse of the fourth power of the wavelength ( \propto \lambda ^ { -4 } ) .
Assuming an extinction dominated by Rayleigh scattering , we derive an atmospheric temperature of 1340 \pm 150 K. If molecular hydrogen is responsible for the Rayleigh scattering , the atmospheric pressure at the planetary characteristic radius of 0.1564 stellar radius must be 410 \pm 30 mbar .
However the preferred scenario is scattering by condensate particles .
Using the Mie approximation , we find that the particles must have a low value for the imaginary part of the refraction index .
We identify MgSiO _ { 3 } as a possible abundant condensate whose particle size must be between \sim 10 ^ { -2 } and \sim 10 ^ { -1 } \mu m. For this condensate , assuming solar abundance , the pressure at 0.1564 stellar radius is found to be between a few microbars and few millibars , and the temperature is found to be in the range 1340-1540 K , and both depend on the particle size .