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The transit spectrum of the exoplanet HD 189733b has recently been obtainedbetween 0.55 and 1.05 microns. Here we present an analysis of this spectrum. Wedevelop first-order equations to interpret absorption spectra. In the case ofHD 189733b, we show that the observed slope of the absorption as a function ofwavelength is characteristic of extinction proportional to the inverse of thefourth power of the wavelength (lambda^-4). Assuming an extinction dominated byRayleigh scattering, we derive an atmospheric temperature of 1340+/-150 K. Ifmolecular hydrogen is responsible for the Rayleigh scattering, the atmosphericpressure at the planetary characteristic radius of 0.1564 stellar radius mustbe 410+/-30 mbar. However the preferred scenario is scattering by condensateparticles. Using the Mie approximation, we find that the particles must have alow value for the imaginary part of the refraction index. We identify MgSiO3 asa possible abundant condensate whose particle size must be between 0.01 and 0.1microns. For this condensate, assuming solar abundance, the pressure at 0.1564stellar radius is found to be between a few microbars and few millibars, andthe temperature is found to be in the range 1340-1540 K, and both depend on theparticle size.

Rayleigh scattering in the transit spectrum of HD 189733b