2008A&A...481L..83L

The transit spectrum of the exoplanet HD189733b has recently been obtained between 0.55 and 1.05µm. Here we present an analysis of this spectrum. We develop first-order equations to interpret absorption spectra. In the case of HD189733b, 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 (∝λ^–4). Assuming an extinction dominated by Rayleigh scattering, we derive an atmospheric temperature of 1340±150K. 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±30mbar. 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 MgSiO₃ as a possible abundant condensate whose particle size must be between ∼10^–2 and ∼10^–1µm. 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 1340-1540K, and both depend on the particle size.