  The transit spectrum of the exoplanet HD 189733b has recently been obtained
between 0.55 and 1.05 microns. 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 (lambda^-4). Assuming an extinction dominated by
Rayleigh scattering, we derive an atmospheric temperature of 1340+/-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+/-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 MgSiO3 as
a possible abundant condensate whose particle size must be between 0.01 and 0.1
microns. 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.
