Rayleigh scattering in the transit spectrum of HD 189733b

TL;DR

This paper analyzes the transit spectrum of exoplanet HD 189733b, attributing the observed spectral slope to Rayleigh scattering, and infers atmospheric properties including temperature, pressure, and particle composition.

Contribution

It introduces a first-order analytical approach to interpret exoplanet absorption spectra, specifically modeling Rayleigh scattering effects and identifying condensate particles like MgSiO3.

Findings

Spectral slope consistent with Rayleigh scattering proportional to lambda^-4

Derived atmospheric temperature around 1340 K with uncertainties

Identified condensate particles with specific size and composition

Abstract

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.