Aerosol properties in the atmospheres of extrasolar giant planets

Abstract

We use a model of aerosol microphysics to investigate the impact of high-altitude photochemical aerosols on the transmission spectra and atmospheric properties of close-in exoplanets, such as HD209458b and HD189733b. The results depend strongly on the temperature profiles in the middle and upper atmosphere that are poorly understood. Nevertheless, our model of HD189733b, based on the most recently inferred temperature profiles, produces an aerosol distribution that matches the observed transmission spectrum. We argue that the hotter temperature of HD209458b inhibits the production of high-altitude aerosols and leads to the appearance of a more clear atmosphere than on HD189733b. The aerosol distribution also depends on the particle composition, the photochemical production, and the atmospheric mixing. Due to degeneracies among these inputs, current data cannot constrain the aerosol properties in detail. Instead, our work highlights the role of different factors in controlling the aerosol distribution that will prove useful in understanding different observations, including those from future missions. For the atmospheric mixing efficiency suggested by general circulation models (GCMs) we find that aerosol particles are small ($\sim$nm) and probably spherical. We further conclude that composition based on complex hydrocarbons (soots) is the most likely candidate to survive the high temperatures in hot Jupiter atmospheres. Such particles would have a significant impact on the energy balance of HD189733b's atmosphere and should be incorporated in future studies of atmospheric structure. We also evaluate the contribution of external sources in the photochemical aerosol formation and find that their spectral signature is not consistent with observations.