Probing exoplanet clouds with optical phase curves

TL;DR

This study uses optical phase curves of Kepler-7b to infer cloud coverage, composition, and particle properties, providing insights into exoplanet cloud characterization through modeling and synthetic phase curves.

Contribution

It introduces a method to connect optical phase curve measurements with cloud properties, including composition and particle size, for exoplanets like Kepler-7b.

Findings

Clouds are composed of poorly absorbing, submicron particles like silicates and silica.

Optical phase curves can constrain cloud composition and particle scattering properties.

Estimated spherical albedo of Kepler-7b is between 0.4 and 0.5.

Abstract

Kepler-7b is to date the only exoplanet for which clouds have been inferred from the optical phase curve -- from visible-wavelength whole-disk brightness measurements as a function of orbital phase. Added to this, the fact that the phase curve appears dominated by reflected starlight makes this close-in giant planet a unique study case. Here we investigate the information on coverage and optical properties of the planet clouds contained in the measured phase curve. We generate cloud maps of Kepler-7b and use a multiple-scattering approach to create synthetic phase curves, thus connecting postulated clouds with measurements. We show that optical phase curves can help constrain the composition and size of the cloud particles. Indeed, model fitting for Kepler-7b requires poorly absorbing particles that scatter with low-to-moderate anisotropic efficiency, conclusions consistent with condensates of silicates, perovskite, and silica of submicron radii. We also show that we are limited in our ability to pin down the extent and location of the clouds. These considerations are relevant to the interpretation of optical phase curves with general circulation models. Finally, we estimate that the spherical albedo of Kepler-7b over the Kepler passband is in the range 0.4--0.5.