Probing exoplanet clouds with optical phase curves

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.close-in giant | exoplanets | atmospheric characterization | clouds | optical phase curves P hase curves provide unique insight into the atmosphere of a planet, a fact well known and tested in solar system exploration (1-3). Disentangling the information encoded in a phase curve is a complex process however, and interpretations can be faced with degeneracies. The potential of phase curves to characterize exoplanet atmospheres, particularly in combination with other techniques, is tantalizing. Phase curves observed over all orbital phases (OPs) are available for a few close-in planets in the optical (passband central wavelengths λ < 0.8 μm) (4-15) and the infrared (1 μm ≤ λ ≤ 24 μm) (16-19). At infrared wavelengths the measured flux from hot planets is typically dominated by thermal emission. In the optical, both thermal emission and reflected starlight contribute, with the relative size of the contributions dependent on the measurement wavelength as well as on the temperature of the atmosphere and the occurrence of condensates (20-25).Kepler-7b (26) is one of the ∼1,000 planets discovered by the Kepler mission. Its inferred mass M p (= 0.44 M J ; J for Jupiter) and radius R p (= 1.61 R J ) result in an unusually low bulk density (0.14 g·cm −3 ) that is inconsistent with current models of giant planet interiors (27, 28). Kepler-7b orbits a quiet G-type star of effective temperature T ⋆ = 5,933 K every 4.89 d (orbital distance a = 0.062 astronomical units) (6, 7), and tidal forces have likely synchronized its orbit and spin motions. Taken together these set a planet equilibrium temperature T eq ≤ 1,935 K.Kepler photometry (0.4-0.9 μm) of the star-planet system has enabled the optical study of 10,14). The inferred geometric albedo, A g = 0. 25-0.38 (4, 6, 7, 10...

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