Closed-formed ab initio solutions of geometric albedos and reflected light phase curves of exoplanets

TL;DR

This paper derives closed-form solutions for geometric and spherical albedos of exoplanets, enabling efficient and self-consistent inversion of reflected light phase curves to retrieve planetary physical parameters.

Contribution

It introduces novel closed-form solutions for geometric albedo and phase functions applicable to any reflection law depending on scattering angle, facilitating Bayesian inversion and analysis of exoplanet light curves.

Findings

Derived closed-form solutions for geometric and spherical albedos.

Applied methods to Kepler-7b, estimating key planetary parameters.

Enabled efficient Bayesian phase curve inversion.

Abstract

Studying the albedos of the planets and moons of the Solar System dates back at least a century. Of particular interest is the relationship between the albedo measured at superior conjunction, known as the ``geometric albedo", and the albedo considered over all orbital phase angles, known as the ``spherical albedo". Determining the relationship between the geometric and spherical albedos usually involves complex numerical calculations and closed-form solutions are restricted to simple reflection laws. Here we report the discovery of closed-form solutions for the geometric albedo and integral phase function, which apply to any law of reflection that only depends on the scattering angle. The shape of a reflected light phase curve, quantified by the integral phase function, and the secondary eclipse depth, quantified by the geometric albedo, may now be self-consistently inverted to retrieve globally averaged physical parameters. Fully Bayesian phase curve inversions for reflectance maps and simultaneous light curve detrending may now be performed due to the efficiency of computation. Demonstrating these innovations for the hot Jupiter Kepler-7b, we infer a geometric albedo of $0.25^{+0.01}_{-0.02}$, a phase integral of $1.77 \pm0.07$, a spherical albedo of $0.44^{+0.02}_{-0.03}$ and a scattering asymmetry factor of $0.07^{+0.12}_{-0.11}$.