Absolute densities in exoplanetary systems: photodynamical modelling of Kepler-138

TL;DR

This study uses photodynamical modelling of Kepler-138 to precisely determine planetary densities and interiors, revealing significant volatile layers and composition differences among the planets.

Contribution

First application of self-consistent photodynamical modelling to Kepler-138, improving density estimates and interior characterization of exoplanets.

Findings

Planet densities are twice as precise as previous estimates.

Kepler-138b has a size between Mars and Earth with a thick volatile layer.

Kepler-138c is likely a purely rocky planet.

Abstract

In favourable conditions, the density of transiting planets in multiple systems can be determined from photometry data alone. Dynamical information can be extracted from light curves, providing modelling is done self-consistently, i.e. using a photodynamical model, which simulates the individual photometric observations instead of the more generally used transit times. We apply this methodology to the Kepler-138 planetary system. The derived planetary bulk densities are a factor of 2 more precise than previous determinations, and we find a discrepancy in the stellar bulk density with respect to a previous study. This leads, in turn, to a discrepancy in the determination of masses and radii of the star and the planets. In particular, we find that interior planet, Kepler-138b, has a size in between Mars and the Earth. Given our mass and density estimates, we characterize the planetary interiors using a generalized Bayesian inference model. This model allows us to quantify for interior degeneracy and calculate confidence regions of interior parameters such as thicknesses of the core, the mantle, and ocean and gas layers. We find that Kepler-138b and Kepler-138d have significantly thick volatile layers and that the gas layer of Kepler-138b is likely enriched. On the other hand, Kepler-138c can be purely rocky.