The curiously circular orbit of Kepler-16b

TL;DR

This paper investigates the formation and orbital characteristics of Kepler-16b, a circumbinary planet, using simulations to understand how its nearly circular orbit is maintained within a protoplanetary disc.

Contribution

The study provides the first high-resolution 3D simulations showing how disc interactions dampen Kepler-16b's eccentricity, indicating formation in a relatively massive disc.

Findings

Kepler-16b's eccentricity is damped by disc interactions.

The minimum gas surface density in the disc is estimated at ~10 g/cm^2.

Kepler-16b likely formed early and migrated inward before disc mass loss.

Abstract

The recent discovery of a number of circumbinary planets lends a new tool to astrophysicists seeking to understand how and where planet formation takes place. Of the increasingly numerous circumbinary systems, Kepler-16 is arguably the most dynamically interesting: it consists of a planet on an almost perfectly circular orbit (e = 0.0069) around a moderately eccentric binary (e = 0.16). We present high-resolution 3D smoothed-particle hydrodynamics simulations of a Kepler-16 analogue embedded in a circumbinary disc, and show that the planet's eccentricity is damped by its interaction with the protoplanetary disc. We use this to place a lower limit on the gas surface density in the real disc through which Kepler-16b migrated of \Sigma_min ~ 10 g cm^-2. This suggests that Kepler-16b, and other circumbinary planets, formed and migrated in relatively massive discs. We argue that secular evolution of circumbinary discs requires that these planets likely formed early on in the lifetime of the disc and migrated inwards before the disc lost a significant amount of its original mass.