The formation of planets in circumbinary disks

TL;DR

This study uses hydrodynamical simulations to explore how binary star systems influence planet formation, showing that the inner disk gap driven by the binary's gravity plays a key role in determining planetary orbits, consistent with observed Kepler systems.

Contribution

The paper demonstrates that the inner disk gap caused by binary stars explains the observed planetary orbits, providing a new understanding of circumbinary planet formation mechanisms.

Findings

Inner disk gap correlates with planetary orbit positions.

Simulated planetary eccentricities are lower than disk eccentricities.

Binary parameters influence planet semi-major axes, testable observationally.

Abstract

We examine the formation of planets around binary stars in light of the recently discovered systems Kepler 16, 34 and 35. We conduct hydrodynamical simulations of self gravitating disks around binary systems. The selected binary and disk parameters are chosen consistent with observed systems. The disks are evolved until they settle in a quasi-equilibrium and the resulting systems are compared with the parameters of Kepler 16, 34 and 35. We find a close correspondence of the peak density at the inner disk gap and the orbit of the observed planets. We conclude, based on our simulations, that the orbits of the observed Kepler planets are determined by the size of the inner disk gap which for these systems results from the binary driving. This mediates planet formation either through the density enhancement or through planetary trapping at the density gradient inversion in the inner disk. For all three systems the current eccentricity of the planetary orbit is less than the disk eccentricity in the simulations. This, together with the long term stability of the orbits argues against in situ formation (e.g. a direct collapse scenario of the material in the ring). Conducting additional simulations of systems with a wider range of parameters (taken from a survey of eclipsing binaries), we find that the planet semi-major axis and binary eccentricity in such a scenario should be tightly correlated providing an observational test of this formation mechanism.