How to build Tatooine: reducing secular excitation in Kepler circumbinary planet formation

TL;DR

This paper demonstrates that the gravity of gaseous circumbinary disks induces precession that suppresses planetesimal eccentricities, enabling planet formation beyond 2-3 AU in Kepler circumbinary systems, contrary to previous expectations.

Contribution

It reveals that disk-induced precession reduces secular excitation, allowing planetesimal growth in regions previously thought inhospitable around binaries.

Findings

Precession driven by disk gravity suppresses planetesimal eccentricities.

Growth of 1-100 km objects is possible beyond 2-3 AU.

The boundary of accretion-friendly regions depends on disk properties.

Abstract

Circumbinary planetary systems recently discovered by Kepler represent an important testbed for planet formation theories. Planetesimal growth in disks around binaries has been expected to be inhibited interior to ~10 AU by secular excitation of high relative velocities between planetesimals, leading to their collisional destruction (rather than agglomeration). Here we show that gravity of the gaseous circumbinary disk in which planets form drives fast precession of both the planetesimal and binary orbits, resulting in strong suppression of planetesimal eccentricities beyond 2-3 AU and making possible growth of 1-100 km objects in this region. The precise location of the boundary of accretion-friendly region depends on the size of the inner disk cavity cleared by the binary torques and on the disk mass (even 0.01 M_Sun disk strongly suppresses planetesimal excitation), among other things. Precession of the orbit of the central binary, enhanced by the mass concentration naturally present at the inner edge of a circumbinary disk, plays key role in this suppression, which is a feature specific to the circumbinary planet formation.