Planet formation in small separation binaries: not so excited after all

TL;DR

This paper shows that the gravity of a massive protoplanetary disk can significantly reduce eccentricity excitation in planetesimals within small separation binary systems, enabling planet formation despite previous challenges.

Contribution

It introduces the role of rapid apsidal precession caused by the disk's gravity as a mechanism to facilitate planetesimal growth in binary systems with small separations.

Findings

Disk gravity reduces planetesimal eccentricities by over an order of magnitude.

Fragmentation barrier is eliminated for planetesimals smaller than 10 km at 2.6 AU.

In-situ planet formation is possible in binary systems with massive disks.

Abstract

Existence of planets is binaries with relatively small separations (around 20 AU), such as \alpha Centauri or \gamma Cephei poses severe challenges to standard planet formation theories. The problem lies in the vigorous secular excitation of planetesimal eccentricities at separations of several AU, where some of the planets are found, by the massive, eccentric stellar companions. High relative velocities of planetesimals preclude their growth in mutual collisions for a wide range of sizes, from below 1 km up to several hundred km, resulting in fragmentation barrier to planet formation. Here we show that rapid apsidal precession of planetesimal orbits, caused by the gravity of the circumstellar protoplanetary disk, acts to strongly reduce eccentricity excitation, lowering planetesimal velocities by an order of magnitude or even more at 1 AU. By examining the details of planetesimal dynamics we demonstrate that this effect eliminates fragmentation barrier for in-situ growth of planetesimals as small as < 10 km even at separations as wide as 2.6 AU (semi-major axis of the giant planet in HD 196885), provided that the circumstellar protoplanetary disk is relatively massive, ~0.1 M_Sun.