Planet Formation in Highly Inclined Binary Systems. II. Orbital Alignment or Anti-alignment and Planet Growth Boost in Intermediate Separation Binaries

TL;DR

This study explores how binary star systems with intermediate separations influence the late stages of terrestrial planet formation, revealing orbital alignment effects and a surprising boost in planet growth due to increased collision rates.

Contribution

It demonstrates that in intermediate separation binaries, orbital (anti-)alignment occurs and the binary companion can enhance planet growth by increasing collision frequency, a novel insight.

Findings

Orbital (anti-)alignment is caused by differential precession and self-damping.

Binary companions at 40-200 AU can promote planet growth.

Close or wide binaries have disruptive or negligible effects.

Abstract

Stars are commonly formed in binary systems, which provide a natural laboratory for studying planet formation in extreme conditions. In our first paper (Paper I) of a series Xie et al. (2011), we have shown that the intermediate stage - from planetesimals to planetary embryos/cores - of planet formation can proceed even in highly inclined binaries. Following Paper I, here we numerically study the late stage of terrestrial planet formation, i.e., from embryos to full planets, in binary systems of various orbital configurations. We identify an orbital alignment or anti-alignment effect; namely, although an inclined binary generally misaligns the planetary orbits with respect to the spin axis of the primary host star (i.e., causing large obliquity), it could align or anti-align the planetary orbits with respect to the binary orbit. Such an orbital (anti-)alignment effect is caused by the combination of orbital differential precession and self-damping, and it is mostly significant in cases of intermediate binary separations, i.e., a_B~40-200 AU for terrestrial planet formation around 1 AU from the primary stars. In such intermediate separation binaries, somewhat contrary to intuition, the binary companion can aid planet growth by having increased the rate of collisions, forming significantly more massive but fewer planets. In the other two ends, the companion is ether too close thus plays a violently disruptive role or too wide to have significant effect on planet formation. Future observations, which can discover more planet-bearing binary star systems and constrain their masses and 3-D orbital motions will test our numerical findings.