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

Abstract

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, 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 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 the planetary orbits with respect to the binary orbit. Such an orbital 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 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. On the other two ends, the companion is either too close, and thus plays a violently disruptive role, or too wide to have a significant effect on planet formation. Future observations that can discover more planet-bearing binary star systems and constrain their masses and 3D orbital motions will test our numerical findings.