Induced Kozai Migration and Formation of Close-in Planets in Binaries

TL;DR

This paper explores how distant stellar companions can induce Kozai cycles in planetary systems, leading to high eccentricities and orbital decay of inner planets, offering an alternative pathway for forming close-in exoplanets.

Contribution

It demonstrates that secular perturbations from binary companions can cause Kozai migration, contributing to the formation of close-in planets in multi-star systems.

Findings

Binary companions induce large mutual inclinations in planetary orbits.

Kozai cycles can lead to high eccentricities and orbital decay of inner planets.

This mechanism provides an alternative formation channel for close-in planets.

Abstract

Many recent observational studies have concluded that planetary systems commonly exist in multiple-star systems. At least ~20%, and presumably a larger fraction of the known extrasolar planetary systems are associated with one or more stellar companions. These stellar companions normally exist at large distances from the planetary systems (typical projected binary separations are on the orders 100-10000AU) and are often faint (ranging from F to T spectral types). Yet, secular cyclic angular momentum exchange with these distant stellar companions can significantly alter the orbital configuration of the planets around the primaries. One of the most interesting and fairly common outcomes seen in numerical simulations is the opening of a large mutual inclination angle between the planetary orbits, forced by differential nodal precessions caused by the binary companion. The growth of the mutual inclination angle between planetary orbits induces additional large-amplitude eccentricity oscillations of the inner planet due to the quadrupole gravitational perturbation by the outer planet. This eccentricity oscillation may eventually result in the orbital decay of the inner planet through tidal friction, as previously proposed as Kozai migration or Kozai cycles with tidal friction (KCTF). This orbital decay mechanism induced by the binary perturbation and subsequent tidal dissipation may stand as an alternative formation channel for close-in extrasolar planets.