Planet-disc evolution and the formation of Kozai-Lidov planets

TL;DR

This study uses hydrodynamical simulations to explore how initially coplanar planet-disc systems in misaligned binaries can evolve to produce eccentric, misaligned planets via Kozai-Lidov oscillations after disc dispersal.

Contribution

It demonstrates that massive, inclined discs can induce Kozai-Lidov oscillations in planets, expanding the range of initial conditions leading to eccentric, misaligned exoplanets.

Findings

Massive discs enable low-inclination systems to undergo KL oscillations.

Tilt oscillations are amplified by secular resonance.

Range of initial misalignments producing eccentric planets is broadened.

Abstract

With hydrodynamical simulations we determine the conditions under which an initially coplanar planet-disc system that orbits a member of a misaligned binary star evolves to form a planet that undergoes Kozai-Lidov (KL) oscillations once the disc disperses. These oscillations may explain the large orbital eccentricities, as well as the large misalignments with respect to the spin of the central star, observed for some exoplanets. The planet is assumed to be massive enough to open a gap in the disc. The planet's tilt relative to the binary orbital plane is subject to two types of oscillations. The first type, present at even small inclination angles relative to the binary orbital plane, is due to the interaction of the planet with the disc and binary companion and is amplified by a secular resonance. The second type of oscillation is the KL oscillation that operates on both the planet and disc at larger binary inclination angles. We find that for a sufficiently massive disc, even a relatively low inclination planet-disc system can force a planet to an inclination above the critical KL angle, as a consequence of the first type of tilt oscillation, allowing it to undergo the second type of oscillation. We conclude that the hydrodynamical evolution of a sufficiently massive and inclined disc in a binary system broadens the range of systems that form eccentric and misaligned giant planets to include a wide range of initial misalignment angles (20 to 160 degrees).