Multi-planet disc interactions in binary systems

TL;DR

This study explores how multi-planet systems in binary star environments evolve, focusing on disc-planet interactions, tilt oscillations, and the potential formation of eccentric and retrograde planets, using secular models and hydrodynamical simulations.

Contribution

It provides new insights into the dynamical evolution of multi-planet systems in misaligned binary systems, highlighting the role of disc mass and tilt in eccentricity and retrograde orbit formation.

Findings

Planets and disc undergo tilt oscillations and nodal precession.

Inner planet's eccentricity can grow significantly due to disc interactions.

Massive discs can induce retrograde orbits in inner planets.

Abstract

We investigate the evolution of a multi--planet--disc system orbiting one component of a binary star system. The planet--disc system is initially coplanar but misaligned to the binary orbital plane. The planets are assumed to be giants that open gaps in the disc. We first study the role of the disc in shaping the mutual evolution of the two planets using a secular model for low initial tilt. In general we find that the planets and the disc do not remain coplanar, in agreement with Lubow & Martin (2016) for the single planet case. Instead, the planets and the disc undergo tilt oscillations. A high mass disc between the two planets causes the planets and the disc to nodally precess at the same average rate but they are generally misaligned. The amplitude of the tilt oscillations between the planets is larger while the disc is present. We then consider higher initial tilts using hydrodynamical simulations and explore the possibility of the formation of eccentric Kozai-Lidov (KL) planets. We find that the inner planet's orbit undergoes eccentricity growth for a large range of disc masses and initial misalignments. For a low disc mass and large initial misalignment both planets and the disc can undergo KL oscillations. Furthermore, we find that sufficiently massive discs can cause the inner planet to increase its inclination beyond $90^{\circ}$ and therefore to orbit the binary in a retrograde fashion. The results have important implications for the explanation of very eccentric planets and retrograde planets observed in multi-planet systems.