Orbital migration of giant planets induced by gravitationally unstable gaps: the effect of planet mass

TL;DR

This study uses 2D hydrodynamical simulations to explore how planet mass influences the gravitational stability of gaps in protoplanetary discs and how this affects the orbital migration of giant planets, revealing mechanisms for outward migration.

Contribution

It demonstrates that more massive planets open gravitationally unstable gaps, significantly affecting their migration paths, and highlights the role of unstable gaps in triggering rapid outward migration.

Findings

More massive planets open more unstable gaps.

Unstable gaps can trigger rapid outward type III migration.

Gap-opening alone does not guarantee in situ formation of wide-orbit giants.

Abstract

It has been suggested that long-period giant planets, such as HD 95086b and HR 8799bcde, may have formed through gravitational instability of protoplanetary discs. However, self-gravitating disc-satellite interaction can lead to the formation of a gravitationally unstable gap. Such an instability significantly affects the orbital migration of gap-opening perturbers in massive discs. We use 2D hydrodynamical simulations to examine the role of planet mass on the gravitational stability of gaps and its impact on orbital migration. We consider giant planets with planet-to-star mass ratio q=0.0003 to q=0.003, in a self-gravitating disc with disc-to-star mass ratio M_d/M_*=0.08, aspect ratio h=0.05, and Keplerian Toomre parameter Q = 1.5 at 2.5 times the planet's initial orbital radius. Fixed-orbit simulations show that all planet masses we consider open gravitationally unstable gaps, but the instability is stronger and develops sooner with increasing planet mass. The disc-on-planet torques typically become more positive with increasing planet mass. In freely-migrating simulations, we observe faster outward migration with increasing planet mass, but only for planet masses capable of opening unstable gaps early on. For q=0.0003, the planet undergoes rapid inward type III migration before it can open a gap. For q=0.0013 we find it is possible to balance the tendency for inward migration by the positive torques due to an unstable gap, but only for a few 10's of orbital periods. We find the unstable outer gap edge can trigger outward type III migration, sending the planet to twice its initial orbital radius on dynamical timescales. This triggered type III migration may be a way of bringing giant planets to large orbital distances. On the other hand, our results also imply gap-opening is not a sufficient condition for the in situ formation of giant planets on wide orbits through disc fragmentation.