Retention of Long-Period Gas Giant Planets: Type II Migration Revisited

TL;DR

This paper revisits the mechanisms of type II migration of gas giant planets, showing that gas flow through gaps and disk surface density profiles can significantly influence planet retention at long orbital periods.

Contribution

It combines hydrodynamic simulations and analytic studies to demonstrate how gap gas flow and surface density profiles affect type II migration, challenging the traditional viscous evolution paradigm.

Findings

Gas continues to flow through depleted gaps, maintaining a quasi-steady state.

Surface density distribution influences the speed and direction of migration.

Migration can be stalled, allowing planets to remain at long orbital periods.

Abstract

During their formation, emerging protoplanets tidally interact with their natal disks. Proto-gas-giant planets, with Hills radius larger than the disk thickness, open gaps and quench gas flow in the vicinity of their orbits. It is usually assumed that their type II migration is coupled to the viscous evolution of the disk. Although this hypothesis provides an explanation for the origin of close-in planets, it also encounter predicament on the retention of long-period orbits for most gas giant planets. Moreover, numerical simulations indicate that planets migrations are not solely determined by the viscous diffusion of their natal disk. Here we carry out a series of hydrodynamic simulations combined with analytic studies to examine the transition between different paradigms of type II migration. We find a range of planetary mass for which gas continues to flow through a severely depleted gap so that the surface density distribution in the disk region beyond the gap is maintained in a quasi-steady state. The associated gap profile modifies the location of corotation \& Lindblad resonances. In the proximity of the planet's orbit, high-order Lindblad \& corotation torque are weakened by the gas depletion in the gap while low-order Lindblad torques near the gap walls preserves their magnitude. Consequently, the intrinsic surface density distribution of the disk determines delicately both pace and direction of planets' type II migration. We show that this effect might stall the inward migration of giant planets and preserve them in disk regions where the surface density is steep.