The Migration of Gap-Opening Planets is not Locked to Viscous Disk Evolution

TL;DR

This paper challenges the traditional view that gap-opening planets migrate at the viscous disk rate, showing through numerical simulations that their migration rate varies with disk and planet parameters and is not locked to viscous evolution.

Contribution

It demonstrates that the migration rate of gap-opening planets is not fixed to viscous drift and depends on specific disk and planet properties, contrary to standard models.

Findings

Migration rate varies with disk mass and parameters.

In massive disks, migration saturates to a non-viscous rate.

In low-mass disks, migration decreases linearly with disk mass.

Abstract

Most standard descriptions of Type II migration state that massive, gap-opening planets must migrate at the viscous drift rate. This is based on the idea that the disk is separated into an inner and outer region and gas is considered unable to cross the gap. In fact, gas easily crosses the gap on horseshoe orbits, nullifying this necessary premise which would set the migration rate. In this work, it is demonstrated using highly accurate numerical calculations that the actual migration rate is dependent on disk and planet parameters, and can be significantly larger or smaller than the viscous drift rate. In the limiting case of a disk much more massive than the secondary, the migration rate saturates to a constant which is sensitive to disk parameters and is not necessarily of order viscous rate. In the opposite limit of a low-mass disk, the migration rate decreases linearly with disk mass. Steady-state solutions in the low disk mass limit show no pile-up outside the secondary's orbit, and no corresponding drainage of the inner disk.