Dust Settling and Rapid Planetary Migration

TL;DR

This paper analyzes how dust settling in protoplanetary discs influences planetary migration rates, showing it accelerates type I migration for low-mass planets but slows type II migration for massive planets, highlighting the need for additional slowing mechanisms.

Contribution

It provides an analytical study of dust settling effects on planetary migration, revealing increased migration speed for low-mass planets and decreased gap-opening mass and migration for massive planets.

Findings

Dust settling speeds up type I migration by up to a factor of 2.

Dust settling reduces the gap-opening mass by a factor of 2.

Dust settling slows down type II migration by a factor of 2.

Abstract

Planetary migration is essential to explain the observed mass-period relation for exoplanets. Without some stopping mechanism, the tidal, resonant interaction between planets and their gaseous disc generally causes the planets to migrate inward so efficiently that they plunge into the host star within the gaseous disc lifetime ($\sim $ 1-3 Myrs). We investigate planetary migration by analytically calculating the migration rate and time within self-consistently computed, radiatively heated discs around M stars in which the effects of dust settling are included. We show that dust settling lowers the disc temperature and raises the gas density in the mid-plane. This inescapable evolution of disc structure speeds up type I planetary migration for lower mass bodies by up to a factor of about 2. We also examine the effects of dust settling on the gap-opening mass and type II migration, and find that the gap-opening mass is reduced by a factor of 2 and type II migration becomes slower by a factor of 2. While dust settling can somewhat alleviate the problem of planetary migration for more massive planets, the more rapid migration of low mass planets and planetary cores requires a robust slowing mechanism.