Buoyancy torques prevent low-mass planets from stalling in low-turbulence radiative disks

TL;DR

This study uses 3D radiation hydrodynamics simulations to show that buoyancy torques in radiative protoplanetary disks cause rapid inward migration of low-mass planets, challenging previous assumptions about migration slowdown.

Contribution

It demonstrates that realistic radiation transport reduces buoyancy damping, leading to faster inward migration of low-mass planets in radiative disks.

Findings

Buoyancy response exerts a negative dynamical corotation torque.

Radiative cooling partially damps the buoyancy response.

Realistic radiation transport is crucial for accurate migration predictions.

Abstract

Low-mass planets migrating inwards in laminar protoplanetary disks (PPDs) experience a dynamical corotation torque, which is expected to slow down migration to a stall. However, baroclinic effects can reduce or even reverse this effect, leading to rapid inward migration. In the radiatively inefficient inner disk, one such mechanism is the buoyancy response of the disk to an embedded planet. Recent work has suggested that radiative cooling can quench this response, but for parameters that are not necessarily representative of the inner regions of PPDs. We perform global three dimensional inviscid radiation hydrodynamics simulations of planet-disk interaction to investigate the effect of radiative cooling on the buoyancy-driven torque in a more realistic disk model. We find that the buoyancy response exerts a negative dynamical corotation torque -- albeit partially damped due to radiative cooling -- resulting in sustained, rapid inward migration. Models that adopt a local cooling prescription significantly overestimate the impact of the buoyancy response, highlighting the importance of a realistic treatment of radiation transport that includes radiative diffusion. Our results suggest that low-mass planets should migrate inwards faster than has been previously expected in radiative disks, with implications for the formation and orbital distribution of super-Earths and sub-Neptunes at intermediate distances from their host stars, unless additional physical processes that can slow down migration are considered.