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

Abstract

Low-mass planets migrating inwards in laminar protoplanetary discs (PPDs) experience a dynamical corotation torque (DCT), 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 disc, one such mechanism is the buoyancy response of the disc 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 3D inviscid radiation hydrodynamics simulations of planet–disc interaction to investigate the effect of radiative cooling on the buoyancy-driven torque in a more realistic disc model. We find that the buoyancy response exerts a negative DCT – 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 discs, 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.