Abstract

Low-mass planets are expected to migrate in the type-I regime. In the inviscid limit, the contrast between the vortensity trapped inside the planet’s corotating region and the background disk leads to a dynamical corotation torque, which is thought to slow down inward migration. We investigate the effect of radiative cooling on low-mass planet migration using inviscid radiation hydrodynamical simulations. We find that for intermediate cooling timescales (β~0.1–100), cooling induces a baroclinic forcing on material U-turning near the planet, resulting in vortensity growth in the corotating region. For longer cooling timescales, the disk buoyancy response has a similar effect. Both mechanisms in turn weaken the dynamical corotation torque, are active for a substantial radial extent of the disk (R∼0.1–50 au), and lead to significantly faster, sustained inward migration. In the innermost few au and for super-Earth-mass planets, which represent the bulk of the Kepler sample, traditional type-I migration gives way to nonlinear effects such as gap opening and turbulence induced by small-scale vortices, with radiative cooling playing a central role in determining the fate of the planet. We finally review the effects of radiation transport and discuss current challenges regarding the migration of super-Earths.