Dusty disks as safe havens for terrestrial planets: Effect of the back-reaction of solid material on gas

Abstract

Previous studies have shown that there is considerable variation in the dust-to-gas density ratio in the vicinity of low-mass planets undergoing growth. This can lead to a significant change in the planetary momentum exerted by the gas and solid material. However, due to the low dust-to-gas mass ratio of protoplanetary disks, the back-reaction of the solid material, is often neglected. We study the effect of the back-reaction of solid material on the torques felt by low-mass planets. We performed locally isothermal, 2D hydrodynamic simulations of planet-disk interactions. Low-mass planets in the range of 0.1-10MEarth accrete only solid material. Simulations were compared with and without taking into account the back-reaction of the solid material on the gas. The solid component was assumed to have a fixed Stokes number in the range 0.01-10. In general, the inclusion of back-reaction results in a greater number of models with positive torque values compared to models that neglect back-reaction. It is clear, therefore, that the simulation of planetary growth and migration via hydrodynamic modeling requires the inclusion of solid-gas back-reaction. As a result of the back-reaction and accretion, a Mars-sized planetary embryo will experience positive total torques from the disk containing coupled solid components St<=0.01. Earth-mass planets also experience positive total torques from the disk containing boulder-sized solid components 2<=St<=5. The accretion of weakly coupled solid material tends to increase the positive torques and decrease the negative torques. Our results suggest that the combined effect of back-reaction and accretion is beneficial to the formation of planetary systems by reducing the likelihood of a young planet being engulfed by the central star.