Saving Super-Earths: Interplay between Pebble Accretion and Type I Migration

Overcoming type I migration and preventing low-mass planets from spiralling into the central star is a long-studied topic. It is well known that outward migration is possible in viscously heated disks relatively close to the central star because the entropy gradient can be sufficiently steep for the positive corotation torque to overcome the negative Lindblad torque. Yet efficiently trapping planets in this region remains elusive. Here we study disk conditions that yield outward migration for low-mass planets under specific planet migration prescriptions. In a steady-state disk model with a constant α-viscosity, outward migration is only possible when the negative temperature gradient exceeds ∼0.87. We derive an implicit relation for the highest mass at which outward migration is possible as a function of viscosity and disk scale height. We apply these criteria, using a simple power-law disk model, to planets that have reached their pebble isolation mass after an episode of rapid accretion. It is possible to trap planets with the pebble isolation mass farther than the inner edge of the disk provided that α _crit ≳ 0.004 for disks older than 1 Myr. In very young disks, the high temperature causes the planets to grow to masses exceeding the maximum for outward migration. As the disk evolves, these more massive planets often reach the central star, generally only toward the end of the disk lifetime. Saving super-Earths is therefore a delicate interplay between disk viscosity, the opacity profile, and the temperature gradient in the viscously heated inner disk.