Planets in Gas Disks: Trapping, Accretion, and Migration
Planets form and grow inside their natal gas disks. Near the disk's inner edge, dust trapped in the pressure bump at the magnetospheric truncation may seed close-in super-Earths late, after the host star's metallicity is already set, potentially explaining their close-in orbits and their insensitivity to stellar metallicity. Farther out, embedded planets continue to exchange mass and angular momentum with the gas around them: in two student-led papers, we showed that accreting objects can reverse the direction of Type I migration, and that well-defined circumplanetary disks need planets massive relative to the disk's thickness, which may explain why few are detected.
Dust trapping in the global pressure bump induced by magnetospheric truncation offers a late formation pathway for close-in super-Earths — one that operates after the host star's metallicity is already established, potentially explaining their insensitivity to host metallicity. Dust grows and piles up efficiently when turbulence is weak, grains are sturdy, and the pebble supply is high (Paper I). Follow-up dust evolution simulations that include opacity, thermal feedback, and evaporation and condensation show that thermal feedback limits the pile-up of small grains, but runaway growth of large solids still retains substantial dust mass, supporting this pathway (Paper II).
Two-dimensional global hydrodynamic simulations of accreting planets and black holes show that the angular momentum deposited by accreted gas can overcome the gravitational torque, reversing migration from inward to outward in the cases studied, whereas standard Type I theory predicts inward migration. The simulations reach a global steady state for mass and angular momentum transport, and caution against applying standard migration prescriptions to accreting objects.
A parameter survey of the flow around embedded planets finds that "diskiness" increases smoothly with planetary mass and decreases with disk aspect ratio; to first order, the transition is set by the ratio of the Hill radius to the disk scale height. Classical rotationally supported circumplanetary disks (CPDs) appear only around significantly superthermal planets. Because the setup nearly maximizes CPD formation, this is a plausible necessary condition, which may help explain why CPDs are rarely detected, especially around wide-orbit planets.