Diffusive migration of low-mass protoplanets in turbulent disks

Torque fluctuations due to magnetorotational turbulence in protoplanetary disks may greatly influence the migration patterns and survival probabilities of nascent planets. Provided that the turbulence is a stationary stochastic process with finite amplitude and correlation time, the resulting diffusive migration can be described with a Fokker-Planck equation, which we reduce to an advection-diffusion equation. We calibrate the coefficients with existing turbulent-disk simulations and mean-migration estimates and solve the equation both analytically and numerically. Diffusion tends to dominate over advection for planets of low mass and those in the outer regions of protoplanetary disks, whether they are described by the minimum mass solar nebula (MMSN) or by T Tauri alpha disks. Diffusion systematically reduces the lifetime of most planets, yet it allows a declining fraction of them to survive for extended periods of time at large radii. Mean planet lifetimes can even be formally infinite ( e. g., in an infinite steady MMSN), although median lifetimes are always finite. Surviving planets may linger near specific radii where the combined effects of advection and diffusion are minimized or at large radii, depending on model specifics. The stochastic nature of migration in turbulent disks challenges deterministic planet formation scenarios and suggests instead that a wide variety of planetary outcomes are possible from similar initial conditions. This would contribute to the diversity of (extrasolar) planetary systems.