  We describe 2D hydrodynamic simulations of the migration of low-mass planets
($\leq 30 M_{\oplus}$) in nearly laminar disks (viscosity parameter $\alpha <
10^{-3}$) over timescales of several thousand orbit periods. We consider disk
masses of 1, 2, and 5 times the minimum mass solar nebula, disk thickness
parameters of $H/r = 0.035$ and 0.05, and a variety of $\alpha$ values and
planet masses. Disk self-gravity is fully included. Previous analytic work has
suggested that Type I planet migration can be halted in disks of sufficiently
low turbulent viscosity, for $\alpha \sim 10^{-4}$. The halting is due to a
feedback effect of breaking density waves that results in a slight mass
redistribution and consequently an increased outward torque contribution. The
simulations confirm the existence of a critical mass ($M_{cr} \sim 10
M_{\oplus}$) beyond which migration halts in nearly laminar disks. For $\alpha
\ga 10^{-3}$, density feedback effects are washed out and Type I migration
persists. The critical masses are in good agreement with the analytic model of
Rafikov (2002). In addition, for $\alpha \la 10^{-4}$ steep density gradients
produce a vortex instability, resulting in a small time-varying eccentricity in
the planet's orbit and a slight outward migration. Migration in nearly laminar
disks may be sufficiently slow to reconcile the timescales of migration theory
with those of giant planet formation in the core accretion model.
