  We study torques on migrating low-mass planets in locally isothermal discs.
Previous work on low-mass planets generally kept the planet on a fixed orbit,
after which the torque on the planet was measured. In addition to these static
torques, when the planet is allowed to migrate it experiences dynamical
torques, which are proportional to the migration rate and whose sign depends on
the background vortensity gradient. We show that in discs a few times more
massive than the Minimum Mass Solar Nebula, these dynamical torques can have a
profound impact on planet migration. Inward migration can be slowed down
significantly, and if static torques lead to outward migration, dynamical
torques can take over, taking the planet beyond zero-torque lines set by
saturation of the corotation torque in a runaway fashion. This means the region
in non-isothermal discs where outward migration is possible can be larger than
what would be concluded from static torques alone.
