  According to current theories, tidal interactions between a disk and an
embedded planet may lead to the rapid migration of the protoplanet on a
timescale shorter than the disk lifetime or estimated planetary formation
timescales. Therefore, planets can form only if there is a mechanism to hold at
least some of the cores back on their way in. Once a giant planet has
assembled, there also has to be a mechanism to prevent it from migrating down
to the disk center. This paper reviews the different mechanisms that have been
proposed to stop or slow down migration.
