Migration has become a standard ingredient in planet formation theory. Newly born planets do not linger at their birth places: they start to change their semi-major axis rapidly and may end up in a totally different place in the protoplanetary disk. Migration has been invoked as the most probable explanation for the "Hot Jupiters": giant planets that are located too close to their central star to have been formed there. However, although migration theory based on simple, isothermal disk models indeed predicts inward migration of planets, it is far too effective: planets should rapidly disappear into the central star. New models of more realistic disk models show that the theory becomes much more complicated when the isothermal assumption is released, and that planets do not move inward in general.
