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A dead zone within a circumstellar disk is an area of low gas viscosity. Planetary migration, which has been much studied since extra-solar planets have been found such that they must have migrated, has proved "too efficient", i.e., models suggest planets in circumstellar disks, where they would be when they first formed, would too-often migrate into the star. Some models attempt to solve this dilemma by modeling the creation of dead zones within the disk, places where the causes of migration are absent. They are areas with low gas viscosity.