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Planetary migration is one of the most serious problems to systematicallyunderstand the observations of exoplanets. We clarify that the theoreticallypredicted type II migration is too fast, as well as type I migration, bydeveloping detailed analytical arguments in which the timescale of type IImigration is compared with the disk lifetime. In the disk-dominated regime, thetype II migration timescale is characterized by a {\it local} viscous diffusiontimescale, while the disk lifetime characterized by a {\it global} diffusiontimescale that is much longer than the local one. Even in the planet-dominatedregime where the inertia of the planet mass reduces the migration speed, thetimescale is still shorter than the disk lifetime except in the final diskevolution stage where the total disk mass decays below the planet mass. Thissuggests that most giant planets plunge into the central stars within the disklifetime, and it contradicts the exoplanet observations that gas giants arepiled up at $r \ga 1$AU. We examine additional processes that may arise inprotoplanetary disks: dead zones, photoevaporation of gas, and gas flow acrossa gap formed by a type II migrator. Although they make the type II migrationtimescale closer to the disk lifetime, we show that none of them can act as aneffective barrier for rapid type II migration with the current knowledge ofthese processes. We point out that gas flow across a gap and the fraction ofthe flow accreted onto the planets are uncertain and they may have a potentialto solve the problem. Much more detailed investigation for each process may beneeded to explain the observed distribution of gas giants in extrasolarplanetary systems.

Do Giant Planets Survive Type II Migration?