Comments on 'Type II migration strikes back - an old paradigm for planet migration in discs' by Scardoni et al.

In the conventional view of type II migration, a giant planet migrates inward in the viscous velocity of the accretion disc in the so-called disc-dominate case. Recent hydrodynamic simulations, however, showed that planets migrate with velocities much faster than the viscous one in massive discs. Such fast migration cannot be explained by the conventional picture. Scardoni et al. has recently argued this new picture. By carrying out similar hydrodynamic simulations, they found that the migration velocity slows down with time and eventually reaches the prediction by the conventional theory. They interpreted the fast migration as an initial transient one and concluded that the conventional type II migration is realized after the transient phase.We showthat themigration velocities obtained by Scardoni et al. are consistent with the previous simulations even in the transient phase that they proposed.We also find that the transient fast migration proposed by Scardoni et al. is well described by a new model of Kanagawa, Tanaka & Szuszkiewicz. The new model can appropriately describe significant inward migration during the initial transient phase that Scardoni et al. termed. Hence, we conclude that the time variation of the transient migration velocity is due to the changes of the orbital radius of the planet and its background surface density during the migration.