  Context. Giant planets open gaps in their protoplanetary and subsequently
suffer so-called type II migration. Schematically, planets are thought to be
tightly locked within their surrounding disks, and forced to follow the viscous
advection of gas onto the central star. This fundamental principle however has
recently been questioned, as migrating planets were shown to decouple from the
gas' radial drift. Aims. In this framework, we question whether the
traditionally used linear scaling of migration rate of a giant planet with the
disk's viscosity still holds. Additionally, we assess the role of
orbit-crossing material as part of the decoupling mechanism. Methods. We have
performed 2D (r, {\theta}) numerical simulations of point-mass planets embedded
in locally isothermal {\alpha}-disks in steady-state accretion, with various
values of {\alpha}. Arbitrary planetary accretion rates were used as a means to
diminish or nullify orbit-crossing flows. Results. We confirm that the
migration rate of a gap-opening planet is indeed proportional to the disk's
viscosity, but is not equal to the gas drift speed in the unperturbed disk. We
show that the role of gap-crossing flows is in fact negligible. Conclusions.
From these observations, we propose a new paradigm for type II migration : a
giant planet feels a torque from the disk that promotes its migration, while
the gap profile relative to the planet is restored on a viscous timescale, thus
limiting the planet migration rate to be proportional to the disk's viscosity.
Hence, in disks with low viscosity in the planet region, type II migration
should still be very slow. Key words. protoplanetary disks; planet-disk
interactions; planets and satellites: formation
