  Gap formation in a gas disk triggered by disk-planet tidal interaction is
considered. Density waves launched by the planet are assumed to be damped as a
result of their nonlinear evolution leading to shock formation and its
subsequent dissipation. As a consequence wave angular momentum is transferred
to the disk,leading to evolution of its surface density. Planetary migration is
an important ingredient of the theory; effects of the planet-induced surface
density perturbations on the migration speed are considered. A gap is assumed
to form when a stationary solution for the surface density profile is no longer
possible in the frame of reference migrating with the planet. An analytical
limit on the planetary mass necessary to open a gap in an inviscid disk is
derived. The critical mass turns out to be smaller than mass M_1 for which
planetary Hill's radius equals disk scaleheight by a factor of at least Q^{5/7}
(Q is the Toomre stability parameter) depending on the strength of the
migration feedback. In viscous disks the critical planetary mass could vary
from about 0.1M_1 to M_1, depending on the disk viscosity. This implies that a
gap could be formed by a planet with mass 1-10 times bigger than the Earth mass
depending on the disk aspect ratio, viscosity, and planet's location in the
nebula.
