  We investigate gap formation in gaseous protostellar disks by a planet in a
circular orbit in the limit of low disk viscosity. This regime may be
appropriate to an aging disk after the epoch of planet formation. We find that
the distance of planet to the gap outer boundary can be between the location of
the $m=2$ and $m=1$ outer Lindblad resonances. This distance is weakly
dependent upon both the planet's mass and disk viscosity. We find that the
evolution of the disk edge takes place on two timescales. The first timescale
is set by the spiral density waves driven by the planet. The second timescale
depends on the viscosity of the disk. The disk approaches a state where the
outward angular momentum flux caused by the disk viscosity is balanced by the
dissipation of spiral density waves which are driven at the Lindblad
resonances. This occurs inefficiently however because of the extremely low gas
density near the planet. We find that the distance between the planet and the
peak density at the disk outer edge is only weakly dependent on the viscosity
and planet mass, however the ratio of the gas density near the planet to that
in the disk (or the slope of density along the disk edge) is strongly dependent
upon both quantities. We find that the disk density profile along the edge
scales approximately with disk viscosity divided by the square of the planet
mass. We account for this behavior with a simple scenario in which the
dissipation of angular momentum from the spiral density waves is balanced
against diffusion in the steep edge of the disk.
