  We consider the minimum mass planet, as a function of radius, that is capable
of opening a gap in an $\alpha$-accretion disk. We estimate that a half Jupiter
mass planet can open a gap in a disk with accretion rate $\dot M \la 10^{-8}
M_\odot$/yr for viscosity parameter $\alpha =0.01$, and Solar mass and
luminosity. The minimum mass is approximately proportional to $\dot M^{0.48}
\alpha^{0.8} M_*^{0.42} L_*^{-0.08}$. This estimate can be used to rule out the
presence of massive planets in gapless accretion disks. We identify two radii
at which an inwardly migrating planet may become able to open a gap and so slow
its migration; the radius at which the heating from viscous dissipation is
similar to that from stellar radiation in a flared disk, and the radius at
which the disk has optical depth order 1 in a self-shadowed disk. If a gap
opening planet cuts off disk accretion allowing the formation of a central hole
or clearing in the disk then we would estimate that the clearing radius would
approximately be proportional to the stellar mass.
