  The processes of planet formation and migration depend intimately on the
interaction between planetesimals and the gaseous disks in which they form. The
formation of gaps in the disk can severely limit the mass of the planet and its
migration toward the protostar. We investigate the process of gap formation
through magnetohydrodynamic simulations in which internal stress arises
self-consistently from turbulence generated by the magnetorotational
instability. The simulations investigate three different planetary masses and
two disk temperatures to bracket the tidal (thermal) and viscous gap opening
conditions. The results are in general qualitative agreement with previous
simulations of gap formation, but show significant differences. In the presence
of MHD turbulence, the gaps produced are shallower and asymmetrically wider
than those produced with pure hydrodynamics. The rate of gap formation is also
slowed, with accretion occurring across the developing gap. Viscous
hydrodynamics does not adequately describe the evolution, however, because
planets capable of producing gaps also may be capable of affecting the level
MHD turbulence in different regions of the disk.
