  Magnetic fields are dragged in from the interstellar medium during the
gravitational collapse that forms star/disk systems. Consideration of mean
field magnetohydrodynamics (MHD) in these disks shows that magnetic effects
produce subkeplerian rotation curves and truncate the inner disk. This letter
explores the ramifications of these predicted disk properties for the migration
of extrasolar planets. Subkeplerian flow in gaseous disks drives a new
migration mechanism for embedded planets and modifies the gap opening processes
for larger planets. This subkeplerian migration mechanism dominates over Type I
migration for sufficiently small planets (m_P < 1 M_\earth) and/or close orbits
(r < 1 AU). Although the inclusion of subkeplerian torques shortens the total
migration time by only a moderate amount, the mass accreted by migrating
planetary cores is significantly reduced. Truncation of the inner disk edge
(for typical system parameters) naturally explains final planetary orbits with
periods P=4 days. Planets with shorter periods P=2 days can be explained by
migration during FU-Ori outbursts, when the mass accretion rate is high and the
disk edge moves inward. Finally, the midplane density is greatly increased at
the inner truncation point of the disk (the X-point); this enhancement, in
conjunction with continuing flow of gas and solids through the region, supports
the in situ formation of giant planets.
