  We use a multi-dimensional hydrodynamics code to study the gravitational
interaction between an embedded planet and a protoplanetary disk with emphasis
on the generation of vortensity (Potential Vorticity or PV) through a
Baroclinic Instability. We show that the generation of PV is very common and
effective in non-barotropic disks through the Baroclinic Instability,
especially within the coorbital region. Our results also complement previous
work that non-axisymmetric Rossby-Wave Instabilities (RWIs, Lovelace et al.
1999) are likely to develop at local minima of PV distribution that are
generated by the interaction between a planet and an inviscid barotropic disk.
The development of RWIs results in non-axisymmetric density blobs, which exert
stronger torques onto the planet when they move to the vicinity of the planet.
Hence, large amplitude oscillations are introduced to the time behavior of the
total torque acted on the planet by the disk. In current simulations, RWIs do
not change the overall picture of inward orbital migration but cause a
non-monotonic behavior to the migration speed. As a side effect, RWIs also
introduce interesting structures into the disk. These structures may help the
formation of Earth-like planets in the Habitable Zone or Hot Earths interior to
a close-in giant planet.
