  A small number of young stellar objects show signs of a halo-like structure
of optically thin dust. This halo or torus is located within a few AU of the
star, but its origin has not yet been understood. A dynamically excited cloud
of planetesimals colliding to eventually form dust could produce such a
structure. The cause of the dynamical excitation could be one or more planets.
This work investigates an inwardly migrating planet that is dynamically
scattering planetesimals as a possible cause for the observed structures. If
this mechanism is responsible, the observed halo-like structure could be used
to infer the existence of planets in these systems. We present analytical
estimates on the maximum inclination reached owing to dynamical interactions
between planetesimals and a migrating planet. A symplectic integrator is used
to simulate the effect of a migrating planet on a population of planetesimals.
It is found that an inwardly migrating planet is only able to scatter the
material it encounters to highly-inclined orbits if the material is on an
eccentric orbit. Such eccentric orbits can be the result of resonance trapping
and eccentricity pumping. Simulations show that for a certain range of
migration rates and planet masses, resonance capture combined with planetary
migration indeed causes the planetesimals to reach eccentric orbits and
subsequently get scattered to highly-inclined orbits. The size distribution of
the resulting dust is calculated determined to find the total mass and optical
depth, which are found to compare reasonably well with the observed structures.
Dynamical scattering of planetesimals caused by a planet migrating in, followed
by the grinding down of these planetesimals to dust grains, appears to be a
promising explanation for the inferred circumstellar dust clouds. Further study
is needed to see if the haloes can be used to infer the presence of planets.
