  Following the optical imaging of the exoplanet candidate Fomalhaut b (Fom b),
we present a numerical model of how Fomalhaut's debris disk is gravitationally
shaped by an interior planet. The model is simple, adaptable to other debris
disks, and can be extended to accommodate multiple planets. If Fom b is the
dominant perturber of the belt, then to produce the observed disk morphology it
must have a mass < 3 Jupiter masses. If the belt and planet orbits are
apsidally aligned, our model predicts a planet mass of 0.5 Jupiter masses. The
inner edge of the debris disk at 133 AU lies at the periphery of Fom b's
chaotic zone, and the mean disk eccentricity of 0.11 is secularly forced by the
planet, supporting predictions made prior to the discovery of Fom b. However,
previous mass constraints based on disk morphology rely on several
oversimplifications. We explain why our constraint is more reliable. It is
based on a global model of the disk that is not restricted to the planet's
chaotic zone boundary. Moreover, we screen disk parent bodies for dynamical
stability over the system age of 100 Myr, and model them separately from their
dust grain progeny; the latter's orbits are strongly affected by radiation
pressure and their lifetimes are limited to 0.1 Myr by destructive grain-grain
collisions. The single planet model predicts that planet and disk orbits be
apsidally aligned. Fom b's nominal space velocity does not bear this out, but
the astrometric uncertainties may be large. If the apsidal misalignment is
real, our upper mass limit of 3 Jupiter masses still holds. The belt contains
at least 3 Earth masses of solids that are grinding down to dust. Such a large
mass in solids is consistent with Fom b having formed in situ.
