Following the optical imaging of the exoplanet candidate Fomalhaut b (Fom b), we present a numerical model of how Fomalhauts 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 bs 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 planets 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 latters 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 bs 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.
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