We propose that the eccentricity and sharpness of the edge of Fomalhaut ’ s disk are due to a planet just interior to the ring edge .
The collision timescale consistent with the disk opacity is long enough that spiral density waves can not be driven near the planet .
The ring edge is likely to be located at the boundary of a chaotic zone in the corotation region of the planet .
We find that this zone can open a gap in a particle disk as long as the collision timescale exceeds the removal or ejection timescale in the zone .
We use the slope measured from the ring edge surface brightness profile to place an upper limit on the planet mass .
The removal timescale in the chaotic zone is used to estimate a lower limit .
The ring edge has eccentricity caused by secular perturbations from the planet .
These arguments imply that the planet has a mass between that of Neptune and that of Saturn , a semi-major axis of approximately 119 AU and longitude of periastron and eccentricity , 0.1 , the same as that of the ring edge .