Predictions for a planet just inside Fomalhaut's eccentric ring

We propose that the eccentricity and sharpness of the edge of Fomalhaut's disc are due to a planet just interior to the ring edge. The collision time-scale consistent with the disc opacity is long enough that spiral density waves cannot 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 disc as long as the collision time-scale exceeds the removal or ejection time-scale 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 time-scale 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.