  The star Fomalhaut hosts a narrow, eccentric debris disc, plus a highly
eccentric companion Fomalhaut b. It is often argued that Fomalhaut b cannot
have significant mass, otherwise it would quickly perturb the disc. We show
that material in internal mean-motion resonances with a massive, coplanar
Fomalhaut b would actually be long-term stable, and occupy orbits similar to
the observed debris. Furthermore, millimetre dust released in collisions
between resonant bodies could reproduce the width, shape and orientation of the
observed disc. We first re-examine the possible orbits of Fomalhaut b, assuming
that it moves under gravity alone. If Fomalhaut b orbits close to the disc
midplane then its orbit crosses the disc, and the two are apsidally aligned.
This alignment may hint at an ongoing dynamical interaction. Using the
observationally allowed orbits, we then model the interaction between a massive
Fomalhaut b and debris. Whilst most debris is unstable in such an extreme
configuration, we identify several resonant populations that remain stable for
the stellar lifetime, despite crossing the orbit of Fomalhaut b. This debris
occupies low-eccentricity orbits similar to the observed debris ring. These
resonant bodies would have a clumpy distribution, but dust released in
collisions between them would form a narrow, relatively smooth ring similar to
observations. We show that if Fomalhaut b has a mass between those of Earth and
Jupiter then, far from removing the observed debris, it could actually be
sculpting it through resonant interactions.
