  Nearly all known white dwarf planetary systems contain detectable rocky
debris in the stellar photosphere. A glaring exception is the young and still
evolving white dwarf WD J0914+1914, which instead harbours a giant planet and a
disc of pure gas. The stability boundaries of this disc and the future
prospects for this white dwarf to be polluted with rocks depend upon the mass
and orbit of the planet, which are only weakly constrained. Here we combine an
ensemble of plausible planet orbits and masses to determine where observers
should currently expect to find the outer boundary of the gas disc. We do so by
performing a sweep of the entire plausible phase space with short-term
numerical integrations. We also demonstrate that particle-star collisional
trajectories, which would lead to the (unseen) signature of rocky metal
pollution, occupy only a small fraction of the phase space, mostly limited to
particle eccentricities above 0.75. Our analysis reveals that a highly inflated
planet on a near-circular orbit is the type of planet which is most consistent
with the current observations.
