  HR 8799 is a four planet system that also hosts a debris disk. By numerically
integrating both planets and a planetesimal disk, we find interactions between
an exterior planetesimal disk and the planets can influence the lifetime of the
system. We first consider resonant planetary configurations that remained
stable for at least 7 Myrs sans debris disk. An exterior debris disk with only
1 per cent the mass of the outermost planet (approximately a Neptune mass) was
sufficiently large enough to pull the system out of resonance after 2 to 6
Myrs. Secondly, we consider configurations which are unstable in less than a
few hundred thousand years. We find that these can be stabilized by a debris
disk with a mass of more than 10 per cent that of the outermost planet. Our two
sets of simulations suggest that estimates of the long term stability of a
planetary system should take into account the role of the debris disk.
