Effects of a planetesimal debris disc on stability scenarios for the extrasolar planetary system HR 8799

HR 8799 is a four-planet system which also hosts a debris disc. By numerically integrating the planets and a planetesimal disc, we find that interactions between an exterior planetesimal disc and the planets can influence the lifetime of the system. We first consider resonant planetary configurations that remained stable for at least 7 Myr sans debris disc. An exterior debris disc with only ∼1 per cent the mass of the outermost planet (approximately a Neptune mass) was sufficiently large to pull the system out of resonance after 2–6 Myr. Secondly, we consider configurations that are unstable in less than a few hundred thousand years. We find that these can be stabilized by a debris disc 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 disc.