Is the HR 8799 extrasolar system destined for planetary scattering?

Abstract

Abstract The recent discovery of a three-planet extrasolar system of HR 8799 by Marois et al. is a breakthrough in the field of the direct imaging. This great achievement raises questions on the formation and dynamical stability of the system, because Keplerian fits to astrometric data disrupt during ∼0.2 Myr. We search for stable, self-consistent N-body orbits with the so-called GAMP (genetic algorithm with MEGNO penalty) method that incorporates stability constraints into the optimization algorithm. Our searches reveal only small regions of stable motions in the phase space of three-planet, coplanar configurations. Most likely, if the planetary masses are in 10 MJ range, they may be stable only if the planets are involved in two- or three-body mean motion resonances (MMRs). We found that 80 per cent systems found by GAMP that survived 30 Myr backwards integrations, eventually become unstable after 100 Myr. It could mean that the HR 8799 system undergo a phase of planet–planet scattering. We test a hypothesis that the less certain detection of the innermost object is due to a blending effect. In such a case, two-planet best-fitting systems are mostly stable, on quasi-circular orbits and close to the 5:2 MMR, resembling the Jupiter–Saturn pair.