Is the HR 8799 extrasolar system destined for planetary scattering?

TL;DR

This study investigates the dynamical stability of the HR 8799 exoplanet system, finding that most configurations are unstable over long timescales and suggesting a possible future scattering event, with some stable resonant orbits identified.

Contribution

The paper introduces a stability-constrained optimization method (GAMP) to identify stable orbital configurations of HR 8799, highlighting the likelihood of planetary scattering and the role of mean motion resonances.

Findings

Most stable configurations involve mean motion resonances.

80% of systems remain stable for 30 Myr but become unstable after 100 Myr.

Two-planet systems near 5:2 MMR are mostly stable.

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 HR 8799 system, because Keplerian fits to astrometric data are strongly unstable during ~0.2Myr. We search for stable, self-consistent N-body orbits with the so called GAMP 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-Jupiter-mass range, they may be stable only if the planets are involved in two- or three-body mean motion resonances (MMRs). We found that 80% systems found by GAMP that survived 30Myr backwards integrations, eventually become unstable after 100Myr. 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-fit systems are mostly stable, on quasi-circular orbits and close to the 5:2 MMR, resembling the Jupiter-Saturn pair.