Orbital Stability of Moons around Giant Exoplanets and Free-Floaters in Planet-Planet Scattering
Résumé
Planet-planet scattering is the best current candidate for explaining the eccentric and inclined orbits of giant exoplanets. It is interesting to know if satellites can survive the violent dynamical history of extra-solar planetary systems. We have conducted a series of numerical simulations using a version of the Mercury symplectic integrator modified by J. Chambers to allow inclusion of moons bound to the giant planets. Through energy and momentum exchange between the perturbing planet and moons during close encounters, moons change orbit or become unbound, generally from outer to inner ones, depending on the geometry of close encounter. The smallest close encounter distance throughout the instability period, which typically ranges from orders of 0.001 to 0.01AU, places an upper bound on the semi-major axis of surviving moons. Whether a moon can survive as the perturber comes close depends strongly on the perturber mass. If a Neptune-mass perturber comes as close as 0.06HR, moons may still remain bound out to 0.35 Hill radii on a Saturn-mass host planet, whereas an equal-mass perturber is more likely to strip away all moons. If the less massive planet is ejected into interstellar space, the most distant moons that can survive have a much smaller semi-major axis. Moons that remain stable may enter orbital resonance with each other, providing a tidal source of heating. Simulated orbits of primordial moons lie partly within the detectable range of microlensing, which may be the best technique for determining the occurrence of exomoons.