Formation of irregular and runaway moons/exomoons through moon-moon scattering

摘要

Gas giant planets in the Solar system host large satellite systems withmultiple regular and irregular moons. Regular moons revolve around their hostplanet in circular, low inclination short period orbits, and are thought toform in-situ through coagulation processes. In contrast, irregular moons havehighly inclined (and even retrograde), typically eccentric and long periodorbits around their host planet. Irregular moons are therefore often thought tobe unbound objects in helio-centric orbits that were later captured to theircurrent orbits around the planet. Here we study the a different possibility inwhich regular moons form in-situ, and dynamically evolve through mutualmoon-moon scattering. We find that such evolution can excite the satellitesinto high eccentricities and inclinations. We find that moons are eitherejected from the host planet to become runaway moons, or stay bound and becomeprograde orbiting irregular moons with inclined and eccentric orbits aroundtheir host planet. Ejected moons, unbound to the planet, can later betemporarily re-captured by the host planet even at retrograde orbits. Suchmoons are eventually re-ejected from the system or collide with the planet, atleast in the absence of dissipative processes (e.g. collisions with existingbound moons, a debris disk or through tidal interactions with the host planet),not currently modeled. Uncaptured runaway moons may eventually be ejected fromthe Solar system, or be captured into stable helio-centric orbits andcontribute to the populations of asteroidal or trans-Neptunian objects. Suchscenarios are potentially relevant both for the gas-giant satellites in theSolar system and for the dynamical evolution of exomoons.