Draft version September 18, 2018 A Preprint typeset using LTEX style emulateapj v. 5/2/11 FORMATION OF IRREGULAR AND RUNAWAY MOONS/EXOMOONS THROUGH MOON-MOON SCATTERING Hagai B. Perets1 and Matthew J. Payne2 Draft version September 18, 2018 ABSTRACT Gas giant planets in the Solar system host large satellite systems with multiple regular and irregular moons. Regular moons revolve around their host planet in circular, low inclination short period orbits, and are thought to form in-situ through coagulation processes. In contrast, irregular moons have highly inclined (and even retrograde), typically eccentric and long period orbits around their host planet. Irregular moons are therefore often thought to have formed as unbound objects in helio- centric orbits that were later captured to their current orbits around the planet. However, such capture scenarios require fine tuned conditions and/or encounter difficulties in producing irregular moon populations around all gas giants. Here we study the possibility that regular moons form in-situ outside the currently observed regular-moon regime, and dynamically evolve through mutual moon-moon scattering (as well as by secular evolution due to perturbations by the Sun). We find that such evolution can excite the satellites into high eccentricities and inclinations. We find that moons are either ejected from the host planet to become runaway moons, or stay bound and become prograde orbiting irregular moons with inclined and eccentric orbits around their host planet. Ejected moons, unbound to the planet, can later be temporarily re-captured by the host planet even at retrograde orbits. Such moons are eventually re-ejected from the system or collide with the planet, at least in the absence of dissipative processes (e.g. collisions with existing bound moons, a debris disk or through tidal interactions with the host planet), not currently modeled. Uncaptured runaway moons may eventually be ejected from the Solar system, or be captured into stable helio-centric orbits and contribute to the populations of asteroidal or trans-Neptunian objects. Such scenarios are relevant both for the gas-giant satellites in the Solar system and for the dynamical evolution of exomoons. 1. INTRODUCTION planetary disk on the growing satellites. The first Satellites of the giant planets in the Solar system are stage lasts a few Myr, until the dissipation of the typically categorized into regular and irregular moons. gaseous circum-planetary disk, following the dissipa- Regular moons orbit their host planet in circular, low in- tion of the gaseous protoplanetary disk which fuels clination short period orbits; irregular moons have highly it. At the end of this stage large moons should have inclined (and even retrograde), typically eccentric and formed (Canup & Ward 2006) in addition to and (possi- have longer period orbits. Similar to planet formation, bly many) smaller moonlets. the formation of satellite systems around giant planets The second stage begins following the gas dissipation. in the Solar system (and in exoplanetary systems) is At this stage the evolution is driven primarily by grav- difficult to study theoretically. Regular satellites are itational interactions and collisions between the moons. thought to have formed through collisional growth of In comparison with the extensive study of the first stage smaller planetesimals in the circum-planetary disk, very of satellite-system formation in a gaseous disk, the dy- similar to planet formation in the coagulation model for namical relaxation through moon-moon scattering at the the terrestrial planets. Irregular satellites are typically later stage have scarcely been explored. Here we study arXiv:1407.2619v1 [astro-ph.EP] 9 Jul 2014 thought to have formed elsewhere in the Solar system (as this dynamical evolution using analytic techniques and asteroid/Kuiper-belt object like planetesimals), and only N-body simulations of moon-moon scattering around gi- later be captured to become moons. The various models ant planets, and suggest that it can play a major role in suggested have major difficuties in explaining the origin the build-up and sculpting of the architecture of satellite of the irregular moons (see Jewitt & Haghighipour 2007, systems, both in the Solar system as well as in (not yet for a review). More succesful recent capture models have detected) exomoon systems. been suggested; these involve planet-planet interactions, This paper is structured as follows. We first provide a and the existence of an additional third ice-giant planet brief overview of the various processes seen in our N-body in the Solar system that have been later ejected from it moon-moon scattering simulations, which are later dis- (Nesvorn´yet al. 2014). cussed in more detail. We introduce the N-body simula- An oversimplified but useful approach used to study tions used to study the satellite systems, and discuss the satellite formation is to divide it into two stages, based initial conditions following the dissipation of the gaseous on the importance of the effects of gas and the circum- circum-planetary disk, followed by a brief discussion of the stability of multi-moon systems. We then provide an analytic context to dynamical relaxation of multi-moon [email protected] 1 Technion - Israel Institute of Technology, Haifa 32000, Isarel systems. We describe several examples of multi-moon 2 Harvard-Smithsonian Center for Astrophysics, 60 Garden systems studied through N-body simulations and present St., Camridge, MA 02138, USA their evolution and outcomes, demonstrating the possi- 2 Perets & Payne ble outcomes. Finally we discuss the evolution of multi- the host planet. Some can interact with the other Solar moon system through scattering and their implications system planets and/or protoplanetary disk and may ob- for satellites systems as well as the populations of minor tain a stable helio-centric orbit. Others might strongly bodies in the Solar system (asteroids, trans-Neptunian interact with the planet and even be ejected from the objects, comets etc). Solar system. Runaway moons can also be re-captured 2. temporarily into orbits around the planet even into ret- OVERVIEW rograde orbits with respect to the orbit of the planet Dynamical evolution of a satellite system goes through around the Sun. Such re-captured moons can potentially various stages. We will first review these stages, and dissipate some of their kinetic energy through scattering then discuss them in more detail both analytically and and/or collisions with other moons orbiting the planet through examples from N-body simulations. and/or tidal interaction with the planet (i.e. some- Instability: Following the formation of the moons in what similar to planet capture from wide eccentric or- an extended circum-planetary disk, they mutually per- bits into close retrograde orbits in planet scattering sim- turb each other gravitationally. As long as a gaseous ulations; Nagasawa et al. 2008; Nagasawa & Ida 2011; disk or a planetesimal disk exist, such disks might dis- Beaug´e& Nesvorn´y2012), and thereby be re-captured sipate the perturbation and stabilize the system. After into permanent orbits . We conjecture that such dissipa- the disk is gone, the mutual perturbations may desta- tive processes could produce retrograde irregular moons bilize the systems on short timescales, if their masses (similar to the models envisioning capture of objects are large and/or the separations between them are small formed independently in the protoplanetary disk). Here enough (similar to the well studied planet-planet scat- we only model purely dynamical interactions and do not tering case;e.g. Rasio & Ford 1996). The perturbations include dissipative processes (beside direct collisions), can then grow and lead to orbit crossing between the var- which will be explored elsewhere. ious moons. Here we study only the evolution of satellite 3. system at the onset of dynamical instability after the N-BODY SIMULATIONS gaseous disk has been gone. Throughout this paper we discuss the results of several Scattering, collsional growth and production of N-body simulations (using the mercury code Chambers prograde irregular satellites: In an unstable system 1999) of satellite systems. In following papers we will the moons will scatter each other gravitationally, leading describe the statistical results of a large sample of simu- to orbital changes, exciting the eccentricity and inclina- lations through the study of a wide range of initial condi- tions of the satellites. When the relative velocities be- tions. The current paper focuses on presenting the basic tween the moons become larger than the escape velocity processes and outcomes of moon-moon scattering pro- from their surface, physical collisions between moons will cess, and only presents results from a small number of occur, which might alleviate further growth of the moons simulations to provide explicit examples. The results of and/or their possible disruption. Moons excited to high these specific simulations are corroborated by the much eccentricities may collide with the host planet. Alter- larger set of simulations (see Payne & Perets; paper I), natively, a moon can be scattered to a large distance for detailed discussion of the extended N-body simula- extending beyond the stable region around the planet tions). Table 1 provides the the initial conditions for (typically 1/3RH or even 2/3RH for prograde and these simulations . We have used a single Neptune like retrograde∼ orbits, respectively),∼ and then be ejected from planet (Neptune mass and separation from the Sun), or- the system to become a runaway moon. Through the bited by a large number of moons (10-50 massive moons, scattering process moons can migrate in the disk and ex- and in some cases up to 1000 massless particles corre- change positions to produce a mixed order, i.e. erasing sponding to low mass moons). In all simulation the out- the signature of the original moon ordering. ermost moon is placed at a distance Rout = 1/6RH Secular evolution and the Kozai-Lidov “bar- (i.e.