The origin and evolution of Transneptunian binaries

Student thesis: Doctoral ThesisDoctor of Philosophy


Transneptunian objects are icy bodies beyond the orbit of Neptune. A large fraction are found in binary pairs, up to ~30% in the Cold Classical population, which generally have components of similar size and colour on widely separated orbits. Understanding what mechanisms have lead to the formation of such a high binary fraction, and their orbital distributions, holds the potential to elucidate the processes of planetesimal formation and evolution in general. Previous work has shown that gravitational instability is an efficient mechanism for producing these transneptunian binaries, where planetesimals and binaries form directly from the gravitational collapse of a pebble cloud. Merging collisions and conservation of angular momentum naturally lead to the formation of similar mass binary objects, and strongly prefers the formation of binaries with low inclination prograde orbits. Furthermore, the components have identical composition (and therefore colour) as they have formed from the same cloud of material. In the first part of this thesis we independently confirm those results using the REBOUND N-body integrator to model a spherical, rotating cloud of particles, in the Hill frame, collapsing under its own gravity with inelastic merging collisions. A fast, accurate, rotating reference frame integrator was used and the robustness of collision detection in all simulations was tested. We perform a deep search for bound particles and conduct a detailed analysis of the properties of all systems formed during cloud collapse. These results are compared to the latest observed transneptunian binaries. We find that gravitational instability is an extremely efficient producer of bound planetesimal systems; it is common for a single cloud to produce several systems with a range of properties. The most massive binary produced by each cloud generally has similar size components (r2/r1 > ~ 0.5); on a wide (median abin ~ 0.2 RHill), low inclination prograde orbit (median ibin ~ 3.5°) with low to moderate eccentricity (ebin < ~ 0.5). Additional binaries are produced during gravitational collapse that display a range of mass ratios and orbital properties, including low system mass and high inclination orbits. These results are particularly relevant with the recent flyby of Arrokoth, a small Cold Classical contact binary for which gravitational instability is becoming the favoured formation mechanism. Assuming that binaries formed early and frequently in Solar System history, they would then have to survive all the processes that lead to the current architecture of the Solar System, most notably planetary migration. In the second part of this thesis we focus on the effects of the outward migration of Neptune on a surrounding disk of binary planetesimals. The REBOUND N-body package was used to set up a forced planetary migration simulation with an additional binary force between planetesimals. We analyse in detail the heliocentric evolution of the planets and planetesimals, and consider how the binary orbits themselves change as a result of the migration process. The evolution of the giant planets was compared to previous work to ensure that they followed suitable migration paths, and the chaotic nature of the planetary architecture was considered. We assessed the survival rate for binary planetesimals during planetary migration and considered which dynamical classes the planetesimals evolved into. For our chosen initial conditions we found that many binaries stayed near their starting locations; binaries that formed in situ in the Cold Classical region could remain there relatively untouched. Other binaries are moved from their formation region; up to ~2.5% of binaries that start interior to the Cold Classical region survive push-out of several AU by the sweeping 2:1 Neptune resonance to become Cold Classicals. Furthermore, we considered how the binary orbits evolved from their initial distribution. We found that during planetary migration the majority of binaries survived and their mutual binary orbits were not significantly perturbed from their primordial orbits. Binary orbit Kozai cycles were identified, and we found that this mechanism affected only widely separated Cold Classical binaries above a critical separation abin ≥ 0.05 RHill. This critical separation coincides with the empirical value commonly used in the literature to divide the observed transneptunian binaries into tight and wide groups with different properties. Tests were conducted that indicate that the onset of Kozai cycles in the simulations for wide binaries has a physical and not a numerical cause, which is independent of perturbations due to planetary migration. In general planetary migration may induce some changes in the binary orbit distribution, however, our results confirm that certain signatures of formation mechanism, such as the ratio of prograde to retrograde orbits, are not expected to change significantly.
Date of AwardJul 2020
Original languageEnglish
Awarding Institution
  • Queen's University Belfast
SponsorsNorthern Ireland Department for the Economy, Royal Astronomical Society & American Astronomical Society
SupervisorAlan Fitzsimmons (Supervisor), Wesley Fraser (Supervisor), Pedro Lacerda (Supervisor) & Stuart Sim (Supervisor)


  • Planetary Science
  • outer Solar System
  • Kuiper Belt
  • Binary Planetesimals
  • Planetesimal Formation
  • Planetary Migration

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