DDA 2015 – On the in situ formation of Pluto’s small satellites

This is one of a series of notes taken during the 2015 meeting of the AAS Division on Dynamical Astronomy, 3-7 May, at CalTech. An index to this series (all the papers presented at the meeting) is here.

Man Yin Woo (University of Hong Kong)


The formation of Pluto’s small satellites – Styx, Nix, Keberos and Hydra remains a mystery. Their orbits are nearly circular (eccentricity $e = 0.0055$ or less) and near resonances and coplanar with respect to Charon. One scenario suggests that they all formed close to their current locations from a disk of debris, which was ejected from the Charon-forming impact. We test the validity of this scenario by performing N-body simulations with Pluto-Charon evolving tidally from an initial orbit at a few Pluto radii. The small satellites are modeled as test particles with initial orbital distances within the range of the current small satellites and damped to their coldest orbits by collisional damping. It is found that if Charon is formed from a debris disk and has low initial eccentricity, all test particles survive to the end of the tidal evolution, but there is no preference for resonances and the test particles’ final $e$ is typically > 0.01. If Charon is formed in the preferred intact capture scenario and has initial orbital eccentricity ~ 0.2, the outcome depends on the relative rate of tidal dissipation in Charon and Pluto, $A$. If $A$ is large and Charon’s orbit circularizes quickly, a significant fraction of the test particles survives outside resonances with $e \gtrsim 0.01$. Others are ejected by resonance or survive in resonance with very large $e$ (> 0.1). If $A$ is small and Charon’s orbit remains eccentric throughout most of the tidal evolution, most of the test particles are ejected. The test particles that survive have $e \gtrsim 0.01$, including some with $e \gt 0.1$. None of the above cases results in test particles with sufficiently low final $e$.

This work is supported in part by Hong Kong RGC grant HKU 7030/11P.


  • Pluto satellite system
    • 5 known
    • Charon dominant
    • all nearly coplanar
    • all nearly circular
    • all near MMR with Charon
    • Brozovic et al. 2015
  • Formation scenarios
    • forced resonant migration
      • Ward & Canup 2006
      • Nix & Hydra formed in same giant impact that formed Charon
      • ruled out by Lithwick & Wu
    • multi-resonance capture
      • unlikely (Cheng et al. 2014)
    • collisional capture of planetesimals
      • Lithwick & Wu 2008, Dos Santos et al. 2012
      • ruled out: capture time << collisional timescale, also Walsh & Levison 2015
    • in situ formation
      • Kenyon & Bromley 2014
      • giant impact produced debris ring
      • problem: outward tidal evolution of Charon
  • Solving the migration problem
    • forced eccentricity — Leung and Lee 2013
    • for $e_C = 0.24$, $e_f \sim 0.01$ to $0.02$ for test particles (small moons)
    • integrate two tidal models, constant $\Delta t$ and constant $Q$
    • For constant $\Delta t$, no preference for resonances and $e \gt 0.01$
  • Conclusion: it is unlikely that all the small satellites formed close to their current position

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