Circumbinary Chaos: Using pluto's newest moon to constrain the masses of nix and hydra

Andrew N. Youdin, Kaitlin M. Kratter, Scott J. Kenyon

Research output: Contribution to journalArticlepeer-review

45 Scopus citations


The Pluto system provides a unique local laboratory for the study of binaries with multiple low-mass companions. In this paper, we study the orbital stability of P4, the most recently discovered moon in the Pluto system. This newfound companion orbits near the plane of the Pluto-Charon (PC) binary, roughly halfway between the two minor moons Nix and Hydra. We use a suite of few body integrations to constrain the masses of Nix and Hydra, and the orbital parameters of P4. For the system to remain stable over the age of the solar system, the masses of Nix and Hydra likely do not exceed 5 × 10 16kg and 9 × 1016kg, respectively. These upper limits assume a fixed mass ratio between Nix and Hydra at the value implied by their median optical brightness. Our study finds that stability is more sensitive to their total mass and that a downward revision of Charon's eccentricity (from our adopted value of 0.0035) is unlikely to significantly affect our conclusions. Our upper limits are an order of magnitude below existing astrometric limits on the masses of Nix and Hydra. For a density at least that of ice, the albedos of Nix and Hydra would exceed 0.3. This constraint implies they are icy, as predicted by giant impact models. Even with these low masses, P4 only remains stable if its eccentricity e ≲ 0.02. The 5:1 commensurability with Charon is particularly unstable, combining stability constraints with the observed mean motion places the preferred orbit for P4 just exterior to the 5:1 resonance. These predictions will be tested when the New Horizons satellite visits Pluto. Based on the results for the PC system, we expect that circumbinary, multi-planet systems will be more widely spaced than their singleton counterparts. Further, circumbinary exoplanets close to the three-body stability boundary, such as those found by Kepler, are less likely to have other companions nearby.

Original languageEnglish (US)
Article number17
JournalAstrophysical Journal
Issue number1
StatePublished - Aug 10 2012


  • Kuiper Belt objects: individual (Pluto, Charon)
  • binaries: general
  • planets and satellites: dynamical evolution and stability
  • planets and satellites: formation
  • space vehicles

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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