Pluto near the edge of chaos

The long term orbital stability of the dwarf planet Pluto is owed to two special properties of its orbit that limit the location of its perihelion in azimuth and in latitude. The azimuthal constraint limits Pluto’s perihelion to ecliptic longitudes more than about 45 degrees removed from Neptune’s; it is understood as arising from Pluto’s libration in Neptune’s 3:2 mean motion resonance. The latitudinal constraint limits Pluto’s perihelion away from the ecliptic, to ecliptic latitudes in the range 12-18 degrees; it is owed to the libration of Pluto’s argument-of-perihelion, attributed (loosely) to the von-Zeipel-Lidov-Kozai effect in the spatial restricted three body problem. We revisit Pluto’s orbital dynamics with a view to elucidating the individual and collective gravitational effects of the giant planets on its perihelion location constraints. We demonstrate with numerical experiments that, while the resonant perturbations from Neptune readily account for the azimuthal constraint on Pluto’s perihelion location, the long term and steady persistence of the latitudinal libration cannot be realized for Pluto’s orbital parameters in the spatial restricted three body model of the Sun-Neptune-(massless)Pluto. This libration requires additional secular forcing of magnitude in a narrow range; Pluto-like orbits would be strongly chaotic beyond this range. We find that the particular orbital architecture of the giant planets fortuitously provides the additional secular forcing within that narrow “Goldilock’s zone”. We also find that Jupiter has a largely stabilizing influence whereas Uranus has a largely destabilizing influence on Pluto’s perihelion librations. Implications of these results for the population of Plutinos and for the orbital migration history and the origin of the orbital architecture of the giant planets will be investigated in future work.

This work was supported by NSF (Grant AST-1824869), the Marshall Foundation of Tucson AZ, and JSPS (Kakenhi Grant JP18K03730/2018-2021); numerical orbit propagations were carried out at the Center for Computational Astrophysics, National Astronomical Observatory of Japan.