A Dwarf Planet Class Object in the 21:5 Resonance with Neptune

We report the discovery of an H-r = 3.4 +/- 0.1 dwarf planet candidate by the Pan-STARRS Outer Solar System Survey. 2010 JO(179) is red with (g - r) = 0.88 +/- 0.21, roughly round, and slowly rotating, with a period of 30.6 hr. Estimates of its albedo imply a diameter of 600-900 km. Observations sampling the span between 2005 and 2016 provide an exceptionally well determined orbit for 2010 JO(179), with a semimajor axis of 78.307 +/- 0.009 au; distant orbits known to this precision are rare. We find that 2010 JO(179) librates securely within the 21:5 mean-motion resonance with Neptune on 100 Myr timescales, joining the small but growing set of known distant dwarf planets on metastable resonant orbits. These imply a substantial trans-Neptunian population that shifts between stability in high-order resonances, the detached population, and the eroding population of the scattering disk.