A dwarf planet class object in the 21:5 resonance with Neptune

We report the discovery of a \(H_r = 3.4\pm0.1\) dwarf planet candidate by the Pan-STARRS Outer Solar System Survey. 2010 JO\(_{179}\) is red with \((g-r)=0.88 \pm 0.21\), 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--2016 provide an exceptionally well-determined orbit for 2010 JO\(_{179}\), with a semi-major axis of \(78.307\pm0.009\) 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 hundred-megayear time scales, 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.