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

We report the discovery of a $H_r=3.4\pm 0.1$ dwarf planet candidate by the Pan-STARRS Outer Solar System Survey. 2010 JO${}_{179}$ is red with $(g-r)=0.88 \pm 0.21$,roughlyround,andslowlyrotating,withaperiodof$30.6$ 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 $78.307\pm 0.009$ 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 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.