  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.
