OSSOS. IV. DISCOVERY of A DWARF PLANET CANDIDATE in the 9:2 RESONANCE with NEPTUNE

We report the discovery and orbit of a new dwarf planet candidate, 2015 RR₂₄₅, by the Outer Solar System Origins Survey (OSSOS). The orbit of 2015 RR₂₄₅ is eccentric (e = 0.586), with a semimajor axis near 82 au, yielding a perihelion distance of 34 au. 2015 RR₂₄₅ has g - r = 0.59 ± 0.11 and absolute magnitude H_r = 3.6 ± 0.1; for an assumed albedo of p_V = 12%, the object has a diameter of ∼670 km. Based on astrometric measurements from OSSOS and Pan-STARRS1, we find that 2015 RR₂₄₅ is securely trapped on ten-megayear timescales in the 9:2 mean-motion resonance with Neptune. It is the first trans-Neptunian object (TNO) identified in this resonance. On hundred-megayear timescales, particles in 2015 RR₂₄₅-like orbits depart and sometimes return to the resonance, indicating that 2015 RR₂₄₅ likely forms part of the long-lived metastable population of distant TNOs that drift between resonance sticking and actively scattering via gravitational encounters with Neptune. The discovery of a 9:2 TNO stresses the role of resonances in the long-term evolution of objects in the scattering disk and reinforces the view that distant resonances are heavily populated in the current solar system. This object further motivates detailed modeling of the transient sticking population.