We report the discovery and orbit of a new dwarf planet candidate , 2015 RR _ { 245 } , by the Outer Solar System Origins Survey ( OSSOS ) .
2015 RR _ { 245 } ’ s orbit is eccentric ( e =0.586 ) , with a semi-major axis near 82 au , yielding a perihelion distance of 34 au .
2015 RR _ { 245 } has g - r = 0.59 \pm 0.11 and absolute magnitude H _ { r } = 3.6 \pm 0.1 ; for an assumed albedo of p _ { V } = 12 % the object has a diameter of \sim 670 km .
Based on astrometric measurements from OSSOS and Pan-STARRS1 , we find that 2015 RR _ { 245 } is securely trapped on ten-Myr timescales in the 9:2 mean-motion resonance with Neptune .
It is the first TNO identified in this resonance .
On hundred-Myr timescales , particles in 2015 RR _ { 245 } -like orbits depart and sometimes return to the resonance , indicating that 2015 RR _ { 245 } 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 modelling of the transient sticking population .