It was announced last week that the Canada-France-Hawaii Telescope (CFHT) had discovered a new dwarf planet beyond the orbit of Neptune. The planet, provisionally named 2015 RR245 by the International Astronomical Union (IAU), has had its orbit measured over several months and from this it has been determined to have a highly elliptical orbit which brings it to within 34 AUs from the Sun, but takes it out as far as 120 AUs. By comparison, Neptune’s orbit is far closer to circular and at about 30 AUs (varying between 29.8 and 30.3 AUs).
From its current distance and brightness, its size has been estimated to be about 700km in diameter (Pluto, in comparison, has a diameter of 2374 km). This is based on an assumed albedo (reflectivity), so if it is more reflective than assumed it could be smaller, if it is less reflective it could be larger.
2015 RR245 was discovered as part of the Outer Solar System Origins Survey (OSSOS), and was spotted in February 2016 on an image taken in September 2015. We thus have images of it spanning nearly 10 months of its orbit, enough to get a decent idea of its orbital parameters.
As of yet, the IAU has not admitted 2015 RR245 to the select club of official dwarf planets. At the moment, there are only five dwarf planets in this ‘official list’; namely Pluto, Eris, Ceres (in the asteroid belt), Makemake and Haumea. Since the creation of the dwarf planet category in 2006, these five are the only objects which have been classified as such. Haumea was the latest object to be added to the list, in September 2008. We shall have to see whether 2015 RR245 makes the list at some point in the future.