The strongest case for demoting Pluto from a planet to a dwarf planet came with the 2005 discovery of Eris—an object roughly the same size as Pluto that orbits farther from the sun in an icy region beyond Neptune known as the Kuiper Belt. The discovery of more objects larger than asteroids but smaller than planets around the same time led astronomers to conclude that there must be hundreds of these Pluto-sized objects in the far reaches of the solar system.
Now we have found another one, currently designated 2015 RR245. Another resident of the Kuiper Belt, RR245's orbit takes it around the sun about once every 700 years. At its most distant point, RR245 is more than 120 times farther from the sun than the Earth is.
The little celestial body was first noticed in February 2016 by J.J. Kavelaars of the National Research Council of Canada while he was looking at September 2015 images taken by the Canada-France-Hawaii Telescope on Mauna Kea, a dormant volcano in Hawaii. This is not Kavelaar's first taste of discovery, as he is credited with a lead role in the discovery of eleven moons of Saturn, eight moons of Uranus, and four moons of Neptune. The telescope is being used as part of the Outer Solar System Origins Survey (OSSOS), an international program involving fifty scientists from around the world.
"The icy worlds beyond Neptune trace how the giant planets formed and then moved out from the Sun," says Michele Bannister, a postdoctoral fellow at the University of Victoria and a member of OSSOS, in a . "They let us piece together the history of our Solar System. But almost all of these icy worlds are painfully small and faint: it's really exciting to find one that's large and bright enough that we can study it in detail."
The new dwarf planet is estimated at roughly 700 kilometers in diameter, which would make it the smallest dwarf planet accepted by the International Astronomical Union (IAU). With the addition of RR245, there would be six official dwarf planets—five in the Kuiper Belt and Ceres in the asteroid belt between Mars and Jupiter.
However, more observations are needed to determine the exact size of RR245 and to refine models of its orbit. "It's either small and shiny, or large and dull," said Dr. Bannister. After these measurements are dialed in from observations planned for the next couple years, RR245 will really join its dwarf brethren by receiving an official name. The OSSOS team will be allowed to submit their preferred name for RR245 to the IAU for consideration.
The number of planets may be fixed at eight (or nine), but the number of dwarf planets is likely to continue growing.