Currently designated 2015 RR245, the giant ball of ice and rock lies nine billion kilometres away in the the most distant reaches of the solar system

A dwarf planet half the size of Britain has been found tumbling through space in the most distant reaches of the solar system.

The giant ball of rock and ice lies nine billion kilometres away on an orbit that swings far beyond the realm of Neptune, the most remote of the fully-fledged planets in our cosmic vicinity.



Astronomers first noticed the new world when it appeared as a bright spot moving slowly across a sequence of images taken in September 2015 by a telescope on Mauna Kea in Hawaii for the Outer Solar System Origins Survey (OSSOS).



“It was really remarkable to see how bright this object was,” said Michele Bannister, an astronomer on the team at the University of Victoria, Canada. “It’s far brighter than the objects we normally find.”



In a formal note released on Monday, the International Astronomical Union (IAU) designated the dwarf planet 2015 RR245. The name will be replaced when astronomers come up with a better one.



While discussions have begun about possible names for the object, Bannister said it was too early to share them. The scientists can propose a name only when the dwarf planet’s orbit has been observed for several years and its trajectory more clearly defined. The name will then be voted on by an IAU committee. “As long as the proposal is reasonable and a bit mythological, it’s generally fine,” Bannister said.



In an act of linguistic gymnastics, the IAU created the term “dwarf planet” in 2006 to describe heavenly bodies that it decided were not proper planets. Pluto became the first dwarf planet that year, when IAU members voted to demote it from full planetary status. A dwarf planet must circle the sun and be large enough to be rendered spherical by its own gravity.

Though five dwarf planets are recognised by the IAU, namely Ceres, Pluto, Haumea, Makemake and Eris, there many be many hundreds of bodies similar to 2015 RR245 on the fringes of the solar system beyond the orbit of Neptune.



The newly-found dwarf planet is on a highly elliptical 700-year orbit that comes as close as five billion kilometres to the sun before heading out twice as far as Neptune. Having spent hundreds of years more than 12 billion kilometres away, the body is swooping inwards for its closest approach in 2096.



Based on the makeup of other dwarf planets, 2015 RR245 is likely to have an exotic landscape covered with frozen water and nitrogen, perhaps some carbon monoxide, and what Bannister described as “hydrocarbon gunk.”



While OSSOS was not designed to discover dwarf planets, Brett Gladman, a member of the team at the University of British Columbia in Vancouver, said he was “delighted to have found one on such an interesting orbit.”



Dwarf planets and other bodies beyond Neptune lurk in the some most frigid regions of the solar system. Unlike the inner planets that are more warmed by the sun, the distant dwarf planets are cold enough at -220 celsius to preserve chemical ingredients that were present when the planets first formed.



“Pluto is the largest known trans-Neptunian object, and the recent images from the New Horizons mission that flew past it show that these worlds are extremely rich and complex,” said Pedro Lacerda of the Astrophysics Research Centre at Queen’s University Belfast. “They are the closest thing to a time capsule that transports us to the birth of the solar system.



“You can make an analogy with fossils, which tell us about creatures long gone,” he added. “2015 RR245 is much smaller than Pluto, about one third as wide, so it tells us things that Pluto cannot.”