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First Look

Beyond Neptune and Pluto, a new dwarf planet

An international team of astronomers has observed a distant dwarf planet, making a 700-year orbit around the sun.

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    A rendering of the orbit of RR245 (orange line).
    Alex Parker OSSOS team
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The newest addition to our solar system’s family of dwarf planets has been discovered by astronomers, orbiting the sun in the distant realms beyond Neptune.

Temporarily assigned the identifier 2015 RR245 until the team suggests a name, the planet is thought to be a mere 435 miles in diameter, but it follows a giant orbit that takes it 7.4 billion miles from the sun – in contrast to Earth's farthest point of 94.5 million miles.

"The icy worlds beyond Neptune trace how the giant planets formed and then moved out from the Sun,” said Michele Bannister of the University of Victoria in British Columbia, who is a postdoctoral fellow in the team who made the discovery. “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."

First to spot RR245 back in February was National Research Council of Canada's JJ Kavelaars, studying images from the previous year taken by the Canada-France-Hawaii Telescope (CFHT) on Mauna Kea, Hawaii, as part of the ongoing Outer Solar System Origins Survey (OSSOS).

Dwarf planets are something of a rarity, the vast majority having been destroyed or ejected from the solar system in the chaos that accompanied the giant planets' settling into their current orbits. Indeed, RR245 now brings the number of dwarf planets circling our sun to six, the others being Ceres, Pluto, Eris, Makemake, and Haumea.

Worlds journeying so far from the sun generally have exotic geology, with landscapes carved from an array of frozen materials, as illustrated by the recent flyby of Pluto by the New Horizons mission.

For OSSOS, it is unusual to detect a dwarf planet, RR245 being its first. By contrast, it has already identified more than 500 smaller trans-Neptunian objects.

"OSSOS was designed to map the orbital structure of the outer Solar System to decipher its history," said Brett Gladman of the University of British Columbia in Vancouver. "While not designed to efficiently detect dwarf planets, we're delighted to have found one on such an interesting orbit."

As Dr. Gladman adds, such discoveries are only possible because of the exceptional capabilities of CFHT, located as it is in one of the best optical observation locations on the planet and equipped with an enormous wide-field imager.

Yet RR245 may be one of the last major worlds to be uncovered until larger telescopes, such as the Large Synoptic Survey Telescope, come online in the 2020s.

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