Dwarf planet with eccentric orbit discovered in the Kuiper Belt

Laurel Kornfeld

CGI rendering of the orbit of 2015 RR245 (orange line). Objects as bright or brighter than RR245 are labeled. The Minor Planet Center describes the object as the 18th largest in the Kuiper Belt.

(Click to enlarge) Image Credit: Alex Parker OSSOS team.

Astronomers working on a study of the orbital structure and history of the outer Solar System announced their discovery of a new dwarf planet beyond Pluto in a 700-year eccentric solar orbit.

Using the Canada-France-Hawaii Telescope on Mauna Kea, scientists working on the Outer Solar System Origins Survey (OSSOS), an international collaboration of 50 scientists at various institutions worldwide, found the dwarf planet, estimated to be 435 miles (700 kilometers) in diameter in February of this year while reviewing images taken in September 2015.

J.J. Kavelaars of the National Research Council of Canada first saw the dwarf planet as a small dot moving against the field of background stars.

The dot moved so slowly that Kavelaars could tell it is about twice as far from the Sun as Neptune, said Michele Bannister of the University of Victoria in British Columbia and an OSSOS postdoctoral fellow.

Designated 2015 RR245, the new dwarf planet has a highly eccentric orbit that takes it more than 120 times the distance between the Earth and the Sun.

The OSSOS team, as the object’s discoverers, will be given the chance to submit their choice for its permanent name to the International Astronomical Union (IAU).

After having spent several centuries more than seven-and-a-half billion miles (12 billion kilometers) from the Sun, 2015 RR245 is now moving toward perihelion, about three billion miles (five billion kilometers) from the Sun, which it will reach in 2096.

Astronomers surmise that the new dwarf planet has been stable in its current orbit for approximately 100 million years.

Dwarf planets in the Kuiper Belt, now recognized as the third zone of the Solar System beyond the terrestrials and gas giants, are difficult to find because they are small, distant, and therefore dim.

Yet they are overwhelmingly larger than the majority of small, shapeless Kuiper Belt Objects (KBOs), which are approximately the size of asteroids, too small to be rounded by their own gravity.

OSSOS has discovered more than 500 KBOs, but 2015 RR245 is its largest discovery and its first dwarf planet.

Because the discovery is so recent, its orbit and size are still estimates. Further study will be required to refine its orbit, uncover its history, and learn about its surface properties and composition.

“It’s either small and shiny, or large and dull,” Bannister said. “The icy worlds beyond Neptune trace how the giant planets formed and then moved out from the Sun. They let us piece together the history of the 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.”

While small KBOs are icy, those large enough to be spherical and, therefore, dwarf planets are composed of more rock than ice, with icy shells covering rocky layers underneath.

During the chaos surrounding the outward migration of the gas giants in the Solar System’s early days, most dwarf planets were either destroyed or ejected from the Solar System entirely.

Of those that remain, the largest are Pluto, with a diameter of 1,473 miles (2,370 kilometers), and Eris, with a diameter of 1,443 miles (2,322 kilometers).

Trans-Neptunian Object (TNO) size/albedo/color comparison. Image Credit: Wikipedia CC

Long thought to be a dead rock, Pluto surprised scientists last year when the New Horizons mission revealed it to be a geologically active world with exotic terrains, layered hazes, and possibly a sub-surface ocean. These findings raised the possibility that other Kuiper Belt dwarf planets, including 2015 RR245, could be experiencing similar processes.

2015 RR245 is smaller than Ceres, the only dwarf planet not located in the Kuiper Belt, which orbits between Mars and Jupiter. Ceres has an equatorial diameter of 587 miles (945 kilometers). This makes the new discovery the smallest known dwarf planet.

“OSSOS was designed to map the orbital structure of the Solar System to decipher its history. While not designed to efficiently detect dwarf planets, we’re delighted to have found one on such an interesting orbit,” said Brett Gladman of the University of British Columbia at Vancouver. “OSSOS is only possible due to the exceptional observing capabilities of the Canada-France-Hawaii Telescope. CFHT is located at one of the best optical observing locations on Earth, is equipped with an enormous wide-field imager, and can quickly adapt its observing each night to new discoveries we make.”

More dwarf planets might be found when the Large Synoptic Survey Telescope (LSST), which will have a 3,200-megapixel camera, becomes operational in Chile during the mid-2020s.