A Dwarf Planet Beyond Neptune Is Bigger Than We Thought

Artist’s impression of the surface of the dwarf planet Makemake. ESO

In the last few years, we have started to realize that the dwarf planets beyond Neptune are just as complex and diverse as the "official" eight planets of the Solar System. Among the dwarf planets is the unnamed 2007 OR10, a small reddish object currently 13 billion kilometers (8 billion miles) from the Sun. And we've now got a better grasp on how big it is.

By combining optical observations from the Kepler observatory and infrared detections from the Herschel satellite, a team of astronomers was able to estimate both its size and its rotational period. The research is published in the Astrophysical Journal.

The team believes 2007 OR10 rotates on its axis roughly once every 45 hours (although a rotation every 22 hours cannot be ruled out), and by using this information they were able to revise the size estimate to 1,535 kilometers (955 miles) across. If the new measurement is confirmed, it makes 2007 OR10 the third largest trans-Neptunian object.

The new size, which is 250 kilometers higher than previous estimates, has also led the researchers to establish that the dwarf planet is actually darker, based on the fact that the same amount of light we observe is now reflected by a larger area. A bigger size also means higher gravity, indicating that the planet is retaining more complex chemicals, hence its dark reddish color.

"Our revised larger size for 2007 OR10 makes it increasingly likely the planet is covered in volatile ices of methane, carbon monoxide, and nitrogen, which would be easily lost to space by a smaller object," said András Pál, lead author of the study, in a statement.

"It's thrilling to tease out details like this about a distant, new world – especially since it has such an exceptionally dark and reddish surface for its size."

Kepler results put 2007 OR10 as the largest unnamed body in our Solar System and the third largest of the dwarf planets. Konkoly Observatory/András Pál, Hungarian Astronomical Association/Iván Éder, NASA/JHUAPL/SwR

The new size measurement is certainly interesting and should allow 2007 OR10 to be finally accepted as a dwarf planet, but some are cautious as it has larger uncertainties than any other confirmed dwarf planet. Makemake, another trans-Neptunian object, is only slightly smaller, so the size might change with future observations. 2007 OR10 is certainly the largest object in the Solar System without a name, though.

The honor of naming it belongs to the discoverers Meg Schwamb, Mike Brown, and David Rabinowitz. Mike Brown, known for being the discoverer of Eris, the object that cost Pluto its planetary status, said in a tweet that the naming will happen soon.

"The names of Pluto-sized bodies each tell a story about the characteristics of their respective objects. In the past, we haven't known enough about 2007 OR10 to give it a name that would do it justice," said Schwamb. "I think we're coming to a point where we can give 2007 OR10 its rightful name."


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