In Brief
Revised estimates of the size of a dwarf planet show that it’s actually the third-largest dwarf planet in the outer Solar System. It’s about two-thirds the size of Pluto, and completes a rotation once every 45 hours.

Size Matters

Sometimes astronomers make mistakes. It happens.

Case in point, a team of astronomers have learned that the size of an object in the far reaches of the outer Solar System, called 2007 OR10, was significantly underestimated—by about 250 km (155 mi).

Of course, they can be forgiven for such mistakes. First, this is how science works. Second, objects in these dim depths of the Solar System are notoriously difficult to measure. The immense distances involved are one problem, and so are the (frequently bizarre) surface compositions of these planetoids. They can often have a very low albedo, or reflectivity, which can make them seem smaller than they really are.

2007 OR10, which was discovered (unsurprisingly) in 2007 by a team scouring the outer Solar System for new members, has quite an erratic orbit, coming almost as close as Neptune but swinging as far away as its current orbit, which is twice the distance to Pluto.

Original estimates suggested the object had a diameter of 1,280 km (795 mi); but by combining data from the ESA/NASA infrared Herschel Space Observatory and the current K2 phase of the planet-hunting Kepler Space Telescope, the team was able to revise that number. They now peg the diameter to 1,535 km (955 mi), which makes 2007 OR10 the third-largest dwarf planet in the outer Solar System.

It’s about 100 km (60 mi) larger than Makemake, the next largest icy runt in that neck of the woods; and it’s about a two-thirds the size of Pluto, which has a diameter of 2,374 km (1,475 mi). The second-largest object, Eris, is only slightly smaller than Pluto, coming in at 2,326 km (1,445 mi).

Artist's rendition of the dwarf planet. Credit: NASA
Artist’s rendition of the dwarf planet. Credit: NASA

Strange World

The new study, published in The Astronomical Journal, also gives us a glimpse of some of the unusual properties of the newly promoted world.

It has a very dark, very ruddy surface. It’s much darker than expected, in fact, because it’s still reflecting the same low levels of light from what’s been found to be a much larger surface area. This distinguishes 2007 OR10 from most other objects of its particular species, as they’re usually brighter. Its red color, which some think might be caused by methane ices on the surface, puts it firmly in the august company of its bigger brothers, Eris and Pluto, which display a similar hue. This suggests its gravity is strong enough to retain volatile methane, carbon monoxide, and nitrogen ices.

It also has a very slow “day,” taking about 45 hours to complete one rotation.

By using the Kepler telescope to research the planetoid’s properties, the team has shown that the K2 mission can do some very important work outside of its primary mission of extrasolar planet hunting—it can tell us a little about what’s going on right here in our own neighborhood.

Meanwhile, it’s about time to give poor, overlooked 2007 OR10 a proper name. That’ll be up to its discoverers—Meg Schwamb, Mike Brown, and David Rabinowitz.

“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,” Schwamb observes.

“I think we’re coming to a point where we can give 2007 OR10 its rightful name,” and it’s about time.