Dwarf Planet 2007 OR10: Big, Dark, and Slow

A Kuiper Belt object discovered nearly a decade ago turns out to be much darker — and thus larger — than thought. It's also an especially slow spinner.

Dwarf planets compared

New results show that 2007 OR10 is the largest unnamed body in our solar system and the third largest dwarf planet. (Pluto is not shown here.)
Konkoly Observatory / NASA / JHU-APL / SwRI

Read the fine print on any newly discovered solar-system object, and you’ll find its size is often just a guesstimate. That’s because all the finders typically know is the magnitude — but is the object big and dark or small and bright? They have to assume an albedo, or surface reflectivity, in order to estimate the diameter.

Recently a team of planetary astronomers worked to pin down the characteristics of dwarf planet 2007 OR10 to learn not just its intrinsic brightness but its size, shape, and surface properties. Surprisingly, they found that it’s bigger and darker than anyone thought. In fact, 2007 OR10 is the largest as-yet-unnamed world in our solar system (though it's been numbered: 225088) and the third largest of the known dwarf planets.

András Pál (Konkoly Observatory, Budapest) led the study, which used data from NASA’s repurposed planet-hunting Kepler space telescope — its mission now known as K2 — along with archival data from the ESA’s infrared-sensing Herschel Space Observatory, whose instruments recorded far-infrared and submillimeter wavelengths (55 to 672 microns). Their results will appear in the Astronomical Journal.

By measuring the heat coming off an object, and then combining this data with its visible brightness, Pál explains, “one can unambiguously obtain the size of the object and the surface albedo as well.”

Unlocking 2007 OR10's Dark Secrets

To get an accurate model of the heat coming off 2007 OR10, however, the researchers had to determine the rotational period of the dwarf planet. They did that by looking at small variations in 2007 OR10’s brightness, which K2 measures very precisely, and looking for regular repetitions.

“If you look very accurately at the brightness of an object as it rotates, then you might see certain bright spots, like the big, bright spot we saw on Pluto with New Horizons, the heart-shaped spot,” says Geert Barentsen (NASA Ames Research Center), a researcher with the K2 mission. “Then as the body rotates, we will see the light go up and down every time the bright spot comes around.”

Watch the video below to see Kepler's observations of 2007 OR10 (indicated by the arrow) over a period of 19 days in late 2014. (The object's apparent movement against the stars is caused by Kepler's changing position as it orbits the Sun, and the diffuse light at right is from Mars, which was near the field of view.)

Konkoly Observatory / László Molnár and András Pál

From the Kepler data, the researchers determined that the best fit to the light curve was a surprisingly slow rotational period of a little bit less than 45 hours, longer than most objects orbiting beyond Neptune.

Meanwhile, the Herschel data showed the team how much sunlight 2007 OR10 absorbs and later radiates as heat. This value, combined with the Kepler data, let them calculate the dwarf planet’s size and how reflective it is.

2007 OR10 turns out to be much larger than thought. A previous estimate using this same technique had suggested a diameter of roughly 795 miles (1,280 km). However, without a handle on the object’s rotation period, past studies were limited in their ability to estimate its overall shape and surface character.

After folding together all these adjustments, the team determined that 2007 OR10 has an albedo of just 9% — half that found in the previous study. The much darker surface implies, in turn, a much bigger diameter, and Pál’s team calculates this to be 955 miles (1,535 km).

A New Name for "Snow White"?

This larger size means 2007 OR10 probably has the gravity to retain icy coatings of volatile chemical compounds such as methane (CH4), carbon monoxide (CO), and molecular nitrogen (N2). Herschel’s infrared spectra also reveal a reddish surface consistent with a covering of methane frost, which creates red-tinged hydrocarbon compounds when bombarded by sunlight and cosmic rays.

Astronomers Megan Schwamb, Michael Brown, and David Rabinowitz spotted OR10 in 2007 as part of a search for distant solar-system bodies. They nicknamed it “Snow White,” thinking it was quite a bit smaller and more reflective. Eventually, they’ll have the honor of giving it an official name.

“It will no doubt get a name soon,” now that more is known about this mysterious object, says Barentsen, “and it might be a name that links back to what we have learned in this study.”