Astronomers have combined data from two space observatories to reveal that an object in the outer solar system – categorized as a dwarf planet and currently labeled 2007 OR10 – is much larger than previously thought. It fact, this little world is now recognized as the largest unnamed body in our solar system. The new study shows that it’s 955 miles (1,535 km) in diameter, or about a third smaller than Pluto. The scientists who determined this size for 2007 OR10 published their work in April, 2016, in the Astronomical Journal.
2007 OR10 is now considered to be the third-largest (or fourth-largest if Makemake turns out to be larger) dwarf planet, smaller only than Pluto and Eris. That means it’s larger than Ceres, the only dwarf planet in the inner part solar system, formerly known as the first asteroid to be discovered in 1801.
Planet? Dwarf planet? Asteroid? What with the 2006 changes in nomenclature by the IAU and the rapid rate of discovery of dwarf planets in the outer solar system, it can be confusing. That’s why astronomer Mike Brown – who helped discover 2007 OR10 and has found several other dwarf planets – does daily updates on a web page titled How many dwarf planets are there in the outer solar system?
But back to 2007 OR10. Another dwarf planet, named Haumea, has an oblong shape that is wider on its long axis than 2007 OR10, but its overall volume is smaller.
The study also found that 2007 OR10 is quite dark and rotating more slowly than almost any other body orbiting our sun, taking some 45 hours to complete its daily spin.
For their research, the scientists used the planet-hunting Kepler space telescope, whose main mission reached a dazzling milestone last week with over 2,000 exoplanets now confirmed. Kepler’s extended mission is called K2. Plus these scientists used archival data from the infrared Herschel Space Observatory.
The video below, from NASA’s Ames Research Center, shows 2007 OR10 as seen by the Kepler spacecraft during for 19 days in late 2014. The apparent movement of the faint dwarf planet (indicated with the arrow) among the stars is caused by the changing position of Kepler as it orbits around the sun.
The Kepler spacecraft job is to search for changes in the brightness of distant objects. That change can be the signature of a planet passing in front of its star, which is how Kepler found so many exoplanets. In its extended mission, K2, the space observatory looks closer to home, observing small bodies such as comets, asteroids, moons and dwarf planets in our own solar system. NASA explained in a statement:
Figuring out the size of small, faint objects far from Earth is tricky business. Since they appear as mere points of light, it can be a challenge to determine whether the light they emit represents a smaller, brighter object, or a larger, darker one.
This is what makes it so difficult to observe 2007 OR10. Although its elliptical orbit brings it nearly as close to the sun as Neptune, it is currently twice as far from the sun as Pluto.
Enter the dynamic duo of Kepler and Herschel.
Previous estimates based on Herschel data alone suggested a diameter of roughly 795 miles (1,280 km) for 2007 OR10. However, without a handle on the object’s rotation period, those studies were limited in their ability to estimate its overall brightness, and hence its size. The discovery of the very slow rotation by K2 was essential for the team to construct more detailed models that revealed the peculiarities of this dwarf planet. The rotation measurements even included hints of variations in brightness across its surface.
Together, the two space telescopes allowed the team to measure the fraction of sunlight reflected by 2007 OR10 (using Kepler) and the fraction absorbed and later radiated back as heat (using Herschel).
Putting these two data sets together provided an unambiguous estimation of the dwarf planet’s size and how reflective it is.
NASA said the newly estimated size of 2007 OR10 makes this world some 155 miles (250 km) wider in diameter than previously thought. The larger size also implies higher gravity and a very dark surface (because the same amount of light is being reflected by a larger body). Most dwarf planets are brighter. Previous ground-based observations found 2007 OR10 has a characteristic red color, and other researchers have suggested this might be due to methane ices on its surface. András Pál at Konkoly Observatory in Budapest, Hungary, who led the research, said:
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.
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.
Astronomers Meg Schwamb, Mike Brown and David Rabinowitz – who spotted 2007 OR10 in 2007 as part of a survey to search for distant solar system bodies – will be the ones to give it a name. Schwamb commented:
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.
I think we’re coming to a point where we can give 2007 OR10 its rightful name.
Bottom line: 2007 OR10 is now the third-largest (or fourth-largest if Makemake turns out to be larger) dwarf planet and the largest unnamed body in our solar system.
Deborah Byrd created the EarthSky radio series in 1991 and EarthSky.org in 1994. Today, she serves as Editor-in-Chief of this website. She has won a galaxy of awards from the broadcasting and science communities, including having an asteroid named 3505 Byrd in her honor. A science communicator and educator since 1976, Byrd believes in science as a force for good in the world and a vital tool for the 21st century. "Being an EarthSky editor is like hosting a big global party for cool nature-lovers," she says.