[meteorite-list] 2007 OR10: Largest Unnamed World in the Solar System

[Ron Baalke]

http://www.jpl.nasa.gov/news/news.php?feature=6509

2007 OR10: Largest Unnamed World in the Solar System
Jet Propulsion Laboratory
May 11, 2016

Dwarf planets tend to be a mysterious bunch. With the exception of Ceres, 
which resides in the main asteroid belt between Mars and Jupiter, all 
members of this class of minor planets in our solar system lurk in the 
depths beyond Neptune. They are far from Earth - small and cold - which 
makes them difficult to observe, even with large telescopes. So it's little 
wonder astronomers only discovered most of them in the past decade or 
so.

Pluto is a prime example of this elusiveness. Before NASA's New Horizons 
spacecraft visited it in 2015, the largest of the dwarf planets had appeared 
as little more than a fuzzy blob, even to the keen-eyed Hubble Space Telescope. 
Given the inherent challenges in trying to observe these far-flung worlds, 
astronomers often need to combine data from a variety of sources in order 
to tease out basic details about their properties.

Recently, a group of astronomers did just that by combining data from 
two space observatories to reveal something surprising: a dwarf planet 
named 2007 OR10 is significantly larger than previously thought.

The results peg 2007 OR10 as the largest unnamed world in our solar system 
and the third largest of the current roster of about half a dozen dwarf 
planets. The study also found that the object is quite dark and rotating 
more slowly than almost any other body orbiting our sun, taking close 
to 45 hours to complete its daily spin.

For their research, the scientists used NASA's repurposed planet-hunting 
Kepler space telescope -- its mission now known as K2 -- along with the 
archival data from the infrared Herschel Space Observatory. Herschel was 
a mission of the European Space Agency with NASA participation. The research 
paper reporting these results is published in The Astronomical Journal.

"K2 has made yet another important contribution in revising the size estimate 
of 2007 OR10. But what's really powerful is how combining K2 and Herschel 
data yields such a wealth of information about the object's physical properties," 
said Geert Barentsen, Kepler/K2 research scientist at NASA's Ames Research 
Center in Moffett Field, California.

The revised measurement of the planet's diameter, 955 miles (1,535 kilometers), 
is about 60 miles (100 kilometers) greater than the next largest dwarf 
planet, Makemake, or about one-third smaller than Pluto. 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.

Like its predecessor mission, K2 searches for the change in brightness 
of distant objects. The tiny, telltale dip in the brightness of a star 
can be the signature of a planet passing, or transiting, in front. But, 
closer to home, K2 also looks out into our solar system to observe small 
bodies such as comets, asteroids, moons and dwarf planets. Because of 
its exquisite sensitivity to small changes in brightness, Kepler is an 
excellent instrument for observing the brightness of distant solar system 
objects and how that changes as they rotate.

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 kilometers) 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.

According to the new measurements, the diameter of 2007 OR10 is some 155 
miles (250 kilometers) larger than previously thought. The larger size 
also implies higher gravity and a very dark surface -- the latter because 
the same amount of light is being reflected by a larger body. This dark 
nature is different from most dwarf planets, which are much 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.

"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 at Konkoly Observatory in Budapest, Hungary, who led the research. 
"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."

As for when 2007 OR10 will finally get a name, that honor belongs to the 
object's discoverers. Astronomers Meg Schwamb, Mike Brown and David Rabinowitz 
spotted it in 2007 as part of a survey to search for distant solar system 
bodies using the Samuel Oschin Telescope at Palomar Observatory near San 
Diego.

"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."

Ames manages the Kepler and K2 missions for NASA's Science Mission Directorate. 
NASA's Jet Propulsion Laboratory in Pasadena, California, managed Kepler 
mission development. Ball Aerospace & Technologies Corporation operates 
the flight system with support from the Laboratory for Atmospheric and 
Space Physics at the University of Colorado in Boulder.

For more information about the Kepler and K2 missions, visit:

http://www.nasa.gov/kepler

More information about Herschel is online at:

http://www.nasa.gov/herschel


News Media Contact

Michele Johnson
NASA Ames Research Center, Moffett Field, Calif.
650-604-6982
michele.johnson at nasa.gov 

Elizabeth Landau
Jet Propulsion Laboratory, Pasadena, Calif.
818-354-6425
Elizabeth.landau at jpl.nasa.gov 

Written by Preston Dyches
Jet Propulsion Laboratory 

2016-127