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 (ESA)
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 California's Silicon Valley.
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,
the Kepler spacecraft 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, California.
"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
2007 OR10, the largest unnamed body in the solar
system, was visible during the K2 mission's campaign three for 19 days
in late 2014. K2 is the new mission of Kepler spacecraft. 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 diffuse light sweeping across is a reflection from the much
brighter planet Mars passing nearby. Credits: Konkoly Observatory/László Molnár and András Pál. Youtube
Media Contact:
Michele Johnson
Ames Research Center, Moffett Field, Calif.
650-604-6982
michele.johnson@nasa.gov
Written by Preston Dyches
Last Updated: May 11, 2016
