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Moon orbits third largest dwarf planet.

2007 OR10

Combining the power of several space observatories, including NASA's Hubble Space Telescope, astronomers have uncovered a moon orbiting the third largest dwarf planet, catalogued as 2007 OR10.

The pair resides in the frigid outskirts of our solar system called the Kuiper Belt, a realm of icy debris left over from our solar system's formation 4.6 billion years ago.

With this discovery, most of the known dwarf planets in the Kuiper Belt larger than 600 miles across have companions. These bodies provide insight into how moons formed in the young solar system.

The team uncovered the moon in archival images of 2007 OR10 taken by Hubble's Wide Field Camera 3. Observations taken of the dwarf planet by NASA's Kepler Space Telescope first tipped off the astronomers of the possibility of a moon circling it. Kepler revealed that 2007 OR10 has a slow rotation period of 45 hours. "Typical rotation periods for Kuiper Belt Objects are under 24 hours," Kiss said. "We looked in the Hubble archive because the slower rotation period could have been caused by the gravitational tug of a moon. The initial investigator missed the moon in the Hubble images because it is very faint."

The astronomers spotted the moon in two separate Hubble observations spaced a year apart. The images show that the moon is gravitationally bound to 2007 OR10 because it moves with the dwarf planet, as seen against a background of stars. However, the two observations did not provide enough information for the astronomers to determine an orbit.

The dwarf planet is about 1529 kms across, and the moon is estimated to be 241 kms to 402 kms in diameter, the researchers determined.

Like Pluto, 2007 OR10 follows an eccentric orbit, but it is currently 3 times farther than Pluto is from the Sun.

The dwarf planet 2007 OR10 is a member of an exclusive club of nine dwarf planets. Of those only Pluto and Erise are larger than 2007 OR10.

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