Millions of miles beyond Pluto in the cold, dark regions of the Kuiper Belt sits another dwarf planet. This one doesn’t have a regular name yet. Astronomers call it 2007 OR10. Amidst the countless chunks of ice and rocket, 2007 OR10 stretches about 950 miles across.

But this dwarf planet isn’t like most Kuiper Belt Objects. NASA’s Kepler Space Telescope took a break from hunting for exoplanets far beyond our solar system and gave the team of astronomers a hint something was different with 2007 OR10.

Your average Kuiper Belt Object rotates one time in under 24 hours. 2007 OR10 is much slower at 45 hours. “We looked in the Hubble archive because the slower rotation period could have been caused by the gravitational tug of a moon,” said Csaba Kiss, lead author of the new study.

Sure enough, the dwarf planet has a buddy traveling with it during its 549 year-long orbit around the Sun.

“The initial investigator missed the moon in the Hubble images because it is very faint,” said Kiss.

The astronomers confirmed the moon in two separate Hubble observations. One in November 2009. The other in September 2010.

moon around 2007 OR10

Credit: NASA, ESA, C. Kiss (Konkoly Observatory), and J. Stansberry (STScI)

But the pair of observations were not enough to figure out the exact orbit of the moon. Or, look into the possible link between 2007 OR10’s slower rotation and its neighboring moon.

The astronomers were able to get an estimate of the moon’s size thanks to the Herschel Space Observatory. Viewing the moon in far-infrared light coughed up estimates between 150-250 miles in diameter.

The existence of this moon and moons around other large dwarf planets gives us clues as to what this region of space looked like billions of years ago. “At the time these bodies formed billions of years ago, collisions must have been more frequent, and that’s a constraint on the formation models,” said Kiss. “If there were frequent collisions, then it was quite easy to form these satellites.”

But not just any collision. One body striking another and becoming a moon isn’t as simple as smashing into each other. It had to happen at just the right speed. Too fast and the collision creates a bunch of debris. Too slow and all you have is an impact crater.

“There must have been a fairly high density of objects, and some of them were massive bodies that were perturbing the orbits of smaller bodies,” said team member John Stansberry. “This gravitational stirring may have nudged the bodies out of their orbits and increased their relative velocities, which may have resulted in collisions.”

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