Moon found round Kuiper dwarf planet

The discovery of a moon around the third largest dwarf planet in the Kuiper Belt adds another piece to the puzzle of what our Solar System was like in its infancy.

A sequence of images of dwarf planet 2007 OR10 captured by the Hubble Space Telescope. The images show that the moon moves with the dwarf planet as it orbits the Sun, proving that it is gravitationally bound to it.
Credit: NASA, ESA, C. Kiss (Konkoly Observatory), and J. Stansberry (STScI)

Astronomers have discovered a moon around the third largest dwarf planet in the Kuiper Belt, adding to our knowledge of what the early Solar System must have been like.

The dwarf planet, named 2007 OR10, resides in the Kuiper Belt, which is a region full of icy debris left over from the formation of the Solar System 4.6 billion years ago.


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The discovery means that most of the known dwarf planets in the Kuiper Belt larger than about 1,000km across have satellites. Studying these bodies reveals clues as to how moons formed in the early Solar System.

The fact that many of them appear to have moons shows that collisions must have been frequent, and also occurring at just the right speed.

If the impacts had occurred at a high speed, they would have created lots of debris that would have escaped from the system. Too slow, and they would have only created impact craters.

"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," says team member John Stansberry of the Space Telescope Science Institute. "This gravitational stirring may have nudged the bodies out of their orbits and increased their relative velocities, which may have resulted in collisions."

The team discovered the moon in images of 2007 OR10 taken by the Hubble Space Telescope’s Wide Field Camera 3. NASA’s Kepler Space Telescope revealed that it has a rotation period of 45 hours, while other Kuiper Belt objects would typically take under 24 hours to rotate.

"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,” says Csaba Kiss of the Konkoly Observatory in Budapest, Hungary.

Observations in far-infrared light by the Herschel Space Observatory enabled the team to calculate the dimensions of both objects. The planet is about 1,500km across and the moon is estimated to be about 240 - 400km across.

2007 OR10 is the third largest dwarf planet in the Kuiper Belt that we know of, smaller only than Pluto and Eris. It was discovered in 2007 by astronomers Meg Schwamb, Mike Brown and David Rabinowitz.


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