Third Largest Dwarf Planet Has A Moon

2007 OR10 and its moon in 2010. NASA, ESA, C. Kiss (Konkoly Observatory), and J. Stansberry (STScI)

Hubble has discovered that the third largest dwarf planet, currently known as 2007 OR10, is orbited by a little moon, which suggests that most of the dwarf planets in the outer Solar System have companions. This discovery will help scientists understand how natural satellites form.

This new moon was discovered in images from 2010 and announced at a specialist conference last fall. The research has now been officially published in the Astrophysical Journal Letters, with estimates of the moon's size at 237 kilometers (147 miles) – over five times smaller than 2007 OR10, whose diameter is 1,535 kilometers (955 miles).

"The discovery of satellites around all of the known large dwarf planets – except for Sedna – means that 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," lead author Csaba Kiss, from the Konkoly Observatory in Budapest, said in a statement.

Beyond the orbit of Neptune, there’s a region called the Kuiper Belt. It is wide and while it appears to be scarcely populated now, it might have been a lot more busy previously, with objects smashing into each other or at least shaking each other’s orbits.

"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," added 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."

Astronomers suspected that 2007 OR10 might have a companion due to its unusually slow rotation. It turns on itself every 45 hours, which is a huge amount of time since most of the other objects in the Kuiper Belt rotate in less than a day.

"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," Kiss stated.

But the observations are not good enough to extrapolate a full orbit for the satellites, so researchers were not able to confirm that it was indeed the moon that caused the slow rotation period.

2007 OR10 is the largest object in the Solar System without a proper name and is one of only nine confirmed exoplanets in the Solar System. It is currently three times farther than Pluto is from the Sun.


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