Now scientists have discovered one that tags along with Uranus. About 38 miles wide, the icy rock runs ahead of the planet.
The object was first spotted in 2011 by a group of Canadian and French scientists led by the University of British Columbia. The team reported the discovery in Friday's issue of Science.
Some 6,000 space rocks are known to follow Jupiter, the most of any planet. Earth shares its orbit with a tiny asteroid.
Scientists say the latest find looks like an asteroid, but its makeup is similar to a comet. A million years from now, the rock will escape back into the outer solar system.
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