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© Chuck Elliot/GettyA victim of planetary pinball
Uranus may have been batted back and forth between Jupiter and Saturn before being flung out to its present location, new simulations suggest.

Previous modelling has shown that Jupiter and Saturn moved out of their initial orbits in the early solar system, scattering nearby objects.

In some simulations, this led to Uranus crossing the path of Saturn, which could then have flung it towards Jupiter, which lobbed it back to Saturn. The process might have happened three times before Uranus was finally ejected beyond Saturn, to where it now resides. Hurling Uranus would have caused Jupiter and Saturn to recoil, further shifting their orbits.

New simulations led by Alessandro Morbidelli of the Côte d'Azur Observatory in France suggest this pinball game, which would have lasted just 100,000 years, fits with observations. In an alternate scenario, Jupiter and Saturn moved to their orbits over 5 million years by simply flinging away space rocks, but this would have visibly scarred the asteroid belt (Astronomical Journal, in press). "The evolution of the giant planets has been more violent than we thought," Morbidelli says.