New simulations suggest that Jupiter and Saturn may have played a bit of planet Pong with Uranus. That is, Uranus got in Saturn's orbit, so it was tossed to Jupiter's orbit and then tossed back and forth until it finally settled.
The back-and-forth game of planet Pong happened three times (and probably lasted 100,000 years) before Uranus was tossed outside of Saturn's orbit, which it then settled in peacefully. Tossing Uranus would have caused Jupiter and Saturn to recoil, shifting their orbits back as well. It's merely a theory, but it's sorta fun to think that God (or the universe or whoever) would bounce a planet around with other planets. [New Scientist]