WASHINGTON (AP) - Uranus is a lopsided oddity, the only planet to spin on its side. Scientists now think they know how it got that way: It was pushed over by a rock at least twice as big as Earth.

  

Durham University astronomy researcher Jacob Kegerreis says detailed computer simulations show that an enormous rock crashed into the seventh planet from the sun. He presented his analysis at a conference this month.

  

The computer simulations show that the collision and reshaping of Uranus happened in a matter of hours.

  

NASA chief scientist Jim Green says it's also possible that the big object is still lurking in the solar system, too far for us to see. That would fit with a theory that a missing planet X is circling the sun well beyond Pluto.