– 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.

Detailed computer simulations show that an enormous rock crashed into the seventh planet from the sun, said Durham University astronomy researcher Jacob Kegerreis, who presented his analysis at a large Earth and space science conference this month.

Uranus is unique in the solar system. The massive planet tilts about 90 degrees on its side, as do its five largest moons. Its magnetic field is also lopsided and doesn’t go out to the poles like ours does, said NASA chief scientist Jim Green. It also is the only planet that doesn’t have its interior heat escape from the core. It has rings like Saturn, albeit faint ones.

“It’s very strange,” said Carnegie Institution planetary scientist Scott Sheppard, who wasn’t part of the research.

The computer simulations show that the collision and reshaping of Uranus — maybe enveloping some or all of the rock that hit it — happened in a matter of hours, Kegerreis said. He produced an animation showing the violent crash and its aftermath.

It’s also possible that the big object that knocked over Uranus is still lurking in the solar system too far for us to see, said Green. It would explain some of the orbits of the planet and fit with a theory that a missing planet X is circling the sun well beyond Pluto, he said.

The collision happened 3 billion to 4 billion years ago, likely before the larger moons of Uranus formed.