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.

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 lops...