The model helps explain a long-standing mystery: Why do Uranus's moons also lie in unexpected positions?

Unlike the other seven planets, Uranus's rotation axis has a bizarre 98-degree tilt relative to the solar system's orbital plane. In other words, the planet seems to roll around on its side as it orbits the sun.

Even odder, the rings and moons of Uranus circle the planet's tilted equator. (Related: "Hubble Reveals New Moons, Rings Around Uranus.")

The widely accepted theory for how Uranus got knocked over is that a rogue Earth-size planet slammed into the ice giant billions of years ago. That lost world was mostly likely destroyed on impact.

But previous computer simulations showed that a single extreme impact wouldn't have affected