Uranus tipped over by several collisions
Uranus was knocked on its side not by a single massive collision, as previously thought, but by a series of small thumps, researchers say.
While other planets are tilted to a small degree compared with their orbital plane around the sun, Uranus effectively lies on its side, with its spin axis inclined by 98 degrees.
It's been believed for some time that this was caused by a collision with a body a few times bigger than Earth. But if this were the case, the planet's moons should have been left orbiting in their original angles - and they too lie at almost exactly 98 degrees.
An international team of scientists led by Alessandro Morbidelli of the Observatoire de la Cote d'Azur simulated various impact scenarios.
They discovered that if Uranus had been hit when still surrounded by a protoplanetary disk - from which the moons would later form - then the disk would have taken on a fat donut shape around the new, highly-tilted equatorial plane.
Collisions within the disk would have flattened the donut, which would then go on to form the moons in the positions we see today.
However, the simulation failed to mirror reality in one crucial way: the moons displayed retrograde motion, orbiting in the opposite direction.
The team tweaked the parameters of the simulation, and discovered that the problem disappeared if Uranus was bumped in at least two small collisions, rather than tilted in one go.
This conclusion, though, is at odds with current theories of how planets form - which may now need adjusting, says the team.
"The standard planet formation theory assumes that Uranus, Neptune and the cores of Jupiter and Saturn formed by accreting only small objects in the protoplanetary disk. They should have suffered no giant collisions," says Morbidelli.
"The fact that Uranus was hit at least twice suggests that significant impacts were typical in the formation of giant planets. So the standard theory has to be revised."