NANTES, France—Knock, knock. That's not the start of a joke but the hard-luck history of Uranus. New research suggests that the giant planet may have suffered two massive impacts early in its history, which would account for its extreme, mysterious axial tilt.

Uranus orbits nearly on its side; its axis of rotation is skewed by 98 degrees relative to an ordinary upright orientation, perpendicular to the orbital plane. Many planetary scientists have sought to explain the odd tilt by invoking a giant impact into Uranus billions of years ago. But the giant planet has a system of moons circling its equator that would have been disrupted by such an impact.

"If Uranus is suddenly tilted, the satellites keep moving like that from north pole to south pole, and [wouldn't be] equatorial at all," Alessandro Morbidelli of the Observatory of Côte d’Azur in Nice, France, reported here Thursday at a joint meeting of the American Astronomical Society Division for Planetary Sciences and the European Planetary Science Congress. [Read more planetary news from the meeting here.]

But what if the tilting was a more gradual process, caused not by one mammoth impact but by two somewhat smaller nudges? Simulations show that the two-strike mechanism appears to solve the problem, knocking Uranus sideways and allowing it to develop equatorially orbiting moons, Morbidelli said.

The key is that the impacts must have come very early, before Uranus's moons had coalesced from a disk of gas and dust surrounding the planet. That disk, supplemented by debris stirred up by the collisions, would have migrated around the planet to form a thin equatorial disk that gave rise to Uranus's five large moons.

In the simulations, the same sort of equatorial migration also worked for the single-impact tilt scenario, but that scenario came with one important and disqualifying caveat: the moons orbited in the wrong direction, counter to Uranus's rotation. "If you tilt Uranus all in one shot, you produce regular satellites on the equator, but they will all be retrograde, and the satellites are actually prograde," Morbidelli said.

The only way for Uranus to have kept its moons in the right place, moving in the right direction, was to have suffered multiple giant impacts. "If we are right, Uranus was hit at least twice by big objects, about the mass of the Earth," Morbidelli said. He noted that Neptune's tilt, although only about one third that of Uranus, is also best explained by a giant impact.

"He solved the problem with the giant-impact hypothesis," said Hal Levison of the Southwest Research Institute in Boulder, Colo. "I've always been worried about this problem." But, Levison noted, "that doesn't mean that the giant-impact hypothesis is right." There are several other ways to change a planet's tilt, or obliquity, including tidal forces and resonances between a planet's spin and its orbit.

But if Uranus did suffer two large collisions, and Neptune absorbed one as well, that would indicate that massive impacts played a significant role in shaping the giant planets. That would be a surprise, given the traditional view that the gas giants grew by sweeping up smaller planetesimals. "This is quite an unconventional scenario for the formation of the giant planets, but I think that the obliquities of Uranus and Neptune point in this direction," Morbidelli said.