What toppled giant Uranus, the seventh planet from the sun, to its tilt is a long-standing puzzle. Scientists suspect it was left spinning on its side after a collision with an object about twice the size of Earth.
But that doesn't explain why Uranus's moons spin sideways, relative to their orbital planes, matching almost exactly their parent planet's 98-degree tilt.
Jupiter's spin axis, by comparison is tilted 3 degrees; Earth's, 23 degrees; Saturn and Neptune, 29 degrees.
The answer, suggests a team of scientists, is that Uranus was pummeled more than once. Computer models show a series of impacts by Earth-sized objects could have left Uranus on its side before its moons formed.
A single crash, the researchers say, would have left any moons accumulating from a cloud of materials surrounding Uranus spinning in the opposite direction from how they appear today.
The research has implications for understanding how the solar system -- and other planetary families beyond our solar system -- formed and evolved.
"The formation history of Uranus and Neptune is one of the most important open problems in planetary science. Having shown that giant collisions had to happen frequently on these planets is an important piece of information on the way to understanding their origin," lead author Alessandro Morbidelli, with the Observatory of Cote d'Azur in Nice, France, wrote in an email to Discovery News.
Better images of Uranus's moons probably would provide more clues about how the bodies formed, Morbidelli added.
The idea that Uranus was toppled by a double-whammy raises the question about where the impacting objects came from, points out astronomer Steven Desch, with Arizona State University.
Previous studies show that if the cores of Uranus and Neptune formed closer to the sun than where the planets are today, there would have been enough time for one object about twice the mass of Earth to form just beyond them. But Desch is skeptical there would have been enough time for more than one object to form and then batter Uranus.
"Is it possible to grow more than one object this large though? It seems unlikely," Desch told Discovery News.
Uranus currently orbits about 19 times farther from the sun than Earth. In Desch's model, it and Neptune would have needed to form about 50 to 75 percent closer to the sun to allow for the formation of the impacting body.
Morbidelli's research was presented this week at a planetary sciences meeting in Nantes, France.