Uranus is a slanted oddity, the only planet that rotates on its side. Scientists now think they know how it happened: it was driven by rocks at least twice as large as Earth.
Detailed computer simulations show that a large rock hit the seventh planet from the sun, said Durham University astronomy researcher Jacob Kegerreis, who presented his analysis at a major earth and space science conference this month.
Uranus is unique in the solar system. This big planet tilts about 90 degrees on its side, just like its five biggest moons. The magnetic field is also tilted and not out of pole like us, said NASA chief scientist Jim Green. It is also the only planet that does not have the interior heat which is detached from the core. It has a ring like Saturn, even though it's faint.
"This is very strange," said Carnegie Institution planetary scientist Scott Sheppard, who is not part of the research.
Computer simulations show that collisions and re-formation of Uranus – perhaps covering some or all of the rocks that hit it – occur within hours, Kegerreis said. He produced an animation that showed a great accident and its aftermath.
It is also possible that large objects dropping Uranus are still hiding in the solar system too far for us to see, Green said. This will explain some planetary orbits and according to the theory that the planet X lost around the sun is far outside Pluto, he said.
Green said it was possible that many smaller space rocks – about the size of Pluto – pushed Uranus, but Kegerreis and Sheppard's research pointed to a large unknown suspect. Green said the single impact "is right thinking."
Collisions occurred 3 billion to 4 billion years ago, possibly before the big months of Uranus formed. Instead there are disks of goods that will eventually come together to form the moon. And when that happens, Uranus's strange slope acts like a gravitational tidal force that pushes the five big moons to the same slope, Kegerreis said.
It will also create an ice shell that keeps the heat in Uranus locked, Kegerreis said. (Uranus's surface is minus 357 degrees, or minus 216 Celsius.)
Ice is key with Uranus and neighboring Neptune. A little more than a decade ago, NASA reclassified the two planets as "ice giants," no longer equating them with other large planets in the solar system, gas giants Saturn and Jupiter.
Pluto, which is tiny, farther from the sun and not even formally a planet anymore, has been explored more than Uranus and Neptune. They only got short flybys from Voyager 2, a space vehicle that entered the interstellar space last month.
Uranus and Neptune "are clearly the least understood planets," Sheppard said.
But that might change. Investigating robots to one or both planets is on the final list of hopes of leading planetary scientists and is likely to be at or near the top of the next list.
Uranus was named after the Greek god of the sky. Its name often results in juvenile humor when incorrectly pronounced like body parts. (Pronounced correctly YUR UR -uh-nus.)
"Nobody laughed when I said Uranus," Green NASA said. "They have to say it wrong to get laughter."