Uranus is a strange oddity, the only planet to turn aside. Scientists now think they know what happened: it was pushed by a rock at least twice as large as Earth.
Detailed computer simulations show that a huge rock fell on the seventh planet in relation to the sun, said Durham University astronomy researcher Jacob Kegerreis, who presented his analysis at a major conference on space and earth science this month.
Uranus is unique in the solar system. The huge planet tilts about 90 degrees to the side, as do its five largest moons. Its magnetic field is also unbalanced and does not leave the poles like ours, said NASA chief scientist Jim Green. It is also the only planet that does not have its inner heat escaping from the nucleus. It has rings like Saturn, though weak.
"It's very strange," said planetary scientist Scott Sheppard of the Carnegie Institution, who was not part of the research.
Computer simulations show that the collision and remodeling of Uranus – perhaps involving some or all of the rocks that hit it – occurred in a matter of hours, Kegerreis said. He produced an animation showing the crash and its aftermath.
It is also possible that the large object that knocked Uranus down is still hidden in the solar system too far away for us to see, Green said. That would explain some of the planet's orbits and fit the theory that an absent planet X is circling the sun well beyond Pluto, he said.
Green said it is possible that a lot of smaller space rocks – the size of Pluto – pushed Uranus, but Kegerreis and Sheppard's research point to a single large unknown suspect. Green said that a single impact "is the right thought".
The collision occurred three to four billion years ago, probably before the largest moons of Uranus formed. Instead, there was a disk of things that would eventually come together to form moons. And when that happened, Uranus's strange inclination acted as a gravitational tidal force pushing those five large moons to the same slope, Kegerreis said.
It would also have created an icy shell that kept Uranus's internal heat locked, Kegerreis said. (The surface of Uranus is minus 357 degrees, or minus 216 degrees Celsius.)
Ice is fundamental to Uranus and its neighbor Neptune. Just over a decade ago, NASA reclassified these two planets as "ice giants," not putting them anymore with the other large planets in the solar system, the gas giants Saturn and Jupiter.
Pluto, which is tiny, farther from the sun and not even officially one more planet, was explored more than Uranus and Neptune. They only received brief flights on Voyager 2, the space probe that entered interstellar space last month.
Uranus and Neptune "are definitely the least understood planets," Sheppard said.
But that can change. A robotic probe for one or both of the planets was at the top of the wish list of top planetary scientists and will probably be at the top of the next list.
Uranus was named for the Greek god of heaven. Its name generally generates juvenile humor when it is mistakenly pronounced as a part of the body. (It is correctly pronounced YUR-uh-nus.)
"No one laughs when I say Uranus," Green said from NASA. "They have to pronounce it wrong to get the laughs."