Big push too much for Uranus Britain/United States
Uranus is a lopsided oddity, the only planet to rotate on its side. Scientists now think they know how it got that way: it was pushed over by a rock at least twice as big as Earth.
Detailed computer simulations show that an enormous rock crashed into the seventh planet from the Sun, said Durham University, England astronomy researcher Jacob Kegerreis, who presented his analysis at a large earth and space science conference this month.
Uranus is unique in the solar system. The massive planet tilts about 90 degrees on its side, as do its five largest moons.
Its magnetic field is also lopsided and doesn’t go out the poles like Earth’s one does.
It also is the only planet that doesn’t have its interior heat escaping from the core. It has rings like Saturn, albeit faint ones.
The computer simulations show that the collision and reshaping of Uranus – maybe enveloping some or all of the rock that hit it – happened in a matter of hours, Kegerreis said. He has produced an animation showing the violent crash and its aftermath.
It was possible that the object that knocked over Uranus was still lurking in the solar system, said Nasa chief scientist Jim Green. It would explain some of the orbits of the planet, and fitted with a theory that a missing Planet X was circling the Sun well beyond Pluto.
Green said it was possible that a lot of smaller space rocks pushed Uranus over, but Kegerreis’s research pointed to a single huge unknown suspect, and a single impact ‘‘is the right thinking’’.
The collision happened 3 billion to 4 billion years ago, probably before the five larger moons of Uranus formed.
Instead, there was a disk of material that would eventually come together to form moons. When that happened, Uranus’s odd tilt acted like a gravity tidal force pushing the large moons to the same tilt, Kegerreis said.
It also would have created an icy shell that kept Uranus’s inner heat locked in, he said. Its surface temperature is minus 216 degrees C.
Ice is key with Uranus and its neighbour Neptune.
A little more than a decade ago, Nasa reclassified them as ‘‘ice giants’’, no longer lumping them with the other large planets of the solar system, the gas giants Saturn and Jupiter.
Uranus and Neptune ‘‘are definitely the least understood planets’’, said Carnegie Institution planetary scientist Scott Sheppard, who wasn’t part of the research.
Pluto, which is tiny, further from the Sun and not even officially a planet any more, has been explored more than Uranus and Neptune. They only got brief flybys by Voyager 2, the space probe that entered interstellar space last month.
But that may change. A robotic probe to one or both of those planets will be at or near the top of the next wish list from top planetary scientists.