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| © The Associated Press This image made from video provided by Durham University astronomy researcher Jacob Kegerreis shows a computer simulation generated by the open-source code SWIFT that depicts an object crashing into the planet Uranus. Kegerreis says the detailed simulations show that the collision and reshaping of Uranus 3 billion to 4 billion years ago likely caused the massive planet to tilt about 90 degrees on its side. (Jacob A. Kegerreis/Durham University via AP) |
From Associated Press
Uranus is a lopsided oddity, the only planet to spin
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 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 ours does, said NASA
chief scientist Jim Green. It also is the only planet that doesn't have
its interior heat escape from the core. It has rings like Saturn, albeit
faint ones.
"It's very strange," said Carnegie Institution planetary scientist Scott Sheppard, who wasn't part of the research.
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 produced an animation showing the
violent crash and its aftermath.
It's also possible that the big
object that knocked over Uranus is still lurking in the solar system too
far for us to see, said Green. It would explain some of the orbits of
the planet and fit with a theory that a missing planet X is circling the
sun well beyond Pluto, he said.
Green said it's possible that a
lot of smaller space rocks — the size of Pluto — pushed Uranus over, but
Kegerreis' research and Sheppard point to a single huge unknown
suspect. Green said a single impact "is the right thinking."
The
collision happened 3 billion to 4 billion years ago, likely before the
larger moons of Uranus formed. Instead there was a disk of stuff that
would eventually come together to form moons. And when that happened,
Uranus' odd tilt acted like a gravity tidal force pushing those five
large moons to the same tilt, Kegerreis said.
It also would have
created an icy shell that kept Uranus' inner heat locked in, Kegerreis
said. (Uranus' surface is minus 357 degrees, or minus 216 Celsius.)
Ice
is key with Uranus and its neighbor Neptune. A little more than a
decade ago, NASA reclassified those two planets as "ice giants," no
longer lumping them with the other large planets of the solar system,
the gas giants Saturn and Jupiter.
Pluto, which is tiny, farther
from the sun and not even officially a planet anymore, 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.
Uranus and Neptune "are definitely the least understood planets," Sheppard said.
But
that may change. A robotic probe to one or both of those planets was
high up on the last wish-list from top planetary scientists and likely
will be at or near the top of the next list.
Uranus was named for
the Greek god of the sky. Its name often generates juvenile humor when
it is wrongly pronounced like a body part. (It's correctly pronounced
YUR'-uh-nus.)
"No one laughs when I say Uranus," NASA's Green said. "They have to mispronounce it to get the chuckles."
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