Uranus Axis Mystery Solved

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What knocked Uranus over? Scientists might finally have answered one of astronomy's longest standing questions. But first, a writer for io9 explains why the planet is such a mystery.

"Uranus is unique among all the planets of the solar system because it essentially orbits on its side, with its axis tilted nearly perpendicular to the Sun. ... Uranus's spin axis lies 98 degrees off of its orbital plane with the Sun. No other planet is tilted more than 30 degrees off its axis..."

The tilt of the planet has always posed a problem for astronomer's vision of the early solar system.

Up until now, the dominant theory is that an impact from a planet several times the size of Earth could have knocked Uranus over. The problem with that theory is that if it were true, the planet's moons wouldn't be in the orbit or rotation they have now. (Video source: The Science Channel)

Basically, if the impact happened early in the planet's formation, its moons wouldn't be spinning the direction they are. If it happened later, the moons wouldn't be orbiting the way they do. But ScienceNow reports new computer models appear to have figured it out.

"This problem would disappear if one impact tilted the planet to, say, 50° or so, and a second blow delivered an additional tilt of 48°. A larger series of smaller impacts is also possible. In each case, the planet and the disk material would remain orbiting in the same direction."

So it sounds simple enough: two separate impacts each knocked the planet halfway to its current rotation. But that creates its own problems with one pre-existing theory. Universe Today quotes the study's author saying Uranus shouldn't have been getting hit at all.

"The standard planet formation theory assumes that Uranus, Neptune and the cores of Jupiter and Saturn formed by accreting only small objects in the protoplanetary disk. They should have suffered no giant collisions. The fact that Uranus was hit at least twice suggests that significant impacts were typical in the formation of giant planets. So, the standard theory has to be revised."

The new theory of multiple impacts could also explain why Pluto has so many large moons -- it was getting pelted as well.

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