Uranus takes a pounding more frequently than thought

Uranus isn't just gassy, it's also tilted completely sideways, such that instead of rotating like a spinning top, it rolls around the plane of the solar system more like a giant ball. Now astronomers think they know how this happened, and it means that Uranus has been pounded really, really hard not once, but twice.

Uranus' axial tilt of 98 degrees means that it's got one pole pointed almost directly at the sun, and one pole pointed out into space. As the planet revolves around the sun, these poles slowly switch places, meaning that if you lived there, you'd get 42 years of sunlight followed by 42 years of darkness, with a short time in between where things would seem almost normal.

Needless to say, this sort of behavior is a bit strange for Uranus, although nobody's quite been able to determine how it happened. New simulations from astronomers at the Observatory of Côte d'Azur in Nice, France may have just figured it out, and the answer seems to be that Uranus has suffered from not one but two giant impacts.

The only reliable way to change the tilt of one planet is to smash it with another planet of comparable size. While planets running into other planets isn't something that happens very often nowadays, back in the tempestuous youth of the solar system, it was relatively common: a planet the size of Mars hitting Earth four and a half billion years ago probably blasted off enough stuff to form the moon, for example. But with Uranus, one collision big enough to knock the entire planet sideways would also have screwed up Uranus' system of moons, which isn't the case.

Replacing that one big impact with two slightly smaller impacts, though, works out perfectly. Each impact would have involved a planet the size of the Earth punching into Uranus several billion years ago, early enough that Uranus' moons could still have made it into their current orbits. Interestingly, an impact of about the same size is also thought to have happened to Neptune, which is making astronomers wonder whether the model they have for the formation of the outer planets is quite right. It's always been assumed that way out around Uranus and Neptune, things were pretty chill, with the gas giants slowly forming by sweeping up gas and dust in their orbits. But if these latest impact simulations are correct, it implies that the outer solar system was a much more violent place than we ever thought.

Europlanet, via Scientific American

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