Uranus’ Tilt Is From A Double-Whupping Upside Its Head.
Astronomi-wizards have long been trying to figure out why Uranus is tilted over on its side. Late night partying? Existential vertigo? Or perhaps a comet-based beat down. By not one, but two rough riders.
Uranus, weirdly, is at 98 °, like it’s rolling around the outer solar system on its side. The best guess is that it got hit hard by something planet-sized long ago, knocking it over (though there are other, more speculative, ideas). The problem with that is that its moons and rings all orbit around its equator, meaning their orbital planes are tipped as well. It’s hard to see how that might have happened, even if you assume the moons formed in that collision (as, apparently, our Moon formed in an ancient grazing impact with Earth by a Mars-sized body).
Well, a team of astronomers have come up with a new idea: maybe Uranus wasn’t hit by one big object. Maybe it was hit by two smaller ones.
It would’ve happened when the planet was still forming, and surrounded by a disk of leftover material that was in the process of forming its moons. A proto-planet could’ve hit it, knocking it over somewhat, and sending up a vast cloud of debris that puffed the disk up into a torus (that’s what us scientist-types call a donut). A second collision some time later would’ve completed the task. After more time elapsed things settled down and Uranus would’ve been rotating sideways, and the torus would’ve flattened back into a disk aligned with Uranus’ equator due to tidal forces.
It’s an interesting, if surprising idea. If there were only one collision at that time, the astronomers found the dynamics would’ve made the moons orbit the planet the wrong way (retrograde, against the spin of the planet). It would’ve taken a second hit to add enough momentum to the debris disk to get the moons orbiting prograde.
Interesting theory. Ah, theories. Read some more Phil Plait explanation over here.