In his prog. , on the outer planets, Brian Cox stated that Uranus spins on its side because it once endured an interplanetary collision.

Such a cataclysm would normally be devastating for both planets. Given that Uranus is a gas-giant, with no solid inner-core, why didn't the second planet simply "float" through the gas clouds?

How does a ball of gas become "turned on its side"?


Astrophysicists at Durham University, in collaboration with scientists at NASA's Ames Research Centre in Silicon Valley, used advanced computing techniques to create the most detailed simulation to date of the suspected impact.

Uranus got tilted so much that it spins on its side, four billion years ago, the authors believe a young proto-planet of rock and ice collided with Uranus, causing its extreme tilt. Instead of rotating like a top spinning nearly upright, as Earth does, the planet rolled on its side as it circles the sun.

For that research paper, see Consequences of Giant Impacts on Early Uranus for Rotation, Internal Structure, Debris, and Atmospheric Erosion


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