What Knocked Uranus over?

The early Solar System was seemingly a violent place. Most versions of its formation involve massive collisions between near-planet sized objects – Mercury’s mantle was stripped by a collision, Venus’s rotation was stilled by a collision, Mars had a hemisphere almost ripped away, Earth made a Moon out of debris from a collision, and last of all Uranus was knocked over onto its side by a massive collision…

Or maybe not. New research by Jacques Laskar and colleagues, discussed at the arXiv blog, reveals another possibility – Uranus was tipped over by a lever. Not a big, long stick but a gravity lever thanks to a ‘moon’ that massed about 1% of Uranus. In Earths that’s 0.15 – a bigger version of Mars – and thus intensely interesting for another reason. There’s good reason to think that a large ‘planet’ is lurking just beyond the Kuiper Belt and its gravitational influence provides a neat explanation for the apparent “Kuiper Cliff” – the sudden end – of the classical Kuiper Belt.

So just how did a tiny thing like a “Macho-Mars” knock over a planet almost 15 times as big as Earth? Chaos. That and the migration of Uranus and Neptune from where they formed – between about 10 and 15 AU – to where they are now, at +19 and +30 AU respectively. How did they migrate? After they formed a large, heavy disk of unaccreted material still surrounded them. Close, hyperbolic flybys of this material gradually tugged the planets away from the Sun, with some nudging from Jupiter and Saturn, themselves caught in an unstable gravitational resonance a few hundred million years after formation… well maybe. There are various issues involved with timing that are yet to be resolved. Just how quick after formation was the migration is a major unresolved issue.

But the principle remains. With a big enough lever, even an Ice Giant can end up flat on his… side

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