Uranus: Why’s It Sideways?

Something’s odd about Uranus. It spins sideways. In more technical lingo, the planet’s axis of rotation is titled approximately 98º in relation to the orbital plane of the Solar System. I feel like I say this a lot on this blog, but there are currently several competing theories to explain why.

One Big Collision: Maybe one large object, more massive than Earth, collided with Uranus, knocking Uranus sideways.


Oc02 Uranus Struck Once

Many Little Collisions: Or maybe a group of smaller objects collided with Uranus, knocking Uranus sideways.

Oc02 Uranus Struck Many Times

Computer simulations seem to favor this idea, mainly because it does a better job accounting for the sideways orientation of not only Uranus but also its rings and moons.

A Lost Moon: Or perhaps long ago, Uranus had an additional moon: an especially massive moon with a large gravitational pull, enough to cause Uranus to tip on its side. Later, this hypothetical moon would have been yanked out of orbit by gravitational interactions with one of the other gas giants.

I’m skeptical of this lost moon idea, mainly because Earth’s large moon supposedly prevents our own planet from wobbling or tipping over in its orbit.  I much prefer the collision or multiple collisions hypotheses.

Is Uranus really as odd as we think?

Consider the fact that Venus’s rotation is also out of whack with the rest of the Solar System. Venus rotates backwards (possibly because of a collision, by the way). That means two out of eight planets rotate out of alignment with the rest of the Solar System. That’s 25%!

We still don’t know much about planets in other star systems, but if this 25% statistic holds true (and given how common collisions are in space, I see no reason why it wouldn’t), then Uranus may not be all that unusual after all.


Series of Bumps Sent Uranus into Its Sideways Spin from Europlanet.

A Collisionless Scenario for Uranus Tilting from arxiv.org.

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Today’s post is part of Uranus month for the 2015 Mission to the Solar System. Click here for more about this series.

8 thoughts on “Uranus: Why’s It Sideways?

  1. I’m writing a Greek mythology short story, and I explain the off kilter rotation due to Eros (the original primordial God, not Venus’ son) being attached during his birth from Gaia. Eros detaches after his birth leaving him unbalanced. In more ways than just physically, he became quite insane. Like all the gods.

    Liked by 1 person

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