Why does Uranus rotate on its side? (And why does Venus rotate backwards?)

Question posed by Jennie.

If you were hovering above the north pole of the solar system, you'd see all of the planets* orbiting the sun in an anti-clockwise direction. If you took a closer look at the planets, you'd see most of them rotating in the same direction. There are two exceptions: Venus and Uranus.

Uranus rotates on its side, with its poles almost in the plane of the solar system, almost like a barrel rolling around the sun, rather than spinning like a top, as the other planets do. Uranus's ring system orbits its equator, so that at certain times they look much like a target to us, with Uranus at the bull's eye.

Venus actually rotates backwards compared to the rest of the planets (except Uranus, of course). That is, if you resumed your vantage point over the solar system's north pole, you'd see that it spins in a clockwise direction, albeit very slowly.

Why do most planets spin in the same direction?
The solar system was formed from a rotating cloud of gas. The direction of this rotation determined the direction of the orbits of the planets as well as their spin direction.

Why are there exceptions?
The early solar system was comparatively crowded, and in such a system collisions were frequent. Most of these collisions would have been between small bodies, or of smaller bodies onto larger ones. Some of them would have been violent enough to cause changes to more massive objects.

Scientists currently believe that both Venus's and Uranus's odd spin properties were caused by two such collisions. The colliding objects must have been very massive and the collision violent to have caused changes to the two planets' direction of spin.

Interesting info

  • Most of the solar system's planets, moons and other objects bear the scars of early solar system bombardment.
  • The most widely accepted argument for the formation of Earth's moon involves a collision with a very massive object early in the solar system's formation.

Have a question about this topic? Comment below! Got an astronomy related question of your own? Ask it here.

* And most of everything else


  1. Thank you for that, most interesting :-)

  2. God made it like that to make evolution look stupid. Law of angular momentum. Simple.


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