Science Questions

Why do planets spin in a particular direction?

Sun, 19th Apr 2009

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Question

Alan Shine asked:

What determines the initial rotation of stellar cosmic objects? If you look at the sun it’s rotating, if you look at the planets going round the sun, they’re all rotating. What makes them decide what rotation to rotate in the first place?

Answer

Dave - I think basically it’s random chance.  If you start off with matter evenly spread out over the universe and everything’s moving round a bit randomly.  Some bit are moving a little bit in one direction, other bits are moving a little bit in another direction.  Some large areas will have ever-so slight spin in one direction and other large areas will have slight spin in another direction.  Then if that collapses with gravitational collapse if you’ve ever tried to climb into the centre of a roundabout in a playground it spins faster and faster.  If you’ve ever watched an ice skater when an ice skater moves all of her mass into the centre then she spins faster and faster.  This minute amount of rotation started off by a huge scale increases and collapses under gravity.  There’ conservation of angular momentum so it speeds up and speeds up.  Some galaxies will be spinning in one direction, others in another direction.

Chris - We should see a roughly equal proportion of each.

Dave - Basically random.

Chris - There should be some systems a bit like our solar system where instead of the planets going one way round the sun they could all be going the other way around the Sun.  But you shouldn’t ever see cosmic billiards going on where one planet’s going one way and the other’s going the other.

Dave - Unless...

Chris - Unless something catastrophic has happened.

Dave - I think on the individual solar system there’s quite a lot of catastrophic stuff which has gone on.  You’re probably talking more about galaxies sort of averaging to make it work.

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