Why do the planets spin ?

[Ian]

I was hoping that someone might give an insight to this on another thread concerning the moon, but no luck there.
Can anyone give me a plausible reason as to why the planets rotate at their particular velocities, e.g. why does the earth spin ~365 times per orbital revolution, or why does Jupiter spin ~1200 times per orbital revolution?
If anyone has an answer can they please back their views up with some maths.

Thanks.

[russ_watters]

The planets spin for the same reason they orbit: the cloud of gas and dust the solar system collapsed from was asymetrical and therefore rotating.

The particular maths are tough though: you need to infer some starting conditions and small changes in starting conditions lead to big changes in the result. People are working on it, but I don't think there yet exists an accurate computer model/simulation of our solar system's formation.

[Ian]

That was a bit quick on the draw! I only posted this a few minutes ago.
Thanks for your ideas. By what you said it sounds as though the rotation is related to the planetary mass or density. If this is so then there ought to be some relation between mass and rotation velocity.

[Nereid]

Maybe, but if you think about how the planets were formed - accretion followed by planetisimal collisions (except the gas giants?) - it would seem quite unlikely that a tight relationship would fall out naturally.

The Earth may be a good example - either way! No doubt there is a fairly well bounded region in the proto-Earth/proto-Moon parameter space that would give rise to an Earth-Moon system, and that region may also pretty much determine the initial rotation period of the Earth (and period of revolution of the Moon) - which, ~4.5 billion years later, gives us today's rates - but the extent to which the parameter space region is determined by the initial conditions of the proto-solar nebula?

[russ_watters]

Nereid's explanation was what I meant when I said it was tough to model: there was a lot going on when the solar system formed.