|MadSci Network: Astronomy|
Good question! The answer lies in something called angular momentum - which is the mass times the velocity times the radius. We say that this is "conserved" - whatever you do to a system, there is always the same amount of angular momentum. The classic example is to think of an ice skater spinning with his arms out. If he pulls his arms in, he spins faster (as the radius - the distance from the centre - has decreased angular momentum would be lost unless he speeded up to compensate). How does this relate to the planets? They all formed when material from a rotating gas and dust disk around the infant sun came together under the influence of gravity. As material fell together and compressed, its radius decreased and so it had to speed up in just the same way as our ice skater. The giant (Jovian) planets have a great deal of mass; in the disk, all of this mass carried its own angular momentum. As it condensed out of the disk, each planet began to spin faster; the more mass the planet had, the more angular momentum it had, and hence the faster it would spin. (This takes place even though the Jovian planets are physically bigger than the terrestrial planets - the key is that they have relatively more mass than radius.) Another good example of rapid rotation comes from pulsars - the cores of dead massive stars that can be observed because they appear to pulse regularly; as the star's material collapses down (a typical pulsar would be about the same distance across as an average city - and you have to fit a whole star in there!) it speeds up and in fact some of these pulsars rotate several thousand times a second! Others are much slower - at about once a second. It's true we don't understand precisely the mechanism of planet formation but this general picture seems to be correct.
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