MadSci Network: Astronomy

Re: Why do the Jovian planets rotate faster on their axis than solid planets?

Date: Thu Nov 1 03:27:56 2001
Posted By: Chris Lintott, Undergraduate, Physics
Area of science: Astronomy
ID: 1004478153.As

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 

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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