Why do most of the celestial bodies rotate about their axis?

  • #1

Main Question or Discussion Point

Every celestial body revolves around a relatively massive celestial body due to the gravitational influence.
But what is the reason behind their rotation about their own axis? What thing initially triggered their rotation?
Does it have something related to the origin of the universe?
 

Answers and Replies

  • #2
phinds
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Every celestial body revolves around a relatively massive celestial body due to the gravitational influence.
But what is the reason behind their rotation about their own axis? What thing initially triggered their rotation?
Does it have something related to the origin of the universe?
When dust and cosmic debris coalesces to form bodies, stuff never falls straight in but rather comes in at an angle. This give the body a spin.
 
  • #3
When dust and cosmic debris coalesces to form bodies, stuff never falls straight in but rather comes in at an angle. This give the body a spin.
Okay! I also thought of some similar reason...
But taking the randomness of the process into consideration...the small spins may tend to cancel each other out...resulting into no (or negligible) net spin.
And in reality, the spin isn't negligible...
 
  • #4
Bandersnatch
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Are you familiar with the principle of conservation of angular momentum?
 
  • #5
Are you familiar with the principle of conservation of angular momentum?
Yes!
 
  • #6
russ_watters
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Okay! I also thought of some similar reason...
But taking the randomness of the process into consideration...the small spins may tend to cancel each other out...resulting into no (or negligible) net spin.
And in reality, the spin isn't negligible...
Note that most planets spin in the same direction they orbit, all orbit in the same direction and the solar system formed from one cloud of gas/dust...
 
  • #7
Note that most planets spin in the same direction they orbit, all orbit in the same direction and the solar system formed from one cloud of gas/dust...
yeah! exactly...
the cloud which formed our solar system should have had a distinct spin direction...which became the spin of the planets which were formed from that cloud...
but using conservation of angular momentum...this should imply that the small planets spin faster and larger planets spin slower...which is not the real case...
where am i going wrong?
and also, what was responsible for the spin of the cloud?
 
  • #8
256bits
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this should imply that the small planets spin faster and larger planets spin slower
Seems reasonable.
But, let's look at planet forming this way.
Earth 1 collects parts of the cloud from a certain volume, and acquires a mass M1 and spin S1.

We now want to build a bigger planet than Earth 1, called X, of mass 2M and spin SX.
We have to do this by collecting from the cloud farther out( the closer parts have already formed M1 ), and acquires a mass M2 and spin S2.
Note that while M2 = M1, the spins are different. .Spin S2 is greater than S1.

Combine Earth 1 and 2 into Planet X.
Planet X, larger than Earth 1 will now have a spin SX which will be greater than that our original Earth 1.

conclusion:
Larger planets can have a greater spin than smaller.
 
  • #9
Bandersnatch
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and also, what was responsible for the spin of the cloud?
Imagine some hypothetical, large, primordial, initially static cloud of gas. Whether universe-sized, galaxy-sized, solar system-sized, or any size really.
There will be fluctuations in density of that cloud. Even if the cloud started out ideally uniform, it would be in a dynamically unstable state, much like a needle standing on its tip. Any deviation from uniformity, even the tiniest one, would destabilise the cloud and lead to irregular clumping.
If you then look at one such over-dense region, you'll notice that it will be attracted by other clumps. Due to their irregular distribution, the resultant forces will locally produce some nett torque, imparting a non-zero angular momentum, as illustrated schematically in the following picture:
Primordial clumping.png

Circles represent concentrations of matter (e.g. particles), extending beyond the picture. Clouds show expected clumping.
(I was eyeballing the forces, so don't take them too literally)

Here, conservation of angular momentum tells us that for each clump that starts rotating one way, there's another (or a number) that rotates the other way, so whatever the angular momentum of the entire system of clumps, it stays unchanged.
But when considering any single clump, which depending on size may go on to become e.g. a group of galaxies, or a solar system, or a planet, it will have non-zero angular velocity.
It doesn't matter how small the rotation is - the conservation law tells us that as it collapses to form a smaller system, the initial rotation will become amplified until the system settles into a stable state, either as a compact rotating body (star, planet) or a collection of separate objects in orbits.

but using conservation of angular momentum...this should imply that the small planets spin faster and larger planets spin slower...which is not the real case...
No, it doesn't imply that.
If the entire cloud collapses to form a smaller object, then it will end up rotating the faster the smaller it gets. But with planets, you're only taking a fraction of the entire mass of the cloud - with the same angular velocity, but smaller radius and mass - and contracting that. I.e., these are different systems, so you can't use conservation laws as if you were describing a single system.
 

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  • #10
Okay! Now I got it!!!:smile:
Thanks a lot:-D
 
  • #11
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Having a zero rotational speed (still) is just one among all the possible (infinite) rotational speeds. The probability of having that zero value instead of any of the others is almost zero, there is no reason to favor one over the other.
That doesn't mean all values are equally probable, but having a value exactly zero is not probable.
 

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