# Do planets have to rotate?

Discussion in 'Astronomy, Exobiology, & Cosmology' started by Gawdzilla Sama, Mar 27, 2019.

1. ### Gawdzilla SamaValued Senior Member

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Is there any requirement that planets have to rotate? I know everything from the DDSSHGB planets up rotate, and some smaller bodies, IIRC. But how come?

3. ### cluelusshusbund+ Public Dilemma +Valued Senior Member

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no requriment that i know of.!!!

5. ### Neddy BateValued Senior Member

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I don't know what DDSSHGB means, and neither does Google apparently. But whether something rotates or not is somewhat relative. Our moon does not rotate when viewed from earth, which is why we never see the far side of the moon from here. But folks who are standing directly on the sun (ouch) would say that our moon does rotate about 13 times in the course of an earth year.

I think it is a matter of conservation of angular momentum. For something free floating in space to be non-rotating, it must have no angular momentum. Imagine an astronaut on a space walk somewhere in deep space, and she lets go of one of her tools. As it drifts away, it will rotate according to the angular momentum it had when it was released, and if it does not rotate then it is because the astronaut happened to release it with no angular momentum. As far as probabilities go, it seems the odds are greater that it would have some angular momentum, rather than none at all.

7. ### Janus58Valued Senior Member

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You have to consider how planets are formed. You start with a cloud that is it rotating itself. (If it wasn't rotating, gravity would just pull everything inward, and you get a star and no planets. The rotation is needed to form the planetary disk from which the planets are formed)

Different as you move outward from the axis of this rotating cloud, the individual parts are orbiting at different speeds, slower as you move outward. Now imagine that some of this material begins to collapse to from a planet. some of that material will come from further in and some from further out. Material coming from inward loses orbital velocity as it climbs and material coming from outward gains its it. As a result of this, the material will have a net angular momentum relative to its center of mass, and a net rotation relative to that center of mass.

Basically, planets can't form without starting with some rotation. For a planet to have no rotation (relative to the stars) would require a unusual sequence of events where just the right outside influences end up robbing it of all its angular momentum.

8. ### Neddy BateValued Senior Member

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By the way, does anyone know what DDSSHGB planets are? I sure don't.

9. ### Gawdzilla SamaValued Senior Member

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Doc
Dopey
Sneezy
Sleepy
Happy
Grumpy
Bashful

i.e., the dwarves.

10. ### Gawdzilla SamaValued Senior Member

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Okay, so the material that formed the planets is rotating around the sun, got that. Does this mean the accreted material will automatically rotate around its axis? Because the accretion wasn't even?

11. ### billvonValued Senior Member

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Effectively yes. What Janus said, plus the only forces that would slow a planet are tidal. With a planet close enough to its sun, you could see a tidal lock happen where the planet always faced one side to the sun. But it would still rotate once per orbit. To stop that completely you'd have to _add_ energy to the planet somehow, to overcome the tidal lock.

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12. ### SeattleValued Senior Member

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I think the process of star formation results in angular momentum. The process of gravity pulling a mass together result in the spinning motion.

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13. ### Confused2Registered Senior Member

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This shifts the origin of the rotation back a step. When a cloud of gas/dust collapses under its own gravity there is no obvious case for there to be any net angular momentum. Just a thought - there may be a parallel here with water draining down a plughole where the angular momentum propagates outwards from the sink hole.
Edit - I don't understand where/how angular momentum is conserved when water flows in this way.

Last edited: Mar 28, 2019
14. ### cluelusshusbund+ Public Dilemma +Valued Senior Member

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I suspect that everthang was spinnin right after the big bang... an angular momentum occured at the very same time as spin.!!!

15. ### YazataValued Senior Member

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Whether or not a body rotates would seem to me be relative to the frame of reference one chooses. If a planet doesn't rotate in the sidereal frame, relative to the "fixed" stars, it would still be rotating once per orbit relative to the body it's orbiting (its star if it's a planet).

Or maybe not. Making rotation simply relative to choice of reference frame would seemingly still leave things like centrifugal and coriolis 'forces' unexplained, conservation of angular momentum and so on. So it gets complicated and a bit above my pay grade.

I'm reminded of Isaac Newton's 'bucket argument' for what he believed was Absolute Space.

16. ### James RJust this guy, you know?Staff Member

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This is why inertial frames of reference have a special status in physics. In such frames, there are no centrifugal forces, for example.