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jambouk

Interstellar "planets"

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Leaving the classification of these bodies aside, if there are planet-like entities outside the reach of solar systems, would they rotate on their axis?

I know very little about physics, but I was under the impression the fact planets orbit a star in some way contributes to their own spin. But would planets between solar systems have this force inflicted upon them? If there was no spin they would surely fall apart...

Anyone clever and able to explain this all to a thickie?

James

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4 minutes ago, bomberbaz said:

The moon doesn't rotate and neither does mercury

Yes, they do rotate, but the rotation is synchronised to the same rotation period around the sun and the earth. 

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I am no scientist but my gut feeling is that the very process of ejection would have given their spin a kick which could last a quite a while if a rocky Planet but maybe degrade quite fast if it was a gas giant but how this effects its ability to hang onto its atmosphere I have no idea.

Alan

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1 minute ago, Knighty2112 said:

Yes, they do rotate, but the rotation is synchronised to the same rotation period around the sun and the earth. 

I thought Mercury had a very slight rotation in relation to the Sun.

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2 minutes ago, Dave Lloyd said:

I thought Mercury had a very slight rotation in relation to the Sun.

Yeah, your right. :) 

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Planets get their rotation from their original formation and will retain it forever unless something interferes.  In the case of moons that show the same face to their planets (tidally locked) they still rotate but their rate of rotation ("Day") has been slowed by tidal effects so that it matches the rate of their orbit ("Year") so they show the same face. They are still rotating though.

I have no idea why you think their rotation has anything to do with holding them together. If anything, rotation would threaten to disrupt if fast enough (Saturn for example is significantly flattened at the poles because of its fast rotation).

 

ps

@Bomberbaz

The Moon rotates just fine (if slowly) as does Mercury:

Rotation period:

Moon: 27.3 days

Mercury: 88 days

The slowest rotational period in our solar system is Venus:  243 days (which is longer than its year)

 

 

 

 

 

 

Edited by digital_davem
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And just to confuse matters... Venus rotates clockwise* and its year is longer than its orbit! :icon_scratch:

 

* the other planets of 'our' solar system rotate anti/counter clockwise.

Edited by Philip R

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41 minutes ago, Knighty2112 said:

Yes, they do rotate, but the rotation is synchronised to the same rotation period around the sun and the earth. 

therefore to us the moon does not rotate, potatoe potatoe i think. ( i will give you mercury) :grin:

Edited by bomberbaz

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A bit off topic but does anyone have an approximation for the numbers of these objects i.e. is it one per solar system or more?

Alan

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My understanding is that as a gas cloud coalesces and starts forming a star there will be some perturbation within the mass that induces spin. Perhaps this could be due to a Coriolis effect borne of the gas clouds motion through it's galaxy or some tidal interaction. Either way, nothing is static in the universe it seems, so some perturbation induces spin which increase as the cloud collapses (conservation of angular momentum).

The cloud doesn't collapse in a spherical manner. The rotation produces a bulge in the plane of rotation which eventually forms a disk. Knots and eddies in the disk eventually produce the planets which acquire their rotation from the Coriolis effect.

Some planets interact gravitationaly and collide or induce violent tidal forces on other planets causing them to be ejected from the system or flip on their rotational axis etc. Other tidal interactions cause synchronous or harmonic orbital relationships.    

It's all very chaotic!

Now, I suppose that those ejected bodies are then doomed to cruise interstellar space foe eternity, carrying their original axial rotation with them.

I'm not sure about the formation of planets outside of star systems. I can't see why a gas/dust cloud couldn't produce a lone planet. though I suspect such a low mass cloud would be slow to collapse under it's own weight. But if it could then there will inevitably be some perturbation in the collapsing mass that would induce rotation. Actually, it seems unlikely that a cloud could collapse so uniformally  that it doesn't rotate.

I think...

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33 minutes ago, bomberbaz said:

therefore to us the moon does not rotate, potatoe potatoe i think. ( i will give you mercury) :grin:

Now now, Copernicus would be spinning in his grave with that interpretation - pun intended.:happy8:

 

Jim

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15 hours ago, bomberbaz said:

therefore to us the moon does not rotate, potatoe potatoe i think. ( i will give you mercury) :grin:

Perhaps a risk of sounding pedantic but I can't accept this description. 

True, all motion is relative and you can seem to negate it by choosing an appropriate reference frame that is matched.

However, in ordinary speak that is not what we mean when we talk about a planetary body's axial rotation.   For example, if you were in a space craft that was in geostationary orbit, your orbital motion in synchronised with the rotational motion of the Earth such that you hover over one spot on the ground.  To you, the Earth would not appear to rotate. But it would be Quixotic to deny that the earth rotates on its axis. Billions of people on the ground experiencing the day/night cycle would disagree with you. And indeed, if you were to raise your eyes from that spot on the ground to observe the background starfield you would see it spinning around you proving that you and the Earth were indeed experiencing axial rotation. When you sit up all night and watch the starts swing across the sky you are experiencing the Earth's axial rotation. You would experience exactly the same thing if you were to try it on the Moon except the night would last (Earth) weeks. Likewise on Mercury (with an even longer night).

The existence of rotational motion of a planet is not dependent on the additional orbital motion of an observer, that merely creates lines of sight "stationary appearance" effects. 

The moon, Mercury, all the planatary bodies all rotate on their axis with respect to the background universe, always have and always will.

Edited by digital_davem

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16 hours ago, digital_davem said:

Rotation period:

Mercury: 88 days

That's its orbital  period, its rotation period is 59 days.

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1 hour ago, goodricke1 said:
1 hour ago, goodricke1 said:
1 hour ago, goodricke1 said:

That's its orbital  period, its rotation period is 59 days.

That's its orbital  period, its rotation period is 59 days.

That's its orbital  period, its rotation period is 59 days.

Oops! Thank you, careless cut and paste error.

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