The possiblity of a solid planetary disk

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Discussion Overview

The discussion revolves around the theoretical possibility of a solid planetary disk and its physical implications, including stability, formation processes, and comparisons to known celestial bodies. Participants explore concepts related to rigid bodies, eccentricity, and the characteristics of fluid and solid forms in astrophysical contexts.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Some participants question whether a planetary disk could exist as a rigid aggregate, noting that such a structure may not be gravitationally stable.
  • One participant suggests that erosion would likely lead to a more spherical form rather than maintaining a disk shape.
  • There is a comparison made to Saturn's rings, with participants discussing the conditions under which a rotating body might break apart or form rings.
  • Questions are raised about the critical limits of failure for rotating rigid bodies and how eccentricity affects stability.
  • Some participants discuss the properties of accretion disks and the effects of tidal forces within the Roche limit.
  • There is a debate on the maximum eccentricity of stable fluid bodies and whether a fluid can form a significantly oblate, self-gravitating spheroid.
  • References to Saturn's oblate shape are made, with discussions on the implications of rapid rotation on planetary form.
  • One participant mentions charge separation in relation to magnetohydrodynamics (MHD) and its relevance to the discussion.
  • A historical reference is made to Maxwell's assertion regarding the impossibility of certain configurations.

Areas of Agreement / Disagreement

Participants express a range of views on the stability and formation of solid planetary disks, with no consensus reached on the feasibility of such structures. Multiple competing ideas regarding the nature of rigid and fluid bodies, as well as their stability under rotation, remain unresolved.

Contextual Notes

Limitations include assumptions about gravitational stability, the definition of rigidity, and the effects of tidal forces, which are not fully explored or agreed upon. The discussion also touches on complex mathematical relationships that are not definitively resolved.

Loren Booda
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Has the existence of a planetary disk condensed as a rigid aggregate ever been postulated or observed?
 
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Loren Booda said:
Has the existence of a planetary disk condensed as a rigid aggregate ever been postulated or observed?
Not that I have ever heard of, until yesterday at 11:06 PM...:biggrin:
 
It would be hard to imagine any physical process creating such an object, because it is not gravitationally stable.

- Warren
 
Basically, you've got a planet that has *two extremely tall mountains* dropping off to two extremely deep valleys. However it got that way, any form of erosion will rapidly reduce it to a more spherical form.

I think you're expecting that, even as a disk, gravity will somehow be normal to the surface. It isn't. Gravity will point to the centre of mass.

[EDIT] correction: *one extremely tall, planet-spanning mountain range*
 
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like saturn's rings?
 
ray b said:
like saturn's rings?


They are moving fast enough to stay in orbit. If the planet spun fast enough to maintain itself against collapse, it would surely break apart - and become rings!:smile:
 
How can one express the critical limit of failure with respect to the eccentricity for a rotating rigid body?
 
Loren Booda said:
How can one express the critical limit of failure with respect to the eccentricity for a rotating rigid body?
OK, now this is a different ball of wax. You're talking about a body rotating so fast that it is being radically deformed. It will not be stable over long periods.
 
DaveC426913,

Actually, I was trying to make a comparison between various bodies of similar rigidity, mass and angular momentum, but different eccentricities. At what eccentricity do such bodies start breaking apart? This reminds me of the problem of a flywheel used for energy storage.

Thank you for helping me form my question.
 
  • #10
If the body is rigid the eccentrivcity has to be zero by definition.

Accretion discs are composed of a myriad of particles each on their own orbit, with different orbital periods. Inner particles orbit more quickly, with a smaller period, than outer particles.

Inside the Roche limit of a planetary/stellar body anybody composed of normal material would disintegrate as tidal forces overwhelm its tensile strength.

Certainly a disc of individual particles could not accrete there.

You might imagine a contrived, and probably therefore necessarily artifical, solid disc around a very small body such as an asteroid where tidal forces would be weak but I doubt you will ever find one outside a SF story.

Garth
 
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  • #11
What is the maximum eccentricity of a stable, fluid oblate spheroid?
 
  • #12
Loren Booda said:
What is the maximum eccentricity of a stable, fluid oblate spheroid?
zero? Did you mean eccentricity or oblateness?

Garth
 
  • #13
Garth,

Please address the question in regards to oblateness.
 
  • #14
Loren Booda said:
Garth,

Please address the question in regards to oblateness.
The galaxy is pretty oblate and a self gravitating body of dust (stars) and gas.

Garth
 
  • #15
Could a fluid also form a stable, rotating, significantly oblate, self-gravitating spheroid?
 
  • #16
Loren Booda said:
Could a fluid also form a stable, rotating, significantly oblate, self-gravitating spheroid?
The shape would depend on the fluid, its density, total mass of the body, speed of rotation, viscosity etc. but the answer is yes! The actual detail would be quite complicated to calculate.

Is this what you are looking for? http://arxiv.org/PS_cache/astro-ph/pdf/0609/0609756.pdf

Garth
 
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  • #17
Loren Booda said:
Could a fluid also form a stable, rotating, significantly oblate, self-gravitating spheroid?
That's what Saturn is, sans rings. Saturn is visibly oblate.

http://www.nasa.gov/worldbook/saturn_worldbook.html"
"The rapid rotation of Saturn causes the planet to bulge at its equator and flatten at its poles. The planet's diameter is 8,000 miles (13,000 kilometers) larger at the equator than between the poles. "

As Saturn is only 75,000 miles in diamter - that's more than 10% oblation.
 
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  • #18
Garth,

Yes, although I had not considered charge separation (mentioned at first in regards to MHD). From what I can tell, this method uses stochastics conventionally and hydrodynamic variables unconventionally.

Can anyone beat the entry of Saturn by DaveC426913 for an oblate fluid?
 
  • #19
Didn't Maxwell say this was impossible?
 

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