# The possiblity of a solid planetary disk

1. Sep 24, 2006

### Loren Booda

Has the existence of a planetary disk condensed as a rigid aggregate ever been postulated or observed?

2. Sep 25, 2006

### Labguy

Not that I have ever heard of, until yesterday at 11:06 PM...

3. Sep 25, 2006

### chroot

Staff Emeritus
It would be hard to imagine any physical process creating such an object, because it is not gravitationally stable.

- Warren

4. Sep 25, 2006

### DaveC426913

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*

Last edited: Sep 25, 2006
5. Sep 25, 2006

### ray b

like saturn's rings?

6. Sep 25, 2006

Staff Emeritus

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!:rofl:

7. Sep 25, 2006

### Loren Booda

How can one express the critical limit of failure with respect to the eccentricity for a rotating rigid body?

8. Sep 25, 2006

### DaveC426913

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.

9. Sep 26, 2006

### Loren Booda

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. Sep 26, 2006

### Garth

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 any body 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

Last edited: Sep 26, 2006
11. Sep 26, 2006

### Loren Booda

What is the maximum eccentricity of a stable, fluid oblate spheroid?

12. Sep 27, 2006

### Garth

zero?

Did you mean eccentricity or oblateness?

Garth

13. Sep 27, 2006

### Loren Booda

Garth,

14. Sep 27, 2006

### Garth

The galaxy is pretty oblate and a self gravitating body of dust (stars) and gas.

Garth

15. Sep 27, 2006

### Loren Booda

Could a fluid also form a stable, rotating, significantly oblate, self-gravitating spheroid?

16. Sep 27, 2006

### Garth

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 [Broken]

Garth

Last edited by a moderator: May 2, 2017
17. Sep 28, 2006

### DaveC426913

That's what Saturn is, sans rings. Saturn is visibly oblate.

http://www.nasa.gov/worldbook/saturn_worldbook.html" [Broken]
"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.

Last edited by a moderator: May 2, 2017
18. Sep 28, 2006

### Loren Booda

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. Oct 1, 2006

### FeynmanMH42

Didn't Maxwell say this was impossible?