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What are the factors that prevent planets from also turning into disks, like the stellar dust in a nebula does?

The Earth is not a perfect sphere, but rather it's squished at the poles and it bulges at the Equator, owing to its rotation working to squish it into a disk.

I'm tempted to say that the Earth couldn't become a disk because of the density of the material in it fighting back against being pressured into a disk in a way that in stellar gas you can't? But then, why isn't the gas around the gas giants a disk? Is it because of the solid nucleus of those planets fighting against that?

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    $\begingroup$ There might be some "cause and effect" reversals here. If you spun a planet fast enough, it would deform into a disk, more or less. But without a bunch of dust having an initial angular momentum, the particles move towards a common center and "try" to get as close as possible, leading to migration in 3 dimensions to a more or less spherical shape. <-- I think this is correct but am not positive. $\endgroup$ – Carl Witthoft Jan 10 '19 at 15:39
  • $\begingroup$ Finally. Plus 1. Tough is true that earth would oblate more if it was made of materials with weaker forces between them. Density is not the point. Without reversal thinking and loosely say density for rigidity the OP now should know the answer. $\endgroup$ – Alchimista Jan 11 '19 at 15:24
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They both form disks, both disks are transient, and both disks are small compared with the central body. The parallels between the Sun and a planet are closer than one may think!

A collapsing nebula forms a star and a circumstellar disk. Nearly all of the angular momentum in the system is in the disk. The disk is unstable and much of the material condenses into small icy or rocky bodies which mostly combine to form planets or are expelled from the system by close encounters with larger bodies. After a few tens of millions of years, a planetary system is all that remains of the disk. (Note that even today, the planets contain 96% of the angular momentum in the Solar System.)

When planets form it seems likely that they also have disks of material around them. Because the planets are mainly forming from planetoids and bits of rubble, the disks don't start out mostly as gas. (The best current theory of the Moon's formation is that it condensed out of a disk formed around Earth from a large, late collision.) These disks also dissipate, falling onto the planet through drag or being ejected from orbit by perturbing bodies.

It's very unlikely that Saturn's (or Jupiter's or any of the others') rings are primordial, but are probably the result of the disruption of a icy body in the (comparatively) recent past.

Note that neither the Sun nor the planets are rotating fast enough for their shape to be much affected: They're all pretty much spherical. In each case, there are mechanisms which carry off angular momentum from the central body and deposits it in the disk or into the bodies which condense from the disk.

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  • $\begingroup$ Ha, that's the insight: Sun + its accretion disk (later, planets) <=> planet + its accretion disk (later, moons), so the differences I was looking to understand aren't really there. There's a bulge that doesn't get flattened in the case of the planets, but also in the case of the star system. And, also in the case of galaxies, as you zoom out. The universe is beautiful. :) $\endgroup$ – Mihai Danila Jan 12 '19 at 16:17
  • $\begingroup$ Still worth understanding why the bulges form. I'm guessing it's happening when gravity overpowers the forces involved in the angular momentum (centripetal force?). $\endgroup$ – Mihai Danila Jan 12 '19 at 16:20
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The circumstellar disk is perhaps about 2%-3% the mass of the central star. Large planets may form in similar ways. The mass ratio of the disk to what it orbits depends a lot on the angular momentum and I would think that only large planets that clear out a considerably larger volume of space are likely to have enough angular momentum to form disks around them. Jupiter's Galilean moons and most formation moons probably form planetary debris disks during their planet's formation.

This ring system around a brown dwarf that orbits a larger star could be compared to a circumstellar disk.

Disks of this nature, both stellar or theoretical-planetary are temporary, so only very young solar systems might have them and we probably don't have telescopes good enough to get a good look at young planets during their formation period. A large part of the reason we don't hear about planetary disks is that we might not have good enough telescopes to observe them.

(Note, if someone with better knowledge than me answers this, I can remove my answer).

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  • $\begingroup$ This is also a very good insight; by pointing out how little matter is in the disk, you're reminding me that the star is also in the picture, and it's the star that the planet should be compared to in this context. $\endgroup$ – Mihai Danila Jan 12 '19 at 16:28
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First point: planet VS nebula Your intuition is correct. Nebula is less dense and is mainly is gaseous state, so it has more fluidity than a solid state like a planet. Think of water can change shape regarding to its container.

Second point: why some gas does not turn into a disk shape? This has to do with the force field that the gas is under the influence, and the initial velocity of the gas. In case of having a solid core, the force field is just gravity pulling everything inward radially to the center. If the pulled stuff has the perpendicular (to the radial direction) component of the initial velocity not equal zero, instead of falling inward as a straight line radially, the stuff spirals into the center. The accumulation of stuff spiralling to the center gives the shape of disk.

For the giant gas cloud, it does not have a disk shape because the influence of gravity of a core (if exists) is negligible due to the outer part of the cloud (that we see) is very far from the core.

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    $\begingroup$ While density distribution relates to the characteristics of a gravitational field, it is worth mentioning that the stiffness due to electromagnetic forces - the chemical bonds in a broad sense - is responsible for holding the shape of a rocky or anyway solid planet. In the Q and somehow A above, density is used loosely as synonym of rigidity.. Think right of ice and water to get the point. A spheroidal of water would be more prone to flatter into an oblate or a disk, as compared to a less dense icy counterpart. $\endgroup$ – Alchimista Jan 10 '19 at 7:58
  • $\begingroup$ "it has more fluidity than a solid state like a planet" - at planetary scale, all bodies are basically fluid. A planet may seem "solid", but it behaves more like an incompressible fluid. $\endgroup$ – Florin Andrei Jan 10 '19 at 19:41
  • $\begingroup$ @Alchimista It's true at a micro level. But macroscopically, the spherical shape of planets is given by an interaction between: A) gravity; and B) the fact that it's essentially an incompressible fluid at that scale, no matter what the surface may seem to be (rocky, liquid, etc). $\endgroup$ – Florin Andrei Jan 10 '19 at 19:43
  • $\begingroup$ Fluidity and density have nothing to do. Consider Hg and diamond if you really need. Gravity itself create a spherical shape. The collapsing matter spread into a disk if rotating. The force opposing spreading it into a disk shaped object is cohesion not density whatever you regards things to be. Again the same: fluidity and density have nothing in common.@Florin Andrei $\endgroup$ – Alchimista Jan 11 '19 at 8:11
  • $\begingroup$ Thank you for the clarifications. It does make sense: density is not a force, and in this context it was used as an indirect reference to a force in light of the fact that at those scales more dense materials have a relevantly higher inward gravity pull and cohesion. $\endgroup$ – Mihai Danila Jan 12 '19 at 16:28
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I like the answers provided by @userLTK and @MarkOlson but let me add my own phrasing here.

The shape in each case is given by the particular characteristics of the scenario.

An irregular shaped nebula is a bunch of gas and dust only very loosely bound by gravity, or unbound. There's not much overall angular momentum (the whole thing is not spinning, typically). It's just a sparse pile of random stuff thrown out by a stellar explosion or something. The shape can be anything, depending on how exactly it came out to be. It could be slowly expanding.

An accretion disk is different. The stuff is bound together by gravity - perhaps the overall density of the cloud is greater, perhaps there's a dense chunk in the middle (protoplanet or protostar), perhaps both. Each bit of cosmic junk is orbiting the common center of mass - bound by gravity. You would think the whole collection could be spherical, but that only lasts for a blink of an eye: particles in the cloud collide with each other all the time, and whichever direction has the most angular momentum ends up winning in the end, so all particles will rotate more or less in the same plane; heretics will slam into the disk and join it in the end; within the disk there are far fewer collisions, so the disk configuration is more stable.

A planet is basically a giant pile of stuff that behaves, at the macroscopic level, like an incompressible fluid. Even "rocky" planets behave like fluids at the scale of the whole body. Of course, as with any fluid, gravity will eventually pull it into a spherical shape. If there's any angular momentum, the sphere will be squished a little, but most planets are close to a perfect sphere (extreme angular momentums are rare).

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