Why are solar systems or galaxies flat?

[skeleton]

Why are solar systems and galaxies flat?

I find it peculiar that astronomical clusters (ie: solar systems and galaxies) are arranged in a flat plane. Can someone explain why?

I understand that: any solar system originates from a random cloud of dust and gas. This cloud will feel its mutual gravitational attraction and the constituents will gradually orbit around a center. I presume that cloud could be originally distributed across the confines of a solid sphere (not a flat circle). As such, the constituents should orbit not just in a plane, but on planes through the great circles on a sphere.

 

[Borek]

What if the original cloud rotates?

 

[skeleton]

Nope; I am still stuck with the understand.

I would presume that a new star emerges from an accretion of dust and gas that previously originated from a nebula. But that nebula had exploded, and when it did, it burst into all directions (spherically) and not just onto a single plane normal to its previous axis of rotation.

I can only guess the answer (but someone please verify):

Perhaps when spherically arranged bodies orbit about a common center, then initially do so amongst all the great circles of a sphere. Then by chance, one of the orbital bodies is more massive than the others. The lighter bodies are inclined to deviate their planar orbits from their initial zenith, and gradually they are drawn down to the equator defined by the more massive object. Of course, the massive object also gets deviated by the lighter ones, but less so. After numerous iterations of the orbits, eventually they all asymptotically converge to a mutual orbital plane.

Yes/No ?

 

[eachus]

Two parts to the answer. Not all galaxies are flat. We have recently learned that not all solar systems are flat either, but for solar systems, it is much more likely. At least based on the data we have now.

For galaxies, I am going to tie several concepts together to end up close to where you are. This is definitely not settled science, and if you would like to help settle it, visit the Galaxy Zoo on line.

As for galaxies, I think the phenomena started when the decoupling of matter from the CMBR light burst. When a (extremely tiny by even today's standards call it a few nanometers wide) volume of space contained only un-ionized atoms or molecules, it would act as if it were shining a brilliant light in all directions on the surrounding plasma. Beyond a critical size, these bubbles would grow, and are today voids between the galaxies some hundreds of megaparsecs in diameter.

What happens when these bubbles expand is an experiment you can perform in the laboratory. For the mix of matter and energy in the universe, you get an open pored foam. Lots of thin lines meeting at junctions that are much thicker than the interconnections.

Galaxies condensed out of the mass ejected from the voids, probably after the matter density reached a critical value in any small region. Now when you go to collapse a galaxy, the moving gas will not only have a direction given by the original bubbles, but it will have a shear. Looked at from above the resulting galaxy the gas on the left will be headed in a slightly different direction than that on the right. Bingo. Except, I am pretty sure that merging of galaxies in the twelve billion or so years since then will probably have erased that signal. Spiral galaxies in our corner of the universe are almost certainly all created from mergers of older galaxies. Two (or more) galaxies merging will almost always have a residual radial momentum vector that results in the spiral shape.

In the early universe we should see traces of the bubble blowing. The Galaxy Zoo should help to determine when we are looking at galaxies resulting from recent collisions and any pristine galaxies that may remain.

 

[skeleton]

Eachus,

The bubble boundaries certainly wasn't intuitive to me, but the coalescence of predecessor galaxies is - now that you have presented it.

Thanks for the succinct explanation.

 

[Ken G]

I think there may be a better way to see why gravitationally condensed structures tend to flatten as they contract. Your intuition is correct that there is no problem with orbits being randomly spread over a sphere, indeed that is just what happens to the comets of the Oort cloud in our own solar system. But the planets and asteroids are mostly in a plane, so why is that? It's because the planets and asteroids are participating in conserving the angular momentum of the original system that contracted to form our solar system.

So if the original system contained no angular momentum, there'd be no reason to form a disk. But just by chance, or perhaps by some process like eachus was describing, the original system will have angular momentum. That means the orbits will prefer, perhaps only to a very small extent, one particular direction over the opposite direction, in regard to some plane perpendicular to the vector angular momentum. Gravity is a central force so cannot change the angular momentum of a closed system, but it can make the system contract, and then an interesting thing happens.

Let's approximate the effect of the presence of angular momentum by imagining that we have two populations, a spherical population with no net angular momentum, and a disk population, all going around the same way, that carries the angular momentum. It isn't literally true, but this picture will serve. As gravity contracts the entire system, Kepler's third law tells us that the characteristic speed of the orbiting gas in both components must scale like the inverse square root of the size of the cloud. (Or if we take account of a dark matter halo, the speed grows even less rapidly than that.) But either way, this is seemingly at odds with conservation of angular momentum, which requires that the disk component, as it contracts, should have characteristic speeds that scale like the inverse of the size of the cloud, not the square root or less of that size. Since the energy requirements that give us Kepler's third law must hold true, it appears the angular momentum is disappearing for no good reason.

Since that cannot be, we find that the only solution is to move mass from the spherical component to the disk component, so the angular momentum per mass can drop without the total angular momentum dropping. This means the only way for both energy and angular momentum to be conserved requires that the cloud become more and more disklike as it contracts. Eventually, the entire cloud would be just a disk, and no further contraction could occur. However, what instead seems to happen is that some central condensation continues to occur (especially for star formation!) because there are processes that do actually transport angular momentum out of the cloud. So it's kind of a Goldilocks situation-- to the extent that angular momentum is not transported away, you get a disklike structure, and to the extent that it is, you get a centrally condensed halo-like structure, or even a central star. Angular momentum is the key attribute to follow.

 

[Radrook]

In order to classify our solar system as flat we would have to disqualify the Oort Cloud as being part of it. But as it stands the Oort cloud is definitely considered part of our solar system and it is not flat.

Excerpt:

Oort Cloud
The Oort Cloud is a collection of icy comets far beyond Pluto. It is the farthest place from the Sun in the Solar System. The Oort Cloud marks the end of the Solar System becuase beyond it the Suns gravity is to weak to hold anything so it will be pulled away from the Sun. It surrounds the planets like a cage. The Oort Cloud is about 8 million million kilometers in diamater.
http://thegalaxyguide.com/galaxy/oortcloud/index.html

 

[Subluminal]

I think it would be useful to ask and understand what initiates the direction and magnitude of that angular momentum. Could some sort of Coriolis effect be at work, curving the path of each particle toward the center of a giant molecular cloud, as the cloud revolves around the center of its home galaxy? Imagine air rushing toward the center of a hurricane - instead of following the spokes of a wheel, the Coriolis effect curves each mass of air to the right as it moves, resulting in an overall CCW rotation of the system (in the northern hemisphere). Maybe the rotation of the home galaxy is the source for the energy and direction of this angular momentum relative to its movement. I'm sure relativistic factors are also at work, since gravity doesn't travel faster than light. Imagine following the forming star system through space - as the dust draws in and accretes into a disk, the star itself is moving away from you, away from the point in local space where gravity was drawing the dust towards moments ago. The effect might be similar to a vortex that forms in a sink as the contents are drawn in towards the drain.

 

[Ken G]

The coriolis effect is none other than the conservation of angular momentum, said in force terms instead of angular momentum terms. So any explanation based on conservation of angular momentum is going to have the same underlying content as an explanation based on the coriolis effect, it really just depends on if you like forces, or like conserved quantities, and if you want to enter into a rotating reference frame to do your analysis.

 

[Radrook]

I think it would be useful to ask and understand what initiates the direction and magnitude of that angular momentum. Could some sort of Coriolis effect be at work, curving the path of each particle toward the center of a giant molecular cloud, as the cloud revolves around the center of its home galaxy? Imagine air rushing toward the center of a hurricane - instead of following the spokes of a wheel, the Coriolis effect curves each mass of air to the right as it moves, resulting in an overall CCW rotation of the system (in the northern hemisphere). Maybe the rotation of the home galaxy is the source for the energy and direction of this angular momentum relative to its movement. I'm sure relativistic factors are also at work, since gravity doesn't travel faster than light. Imagine following the forming star system through space - as the dust draws in and accretes into a disk, the star itself is moving away from you, away from the point in local space where gravity was drawing the dust towards moments ago. The effect might be similar to a vortex that forms in a sink as the contents are drawn in towards the drain.

Some maintain that the non-earthian corialis effect is produced by a spinning universe and is responsible for pinwheel galaxies.