Why is the Solar System Flat Instead of Spherical Like Atomic Structures?

[hafiz ns]

hi guys,
why our solar system is flat? why planets are not revolving arround the sun as atoms revolve arround the nucleus? why galaxys are flat? why black holes are flat?

 

[ShayanJ]

Hi
They aren't!
http://en.wikipedia.org/wiki/Tests_of_general_relativity#Perihelion_precession_of_Mercury
http://en.wikipedia.org/wiki/Two-body_problem_in_general_relativity
http://en.wikipedia.org/wiki/Apsidal_precession

 

[Drakkith]

By "flat" do you mean why are they shaped like disks?

 

[ShayanJ]

I should say that my answer was only about the solar system.
About Galaxies, only some types are flat. Of course even in those types there are stars or smaller systems inside the galaxy that don't move in the plane of the galaxy but I don't think even a thousand of such deviations causes a change in the shape of the galaxy. I should mention again that there are galaxies which aren't flat and they're abundant too!
But your question doesn't make sense about black holes. What do you mean by "flat black hole" ?

 

[Bandersnatch]

There's a great video on MinutePhysics explaining just that:


There's also a fun gravitational collapse simulation on Khan Academy that shows in detail what the video mentions at 1:05
https://www.khanacademy.org/science...al/v/accreting-mass-due-to-gravity-simulation

 

[snorkack]

One important reason why solar system is flat is Kozai resonance.

 

[Ken G]

There's a great video on MinutePhysics explaining just that:


There's also a fun gravitational collapse simulation on Khan Academy that shows in detail what the video mentions at 1:05
https://www.khanacademy.org/science...al/v/accreting-mass-due-to-gravity-simulation

That's a nice short blurb, keeping things pretty simple. If one wishes to go a level deeper into the mathematics, I would point to the fact that it is much easier for a system to lose energy than it is to lose angular momentum (though both are conserved quantities in total). Hence it is significant that the state of minimum kinetic energy, for a given angular momentum, is a disk. Systems that show disks have found a way to lose energy but keep angular momentum, leading them to that state. Electrons in atoms, on the other hand, are fermions, so obey the Pauli exclusion principle, which limits the energy they can lose even more stringently than the angular momentum they can lose. By the time they reach their ground state, they typically lose all their angular momentum, but still have lots more energy to lose that they are forbidden to lose by the PEP. So the story is all about how easy is it to lose angular momentum while you are losing energy.

 

[hafiz ns]

By "flat" do you mean why are they shaped like disks?

yes

 

[hafiz ns]

thanks Bandersnatch. i had watched this video before i don't think so its 100% correct

 

[collinsmark]

The video is pretty much right on the money when it comes to "clumping." Combine this conservation of angular momentum, and you end up with a roughly flat solar system, and roughly flat galaxies.

When baryonic matter collides, much of the energy is converted to heat (inelastic collisions), and ultimately radiated as electromagnetic radiation. Angular momentum is conserved, however. The result is the system "flattens out," so to speak, as the baryonic matter clumps into larger dust particles, planets, asteroids; and in the case of galactic scales, stars.

Contrast that with cold dark matter (CDM). According to most CDM theories, the dark matter particles do not interact with each other in any way except gravitationally. They don't interact with baryonic matter electromagnetically, or even with other CDM particles. The only way CDM particles lose energy is through gravitational waves, combined with virialization (see the virial theorem). Although CDM density is thought to be negligible on scales of our solar system, it dominates on the scale of galaxies.

A point of interest here is that since CDM doesn't "clump" in the same way that baryonic matter does, the CDM cloud making up most of our galaxy (and other galaxies) is though to be shaped in more of an oblate spheroid, rather than a disk. It's density is distributed -- not uniform -- being denser at the center and becoming less dense the farther way one is from the center, but in the general shape of the density distribution is of an oblate spheroid.

But the baryonic matter that we see does in fact "clump" much easier than this CDM. That's why galaxies are pancaked shaped when taking into account only the baryonic matter. But if you consider the dark matter (according to many popular CDM theories), galaxies are anything but flat.

[Edit: I admit I might be oversimplifying things a little, but my point is that the baryonic matter "clumping," due to inelastic collisions, combined with conservation of angular momentum, is what results in "flat" objects like solar systems, the rings of Saturn, the accretion disk of a black hole, and even galaxies (although the CDM in galaxies is a special case, and is not thought to be flat simply because CDM is not thought to undergo inelastic collisions with the exception of emitting gravitational waves, before virialization is complete).]

 

[ShayanJ]

The video is pretty much right on the money when it comes to "clumping." Combine this conservation of angular momentum, and you end up with a roughly flat solar system, and roughly flat galaxies.

When baryonic matter collides, much of the energy is converted to heat (inelastic collisions), and ultimately radiated as electromagnetic radiation. Angular momentum is conserved, however. The result is the system "flattens out," so to speak, as the baryonic matter clumps into larger dust particles, planets, asteroids; and in the case of galactic scales, stars.

Contrast that with cold dark matter (CDM). According to most CDM theories, the dark matter particles do not interact with each other in any way except gravitationally. They don't interact with baryonic matter electromagnetically, or even with other CDM particles. The only way CDM particles lose energy is through gravitational waves, combined with virialization (see the virial theorem). Although CDM density is thought to be negligible on scales of our solar system, it dominates on the scale of galaxies.

A point of interest here is that since CDM doesn't "clump" in the same way that baryonic matter does, the CDM cloud making up most of our galaxy (and other galaxies) is though to be shaped in more of an oblate spheroid, rather than a disk. It's density is distributed -- not uniform -- being denser at the center and becoming less dense the farther way one is from the center, but in the general shape of the density distribution is of an oblate spheroid.

But the baryonic matter that we see does in fact "clump" much easier than this CDM. That's why galaxies are pancaked shaped when taking into account only the baryonic matter. But if you consider the dark matter (according to many popular CDM theories), galaxies are anything but flat.

[Edit: I admit I might be oversimplifying things a little, but my point is that the baryonic matter "clumping," due to inelastic collisions, combined with conservation of angular momentum, is what results in "flat" objects like solar systems, the rings of Saturn, the accretion disk of a black hole, and even galaxies (although the CDM in galaxies is a special case, and is not thought to be flat simply because CDM is not thought to undergo inelastic collisions with the exception of emitting gravitational waves, before virialization is complete).]

So what about elliptical galaxies?
What you say implies that all "flat" types of galaxies are made up of only baryonic matter and its only elliptical galaxies having some dark matter. Also this means the existence of elliptical galaxies is a proof of the existence of dark matter!
These are at odds with my knowledge about dark matter. Of course my knowledge could be too narrow here!

 

[collinsmark]

So what about elliptical galaxies?
What you say implies that all "flat" types of galaxies are made up of only baryonic matter and its only elliptical galaxies having some dark matter. Also this means the existence of elliptical galaxies is a proof of the existence of dark matter!
These are at odds with my knowledge about dark matter. Of course my knowledge could be too narrow here!

No, that's not what I mean. Galaxies, ours and the rest, are made mostly of dark matter. Don't forget that for ever pound of bayonic matter there is four or five pounds of dark matter (according to CDM theories). When galaxies were formed, dark matter played the fundamental part of their creation. Baryonic matter came along "for the ride," so to speak. Baryonic matter clumped when the dominant dark matter went through virialization. Dark matter did not clump since it doesn't interact directly. Baryonic matter did.

But maybe that's not relevant to the question at hand here. When you see an elliptical galaxy, it's elliptical because of its orientation, with respect to our viewing angle.

[Edit: Dark matter may be dominant on galactical scales and larger, but is not necessary for scales such as star systems or smaller, where dark matter density is negligible. By that I mean dark matter particles shoot through our solar system almost as though our solar system didn't even exist, as they orbit around the center of our galaxy, more or less.]

 

[collinsmark]

Oh, and I should point out that when two or more, otherwise "flat" galaxies of similar size merge, the result (elliptical galaxy) may not start out flat if the constituent galaxies' planes were different. In time, the result will flatten, but may take longer had the original galaxy formed from a single cloud (maybe a lot, lot longer -- once the galaxy is already clumped into stars, collisions are very unlikely). Still, it's the baryonic matter's inelastic collisions that are the primary contributor to the eventual flattening. My point is that the "clumping" tendency of baryonic matter, combined with conservation of angular momentum, is what causes such things to flatten. Remove the inelastic collisions, and the clumping (i.e., CDM, neglecting gravity), and you don't get the flattening.

 

[Ken G]

Ellipticals and spirals differ in one very important way-- their ratio of angular momentum to kinetic energy. Or more correctly, since angular momentum and energy have different units, we can get a unitless result by taking the ratio of angular momentum to kinetic energy times characteristic orbital period. That ratio is always the key to how disklike any gravitationally bound object is. The stars in a spiral galaxy have a large such ratio, the stars in an elliptical have a small such ratio. So it's not the viewing angle, there's a fundamental difference there. Mergers can indeed cause angular momentum cancellation, reducing that ratio, but I think an important consideration is how the baryonic gas can lose angular momentum to the dark matter. A key difference between ellipticals and spirals is that only the latter has a lot of gas and dust, and that gas and dust forms the young stars that have the high ratio of angular momentum to energy times period and gives the spiral galaxy its appearance. The idea that spirals evolve into ellipticals has been bounced around over history, I'm not sure what is the current thinking on that. It's possible you can also form ellipticals from the outset, if the gas loses it angular momentum somehow.

 

[collinsmark]

That's a good point, Ken G. If you don't have significant angular momentum perhaps because of numerous mergers, combined with less gas/dust due to the galaxy being older (most of the potential baryonic clumping has already clumped), then there's not much to push the galaxy to flatten: the baryonic matter is [more-or-less] virial too in such cases. Good point.