Why is the solar system, & galaxies seemingly rotate on a 2-D plane?

[Techno-Raver]

Is this true and why?

 

[chroot]

It's not exactly a plane. Every planet's orbit has a slight inclination with respect to the others. Pluto, the Kupier belt objects, and comets all have orbits of pretty much random inclination. The reason the planets' orbits approximately planar is because they all formed from the same nebula, rotating in the same direction, and this formation, orbiting around a common axis, conserves the nebula's original angular momentum.

In the same way, a spinning ice skater who pulls her arms inwards spins faster, but does not suddenly acquire a tilt.

- Warren

 

[Soul Surfer]

That's really only part of the story chroot. I think it is important to explain why a rotating blob collapsing under gravity tends to become a disc.

The anwer is as I believe that as various parts interact gravitationally or collide they tend to be disturbed and thrown into different orbits but those that are in a disc tend interact rather less than those in the blob and so the disc structure gradually dominates.

 

[tony873004]

The rotation of a spherical cloud about an axis prevents collapse towards the axis, but does not prevent the cloud from collapsing parallel to the axis of rotation, so it flattens into a disk.

 

[Astronuc]

And then we have globular and open (galactic) clusters:

http://en.wikipedia.org/wiki/Globular_clusters

http://en.wikipedia.org/wiki/Open_cluster

which are distributed more in 3-D than 2-D.

 

[Soul Surfer]

I agree Astronuc that globular clusters tend to be an exception to this rule. Open clusters are generally not stable relaxed structures and can be discounted.

I have looked many times for a published theory of the origins of globular cluster structures and not found one.

I have my own and that is as follows:-

Early in the univese very large clouds started to condense and got well on the way to forming a star but the centre collapsed quicker and formed a very large star which rapidly evolved and exploded while the outer parts of the cloud were still collapsing. The violent explosion of the first star did not disperse the cloud it was to gravitationally bound for that but the turbulent shock wave propagated outwards and precipitated the formation of many stars. Once the stars had formed they are so small that the probability of interaction is low enough to prevent the collapse into a disc or delay it for so long that they are still largely globular.

 

[SpaceTiger]

The rotation of a spherical cloud about an axis prevents collapse towards the axis, but does not prevent the cloud from collapsing parallel to the axis of rotation, so it flattens into a disk.

This is basically correct. As long as a cloud is in the form of gas, it is relatively easy for it to lose energy (by, for example, viscous forces), but not angular momentum. As it loses energy, it collapses (see a recent thread on the https://www.physicsforums.com/showthread.php?t=121847"), and as it collapses, the faster it rotates. The end result is a disk. Examples of this are quasar accretion disks, protoplanetary disks, and spiral galaxies (where the disk stars formed from gas that had already collapsed to a disk).

If the system is composed entirely of stars or dark matter, however, then it is hard for it to lose either energy or angular momentum. This means that the collapse will be slow and the rotation rate relatively small. Such is the case with globular clusters and elliptical galaxies.

 

[laurelelizabeth]

In the same way, a spinning ice skater who pulls her arms inwards spins faster, but does not suddenly acquire a tilt.

True :)

If we are trying to do something like a layback spin and don't keep balanced as we're spinning it goes really weird for some reason, unlike if you'er just standing still and trying to do a layback position. I guess that's just because it's hard to tell which way you're falling when you are still spinning. Sorrry if that was completely irrelevant.

Would the planets that aren't exactly on the same plane have been hit by a meteor or something, that would knock it sort of to the side?

 

[stevebd1]

I remember being asked the same question about a year ago in respect of a video I posted on another website of the eventual collision between our galaxy and the Andromeda which promted me to look for an answer. I find a website called 'Curious About Astronomy' that had made some attempt at answering the question. I've cut and pasted the question & answers below directly from the website (hopefully this is within Physics Forums policies). The answers certainly shed some light on the subject matter.

Curious About Astronomy website- http://curious.astro.cornell.edu/


Extract 1-

Expert: Tom Whiting
Date: 5/23/2006
Subject: flat solar systems and galaxies


Question -
This is something I've wondered about for a while: Why is it that solar systems and galaxies are generally flattened - shouldn't gravity pull everything uniformly together into just one big ball? I believe I've heard it has to do with the spinning motion, but I don't understand how that works.


Answer -
Well, not all galaxies are flat, in fact most galaxies ARE of the elliptical kind, are all pretty much spherical in shape...check out M's 84, 86, 87, etc in the Virgo Supercluster. And most of the dwarf galaxies surrounding the Milky Way..Leo I, Maffei
I and II, etc. are spheroidical.
70 to 80% of all galaxies are of the elliptical type (spherical in shape); only 20-30% are the spiral, or barred-spiral type.

Yes, the spin, or rate of spin, is the key.
More rapid spinning objects, relative to their size, will evolve into a flat pancake-like plane...Saturn's rings, the Solar System, a spiral galaxy.
Very slow spinning, or non-rotating objects, will assume a spherical form...globular clusters and old elliptical (spherical) galaxies for example...
and the entire known visible Universe itself, believed to
have a very slow spin rate, thus it is thought to be spherical
in shape too.
With the rapid spinning objects, some of the particles tend to migrate into the equatorial plane of the object instead of
"falling" toward the main body (many do in fact fall right into
the main body)...thus producing the flat pancake shape.
But realize with say, our Solar System, we can be certain
that many planetisimals also fell directly into the sun too,
4.6 billion years ago. But some of the outer planetisimals would migrate toward the equatorial region of the body...
and once a ring of material forms, that gravity of the ring would induce even more material to fall down into the ring structure, as opposed to falling directly into the main body.

So yes, the spin rate is responsible for some of the material
falling into an equatorial plane and forming a flat ring.
Clear Skies,
Tom Whiting
Erie PA


Extract 2-

Expert: Tom Whiting
Date: 5/24/2006
Subject: flat solar systems and galaxies


Question -
This is something I've wondered about for a while: Why is it that solar systems and galaxies are generally flattened - shouldn't gravity pull everything uniformly together into just one big ball? I believe I've heard it has to do with the spinning motion, but I don't understand how that works.


Answer -
With the rapid spinning objects, some of the particles tend to migrate into the equatorial plane of the object instead of
"falling" toward the main body (many do in fact fall right into
the main body)...thus producing the flat pancake shape.


Question -
Thanks for the prompt response, but WHY do particles tend to migrate towards the equatorial plane in rapidly rotating objects?


Answer -
Hi Aryeh,
Well, it's one of those 3-D spherical trigonometry vector
thingy's...(Sorry, I slept through that class 40 years ago)! ;-)

Take any particle, say at the 10 O' Clock position on a 3-D diagram of an oblate spheroid rapidly rotating...
You'll have one vector of gravity pulling the particle
inward toward the center of the spheroid;
You'll have another orbiting vector, centripetal force, going tangent to the rotating sphere, and;
You'll have a third small vector down toward the
equatorial plane, which will grow in size (length) as more and
more matter gathers onto the orbiting ring structure in the
equatorial plane.

The resultant force of those 3 force vectors leads the body,
eventually, down onto the equatorial plane orbiting the
rotating "football"...
I'm not much into the higher math, but that's basically
how material, eventually, migrates down into a flattened
pancake around the rotating "football"... it's all vector
relationships.
Obviously it works, because we DO have Saturn's rings,
the Solar System, and the Spiral galaxies, etc. as examples in nature.
Hope all this helps,
Clear Skies,
Tom Whiting
Erie, PA


regards
Steve

 

[Hugh Rutledge]

The math is interesting, but it would be nice if someone modeled this experimentally.

 

[Vanadium 50]

This thread is four years old.