What do planets and galaxies exist as planes?

RolandIRL

hi all,
just wondering why do the planets and galaxies exist in flat planes?
i think my maths physics lecturer mentioned it in passing but i can't remember. why don't the planets orbit like eg the infinity symbol around the sun instead of elliptical orbits?

thanks

WBC

Not all galaxies are in flat planes for a start. The ones that are in a flat plane is simply to do with them rotating. As they rotate the galaxies takes on the shape of a disc (gravity can't hold them in the shape of a sphere as their momentum causes them to spread out), this is similar to the formation of a solar system such as ours.

The orbit of the planets can be derived from Kepler's planetary motion equations and Newton's universal gravity law. From these equations you arrive at the conclusion that we get elliptical orbits. Also from the above solar system form from interstellar dust and gas clouds. These coalesce into a more dense structure under the effect of gravity and begin to spin (this is due to conservation laws).

The cloud of dust and gas due to the rotation has spread out into a thin disc. As the centre the density reaches a critical point where a star can form. The star formation trends to blow away a lot of gas and debris from near the centre and leaves the heavy particles behind (ie the rocky planets). From the remains forms the planets which maintain their original orbits which we see.

Apologies if some of this doesn't make sense as I am a bit sleepy crawly boards at 1.30 in the morning :P

de_Selby

WBC wrote: »

Not all galaxies are in flat planes for a start. The ones that are in a flat plane is simply to do with them rotating. As they rotate the galaxies takes on the shape of a disc (gravity can't hold them in the shape of a sphere as their momentum causes them to spread out), this is similar to the formation of a solar system such as ours.

You've kind of dodged the explanation a little bit here, if a galaxy is rotating and is in the shape of a sphere then objects off the axis of rotation will get attracted towards that axis, while for objects on the axis of rotation there will be a centrifugal force acting against it. So those objects off the axis will oscillate up and down as well as rotate around the centre, but they will also be braked in the direction perpendicular to the plane as they pass through it (from collisions with all the other particles, magnetic fields from those particles etc) => you end up with a disk. If there was no friction that material would be perfectly happy to oscillate away in a spherical shape. Those friction forces are most efficient when the material is gaseous and dusty, not when you have a lot of stars and rocks (because collisions are more likely, and the particles are smaller so magnetic fields can affect their motion more easily) so all this happens when the galaxy is relatively young.


The same holds for the clouds of dust from which solar systems form.

(By the way, elliptical galaxies, or non-spiral galaxies, consist of stars that are rotating in random planes. They are thought to be the result of a collision between two spiral galaxies though, and they are observed not to have a lot of interstellar medium, so there isn't efficient breaking occurring.)

So that explanation is fine, except you need the galaxy to be rotating in a preferred direction in the first place. If you think about it, if you have any cloud with all the particles moving in some random directions, firstly gravity will make all the material orbit around the centre of mass. Now if there is any instability at all in the direction of all that random rotation around the centre of mass it will quickly grow, because it will attract more material to that plane of rotation and because of the breaking effects. So you really just need a seed or a really small push to start the rotation in a preferred direction. Something which might cause that is the intergalactic magnetic field, which is a weak magnetic field that permeates all of space, even outside galaxies. No one really knows what causes the intergalactic magnetic field.

So in a way a short answer to that question could have been "We don't know"! (Which is always the best answer anyway).