The Basics Regarding : Planetary Orbits and Whatnot

[JulianBoolean]

Q: Why is it that objects in orbit around other objects, tend to do that in elliptical patterns, and not circular patterns? I can picture objects of great mass, planets and stars warping the fabric of space time, and how moons and whatnot get caught spinning in the orbit. But can't wrap my head around the elliptical aspect of the deal.

Q: Why is it that our galaxy is (very loosely speaking) disc shaped? I can picture how the spiral arms are created from the spin, and I can picture how the black hole at the center (perhaps with the aid of dark matter) keeps everything together. But why is it pancake-like, rather than sphere-like, in shape?

Thanks!

 

[Forestman]

That is a good question, I hope that someone answers it, I would like to know that one myself.

More than likely though it probably takes a deep understanding of the mathematics of general relativity. Maybe someone here can explain it simply though.

If I had to guess though I would say that it has to do with the fact that things many times don't start out in circular orbits, so they end up as ellipses.

 

[russ_watters]

Q1: A circle is just a special case of an ellipse, so a circular orbit would still be elliptical. Being such a special case would be unlikely given all the things that work to cause orbits to not be perfect (such as Jupiter).

Q2: There are some complicated dynamics at work, but a couple of easy to visualize issues:
1. If the objects were on different planes, they would collide with each other.
2. If a big cloud is rotating uniformly, the objects way above it would just be making little circles, they wouldn't be orbiting the center.
3. If objects orbit above or below the plane, the gravity of the plane will tend to pull them toward the plane.

 

[marcus]

... But why is it pancake-like, rather than sphere-like, in shape?
...

What Russ said. Plus you should know that not all galaxies are disk shape. Not every galaxy looks like a pancake.

How a galaxy turns out depends to some extent on its past history---the circumstances under which it formed. There is a lot to this topic. It is an interesting question, how galaxies get their various shapes.

 

[JulianBoolean]

That is a good question, I hope that someone answers it, I would like to know that one myself. More than likely though it probably takes a deep understanding of the mathematics of general relativity. Maybe someone here can explain it simply though. If I had to guess though I would say that it has to do with the fact that things many times don't start out in circular orbits, so they end up as ellipses.

Forestman, Thanks that got me thinking on the right track. :)

What Russ said. Plus you should know that not all galaxies are disk shape. Not every galaxy looks like a pancake.How a galaxy turns out depends to some extent on its past history---the circumstances under which it formed. There is a lot to this topic. It is an interesting question, how galaxies get their various shapes.

Cool, I thought all galaxies were disc shaped. Is is possible to say that most are?

Q1: A circle is just a special case of an ellipse, so a circular orbit would still be elliptical. Being such a special case would be unlikely given all the things that work to cause orbits to not be perfect (such as Jupiter).

Q2: There are some complicated dynamics at work, but a couple of easy to visualize issues:
1. If the objects were on different planes, they would collide with each other.
2. If a big cloud is rotating uniformly, the objects way above it would just be making little circles, they wouldn't be orbiting the center.
3. If objects orbit above or below the plane, the gravity of the plane will tend to pull them toward the plane.

Q1 : I think my thought experiment was correct to a certain point, but flawed at the very end. Planets are round, and the fabric of space time is being deformed around it, which implies that the fabric of space time around a round planet will be circular not elliptical. However, just because the fabric of space might be circular, it doesn't meant that object must follow the warp shape. I'm picturing a bowling ball on the center a trampoline. Shoot a marble at just about any trajectory and it's going to follow an ellipse. Getting a perfectly circular orbit would be possible, but you'd have to shoot it at just the right speed and direction.

Q2 : Still struggling with that one a bit. Can't really grasp the three things you are pointing to

Point 1 : Seems like maybe a typo? How would objects on dif planes collide?
Point 2 : Not getting that at all.
Point 3 : Well that would certainly do it. I can accept that new info (gravity at the center plane is stronger) at face value, but kinda curious as to how that happens? And is it the same phenomena that have forced the rings of Saturn and Jupiter to the center?

Thanks! awesome conversation, much appreciated

-Julian

 

[Nik_2213]

Just a small point on your Q1: Your planet may be 'round', but it may not be *spherical* due to rotation, tectonic mass distribution, solar tides etc etc. The effect of these, plus axial tilt etc etc mean that even a perfectly circular initial orbit will soon become slightly elliptical...

 

[redwood973]

Centrifugal force always tends to flatten things doesn't it? I would assume galaxies to be flat due to that force.

 

[FtlIsAwesome]

Look here for information on types of galaxies.

http://en.wikipedia.org/wiki/Galaxy_morphological_classification"

The most basic division is spiral/disk galaxies and spherical galaxies, but the classifications shown in the link are more detailed.

Also there are irregular galaxies, ones that have randomish shapes. The two Magellanic Clouds are notable examples. Though the Magellanic Clouds have been found to be a sort of hybrid of irregular and spiral types.

Centrifugal force always tends to flatten things doesn't it? I would assume galaxies to be flat due to that force.

Well, that's more for solid objects. Galaxies are several separate objects.
I would also like to point out that a few of the exosolar systems we've discovered seem to have planetary orbits not on the same plane as our solar system, though I can't think of any specific examples.

 

[Chronos]

Due to distance from the central mass and the finite speed of light, all orbital bodies are subject to frame dragging effects. This forbids perfectly circular orbits.