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I've been wondering for a while now why some objects in the universe (such as stars, planets, etc.) are round, while others are flat (galaxies, planetary rings and protoplanetary disks).

I know these shapes come to be because of gravity. In planets, the gravity from the core pulls everything to the centre, thus, rounding the planet (same with stars). But with Saturn, for example, why have its rings formed as rings and not as a spherical layer of ice surrounding the planet?

The same with the Kuiper belt and asteroid belt, I would presume the mechanics there are the same as with a planetary orbit, hence, why they are belts and not clouds. But then what about the oort cloud? Why is that a huge cloud in the outermost solar system and not a huge ring of ice.

As for the solar system itself, I've read that the solar system is kind of egg-shaped, but shouldn't it be just like the galaxy (or the other way round)?

I've tried looking into this and wondering about what different laws of physics may apply in each case, but it's all really confusing.

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I tend to see it as simply a consequence of symmetry, symmetry breaking and the balance between them. The simplest is the spherical symmetry where every direction are the same, as good as another one (isotropy). It's the case for the Sun, planets, globular star clusters, ... If there is some (sufficent) rotation involved, this isotropy will be broken, there will be a distinguished direction and this will give planar objects: spiral galaxies, planetary rings, ... or even sometimes linear objects like a relativistic jet. Of course, when there is some rotation involved but not much strong you will have some intermediate objects: the Earth is an ellipsoid, elliptical galaxies, ...

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Preliminary note: there is a continuity of more or less flattened shapes. No macroscopic objects are really spherical, or really flat. Take Jupiter: it has a rotation period around its axis of 10 hours, and you can clearly see that it is an ellipsoid. It's the centrifugal force, combined with the gravitational attraction, that shapes these objects.

Now to your question: Planets and stars are dense bodies, and rotate as single objects. On the other hand, the Oort cloud, Saturns rings, stellar clusters and galaxies are made of independent particles, free-floating in lots of empty space. Depending on the formation history and gravitational environment (e.g., Saturn and its moons) of these entities, the orbits of these individual particles can be more or less co-aligned, and form a more or less spherical clouds.

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  • $\begingroup$ So essentially, it's inertia combined with gravity that causes a planet to have it's rounded shape. So the outside material is travelling fastest and it acts like it's orbiting the core, except all the material is held together. That makes sense to me. But it terms of large groups of 'independent particles' as you mentioned, that model sounds quite similar to an atom. In an atom, the electrons orbit all around the nucleus, giving it an apparently spherical shape. So why is it not the same with galaxies? Why do some stars not orbit vertically rather than horizontally (from our reference frame)? $\endgroup$
    – AkThao
    Sep 1, 2016 at 10:31
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    $\begingroup$ Many galaxies do contain stars with "randomly" oriented orbits, and without any disk structure. They are called elliptical galaxies (click for images). The reason for the diversity in stellar orbits is the formation history of galaxies. The scientific consensus is that galaxies are generally formed by "mergers" of smaller galaxies, often many of them over billions of years. Such mergers happen in random directions (not only in the plane of a disk), and therefore often result in a "mess" of orbits. $\endgroup$
    – mtewes
    Sep 1, 2016 at 18:24
  • $\begingroup$ Oh ok, that makes better sense, so it's all to do with how they form and in what direction they form. $\endgroup$
    – AkThao
    Sep 1, 2016 at 20:21

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