Saturn and its rings, our solar system, the Milky Way – Why are they (and the bodies orbiting them) in such a neatly flat shape and not more akin to how electrons are often depicted orbit an atom in all directions?
In: Physics
If you only have *one* object in orbit, it obviously orbits in a single circle — actually an ellipse — and that ellipse forms just one plane.
If you have more than one object in orbit, they could start out in a single plane, but if their orbits cross, then eventually those two objects will smash into each other and become just one object, bringing you back to one plane. (Unless they bounce. We’ll get to that in a moment.)
If the orbits don’t intersect, they will never hit each other. The most likely reason they won’t intersect is because they are *already* in the same plane, orbiting parallel to each other, or close to it.
So a bunch of objects orbiting in the same plane, just at at different distances, are pretty stable — because none of the objects are crossing and hitting each other.
So what about when two objects hit each other and then bounce? Well, what directions do they go after they bounce? It turns out that *on average*, they will bounce most in the direction they would go if they just smashed and stuck to each other.
So if you start out with a sphere cloud of objects orbiting around the center randomly, after they hit each other over and over again millions of times, they will eventually *all* go in their *average* orbital direction, at least until you get to the point where they stop hitting each other. So by following the steps above, the cloud eventually becomes the disc shape that was the original “average”.
Sometimes you can get [objects from outside the system and don’t stay in orbit](https://solarsystem.nasa.gov/news/473/what-we-knowand-dont-knowabout-oumuamua/) (in other words, they’re not part of that average). Such an object can can knock one of the things a little bit out of the plane, because it changes the average. If the system is so mature (like the planets in our solar system are now) that that object no longer crashes into the other objects, it can stay out of kilter.
Finally, let me say that those pictures of electrons “in orbit” no longer represent our best estimates of how electrons behave. We don’t think that electrons anymore like little stones that travel continuously around the nucleus like planets orbit the Sun. Instead we now model electrons as winking in and out of existence in different locations (like Whack-a-Moles) in probabilistic “clouds” that can have various different shapes depending on how many other electrons are nearby. We still use the term “orbit” to describe the locations of those clouds, but we should probably stop doing that, because they don’t involve movement that is in any way like the orbits that are produced by gravity. The behavior of electrons in those clouds is much more difficult to get your mind around, but when you finally understand it, it’s super, super cool. Electrons travel kindof like Dr. Who in his tardis.
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