# Planetary orbits and the ecliptic

Last night Neil deGrasse Tyson was on The Daily Show, talking about water on the moon and methane on Mars and other such things (it was an excellent episode, worth watching btw). Anyway, it got me thinking about the little astronomy I’ve studied, and a few minutes with Google confirmed the idea in my head that those diagrams you see of the solar system with more or less in the same plane are in fact correct (like this): there’s really only a few degrees of variation between any one planet’s orbit and the ecliptic plane.

I suspect it has something to do with gravity, but all I remember of my studies along those lines are getting hopelessly, helplessly lost trying to work my way through Newton’s work on the three-body problem (the best summary I can dredge up is "the gravity of a body will effect every other body within a ‘system’, and shit gets weird when you have three bodies interacting with each other). So, preferably in smallish words: why’s everything hanging out around the ecliptic?

When the cloud of stuff that later formed the solar system collapsed on itself to form a protostar that later became the sun, it generated rotational momentum. The planets were all essentially spun out of this and put things out along the same plane

Centrifugal effects are the reason.

The solar system started from a nebula (lump of gas). Over time gravitational attraction caused it to collapse in on itself.

The gas in the cloud, while moving fairly randomly, had a net bias in a particular direction. As the gas cloud collapsed this angular momentum is conserved and the rotation sped up (ala a figure skater pulling their arms in speeds up). In time everything gets spinning in the same direction and continuing to speed up.

As the collapse proceeds the centrifugal effect flattened out the gas into a disk. Same as a guy tossing pizza dough gets a flatter and flatter disk.

From that the planets form and voila!

How did the “pizza” become several clumps of “dough”, with just one clump in each orbit?

Gravity, essentially. As tiny rocks tumbled through the gas, they collected things in their path, like a vacuum cleaner. Once they got big enough to have a respectable gravity, they started pulling in other junk.

Aaand voila, I have yet another addition to “Things I assume I don’t know because I assume the solution is far more complicated than the obvious” list, and I feel a bit foolish.

I once heard someone answer this with a snowball, literally. You have a little rock or something just rolling around in the cosmic ‘snow’ and stuff naturally clings to it. The larger it gets, the more stuff clings to it. Let your rock roll around a really big field for a really long time, and eventually you’ll end up with a pretty-much bare field and a really big snowball. And if a smaller (just big instead of really big) snowball wanders into your field, you may get lucky and it’ll just follow your giant snowball around like a puppydog, and now you’ve got a moon! (This last sentence may not be entirely scientifically accurate.)

The technical term for this theory is the Kant-Laplace nebular hypothesis.

Yes, it arises from the way the solar system was formed, not from anything inherent in Newton’s laws. Although the orbital plane of all the proper planets is close to the ecliptic, Pluto’s orbital plane, and, I believe, those of many comets, are not.

You’d be amazed by how much of physics falls into this category. I spend at least as much time un-teaching as I do teaching, shaking out all of the bad ideas students have developed because “it can’t be that simple”.

Actually, having everything in a single plane is the only situation in which the gravitational forces between objects all act within the plane as well.
If you have two planets orbiting at right angles to each other, and they have a gravitational encounter, the planes of both planet’s orbits will be shifted.

Averages, more or less, is the easiest way to explain why they’re all on the same plane.

A big circular ball of gas has an average momentum, a very slow circular momentum. As it condenses, it speeds up, but most of the gas doesn’t have enough momentum to do anything but fall into the center (which is why the Sun is by far the largest thing in our solar system). However, some of it is either going fast enough, or will go fast enough, to maintain an orbit (or multiple orbits), these orbits then normalize with each other because one pulls another “up” while it’s being pulled “down” (up and down are relative, considering, you know, space…), they eventually reside on the same plane, no longer pulling up or down.

The stuff in orbit eventually clumps together due to static, random collisions and eventually gravity (the first two are a far larger factor in dust-clouds, where the latter becomes a larger factor later on, with larger bodies) smooshing stuff together. As bodies gets bigger, they attract more stuff.

And so on, and so on, until you’re left with a (relatively) clean solar system. In fact, other than (4, I’m relatively certain) clumps of non-planetary bodies, most of our Solar System is relatively tidy. (1. Asteroid Belt, 2. Trojans, 3. Greeks, 4. Hildas – I’m excluding the Kuiper Belt because it’s reeeeeehaly far away… but maybe we’ll call it 5.)

And all of those (except the kuiper belt) are actually caused by Juipter’s Lagrange points.
As for moons, a lot of times things get ‘caught’ (Irregular satellites), but often they form where they are because of accretion.

If they form where they are, there’s two possibilities. 1) Coalescence from an impact, like our moon. A very large object hits with enough force to eject matter into orbit, which then clumps together (in the same way planets form) to former progressively larger objects until they usually smash together to form a single large object (like our moon). Or, 2) they form where they are when the planet itself is formed (not exactly when, but in the same time frame), much like planets do around stars.

Quite a few Trans-Neptunian Objects are outside the ecliptic. Just check out Eris, for instance.

It’s fascinating how many there are out there, and how big many of them are. We spent decades wondering whether we’d find a “10th planet,” and in recent years we’ve discovered dozens, and nobody notices (not even sci-fi writers).

Of course, the question is whether those are original planets formed along with the Sun, or interstellar drifters that got captured, however tenuously, by the Sun’s gravity (hence their bizarre orbits).

I think the most common opinion is that they’re Kuiper Belt objects.

Because the lack of considerable density that far out, there’s not enough gravity to normalize their orbits.

Many are, but Eris is technically not-- the Kuiper is more or less on the ecliptic, while Eris more properly belongs in what’s known as the scattered disc.

Weird, distant, cold place out there…

Will the shifts end up with the planets closer to being coplanar? Such that even if a body formed with a tilted orbit, it would end up in the ecliptic if it’s orbit brought it close to other bodies?

Statistically, I think each encounter between any pair of objects will bring their orbital planes into closer alignment. However each individual encounter also has the potential to go horribly nonlinear, perhaps even resulting in ejection of one of the bodies from the solar system.
No doubt there exists an elegant method to describe exactly which factor dominates at various body sizes, particle densities, and orbital periods, but I don’t know what that method is. I don’t think anyone else does either.

Indeed, and I’ve had that talk with professors before. I’ll just blame it on the fact that the last physics class I took was basically quantum mechanics 101, which left me permanently confused about pretty much everything.