This is my first post here (yay for me), I'm just wondering how exactly a planet (or for that matter, two stars in a binary star system, stars in a galaxy, etc, etc) are able to maintain a roughly constant orbit (same maximum/minimum distance from the centre of mass of the system). I'm guessing it's due to the fact that as they approach the centre of mass of the system their gain in velocity causes them to shoot past each other in a sense, even if this is on a nearly circular orbit (eg the earth around the sun). Lastly, is there any reason why the planets (with the exception of pluto) orbit the sun in a nearly circular path, and not one a bit more stretched to the side? Thanks for reading and hope someone can help.

I'm not really sure about this, but I think there may be two questions in your question. One is simply about orbital mechanics, i.e. why an object remains in the same orbit? And the second is why the planets, specifically, got in the more or less circular orbits that they have.

I think the generally accepted answer to the second question is that the solar system started as a nebula which condensed, so all the planets formed from a rotating cloud. Generally, things with highly eccentric orbits (things that come close to the sun and then go far out, like comets) may be "captured objects."

One mystery about the idea that the sun and planets all coalesced out of a rotating cloud is why most of the rotational energy is in the planets (mostly Jupiter) and not the sun. The sun has almost all the mass of the solar system, but only a small part of the angular momentum.

And about the first question, I'm not really sure if this is the answer you're looking for, but in fact there are objects that orbit other objects (like the sun) in very eccentric orbits, and their orbits don't necessarily get more circular. So in a sense, the planets "happen" to be that way. BTW, Pluto's orbit is more eccentric than those of other planets, so sometimes it is closer to the sun than Neptune.

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Welcome to the BABB Managore

It sounds like you are asking about the two body problem, where two bodies orbit each other.

A single planet in a circular orbit about a star will go in a circle around the center of mass--and so will the star, but the star will make a much smaller circle, staying opposite of the planet. Of course, adding just one more body makes the situation much more complicated.

I had a similar question about this, and didn't really want to create another post. Why are almost all the planets (except for pluto) in the same plain. You know? Why isn't there anything rotating around the sun at a 90 degree angle from the plane all the other planets rotate in? I hope you understand what I'm asking. Thanks!

Oh. This is my first post!! =D>

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Welcome to the board!

The accepted theory for many years has been that the planets condensed ('accreted') out of a rotating disk of gas and dust. That theory is now largely borne out by direct observation of 'circumstellar disks' around young stars like Vega and Fomalhaut. So the central part of the solar system is orbiting roughly in the plane of the disk from which it formed.

Of course, as you are about to point out, this only shuffles the problem along one. Given that stars and their attendant solar systems form from non-planar globules, why do they collapse into a disk in the first place? (Seems to be the natural way of things - it's what galaxies do too - we now think many elliptical galaxies are the result of colliding spirals that mutually disrupt their neat disks.)

Anyway, that's a harder answer (someone will have a better one than me, I'm sure, so don't take my answer as read) and I suspect chaos theory creeps in somewhere.

I think it is ultimately because of galactic tidal effects, causing the initial cloud to rotate. If you rotate an object, it will bulge around its equator, as does Jupiter in our system. The cloud is far larger but vastly more diffuse than Jupiter, and it is also contracting, which will amplify the rotation (the old 'skater's arms' effect.) We are conserving something called angular momentum here.

Eventually the 'bulge' in the star-forming cloud becomes quite extreme - a disk. And in fact the disk of the solar system contains most of the angular momentum in the system (though other processes may also contribute to that).

As for objects outside the plane, 'Xena', the new planet, is one in fact - inclined at 45 degrees or so. The long-period comets, which are thought to come from a vast sphere called the Oort Cloud, also have no preferred inclination. But it is thought that all these 'out-of-plane' objects were ejected from the inner system by gravitational interactions ('slingshots') later on.

[Edited for sense, duh.]

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Parts of the rotating cloud near the poles have little or no lateral velocity, so as the cloud contracts they fall toward the center unimpeded. Whereas parts of the cloud near "rotational equator" have significant lateral velocity, and eventually end up in orbit. All thus resulting orbits are more or less in the same plane.

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Actually, this makes a certain amount of sense. The bits of matter that had an initially high angular momentum, and didn't lose it through collisions, remain far away. To get close to the center, though, a bit of matter would have to have a small angular momentum, so all of the bits that eventually formed the Sun ended up being precisely those bits that did not have a large angular momentum.