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I always see pictures of the solar system where our sun is in the middle and the planets surround the sun. All these planets move on orbits on the same layer. Why?

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Possible duplicate: physics.stackexchange.com/q/8502/2451 Related: physics.stackexchange.com/q/12140/2451 and links therein. –  Qmechanic Sep 6 '13 at 8:21
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up vote 31 down vote accepted

We haven't ironed out all the details about how planets form, but they almost certainly form from a disk of material around a young star. Because the disk lies in a single plane, the planets are broadly in that plane too.

But I'm just deferring the question. Why should a disk form around a young star? While the star is forming, there's a lot of gas and dust falling onto it. This material has angular momentum, so it swirls around the central object (i.e. the star) and the flow collides with itself. The collisions cancel out the angular momentum in what becomes the vertical direction and smear the material out in the horizontal direction, leading to a disk. Eventually, this disk fragments and forms planets. Like I said, the details aren't well understood, but we're pretty sure about the disk part, and that's why the planets are co-planar.

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Very interesting, I've been wondering about this everysince I learned what a solar system was. +1 –  OmnipresentAbsence Mar 16 '13 at 14:20
    
I find this clearer: With a heavy enough cloud left around the just collapsed cloud (=sun), 2 reasons often evade further collapse into the sun. 1. Solar wind 2. The angular momentum. Subtracting this attraction towards the center leaves a kind of "net attraction" perpendicular to it (in each point of the cloud). So logically this eventually forms a disk with the net angular momentum. (There is a chance for some chunks to not follow the disk, but it becomes smaller with its deviation and with each additional one. Pluto?) Similar:en.wikipedia.org/wiki/Protoplanetary_disk#Formation –  user41785 Mar 4 at 11:16
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It is a plane because we are orbiting the center of the galaxy very fast. Our eliptic is realy a spire with the sun the most massive object of our solar system draging us around with it.

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protected by Qmechanic Sep 6 '13 at 8:14

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