Take the 2-minute tour ×
Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. It's 100% free, no registration required.

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?

share|improve this question
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

2 Answers 2

up vote 35 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.

share|improve this answer
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 '14 at 11:16
My question is why can there not exist, say, two orthogonal axis of rotations, for various planets to revolve around and not colliding? –  Hans Jul 9 '14 at 18:04
@Hans Then the discs would intersect and their material would interact, dispersing and rearranging the material into a more stable configuration. What that configuration is depends on the initial conditions. –  zibadawa timmy Oct 24 '14 at 6:46
@zibadawatimmy: Why do the planets belonging to two orthogonal axis of rotation have to be "discs" as you say and interact? The simplest counterexample is two concentric circles of different radii perpendicular to each other. –  Hans Oct 24 '14 at 14:21

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.

share|improve this answer

protected by Qmechanic Sep 6 '13 at 8:14

Thank you for your interest in this question. Because it has attracted low-quality answers, posting an answer now requires 10 reputation on this site.

Would you like to answer one of these unanswered questions instead?

Not the answer you're looking for? Browse other questions tagged or ask your own question.