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?


1 Answer 1


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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    $\begingroup$ Very interesting, I've been wondering about this everysince I learned what a solar system was. +1 $\endgroup$ Mar 16, 2013 at 14:20
  • $\begingroup$ 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 $\endgroup$
    – e-motiv
    Mar 4, 2014 at 11:16
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    $\begingroup$ My question is why can there not exist, say, two orthogonal axis of rotations, for various planets to revolve around and not colliding? $\endgroup$
    – Hans
    Jul 9, 2014 at 18:04
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    $\begingroup$ @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. $\endgroup$ Oct 24, 2014 at 6:46
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    $\begingroup$ @Hans Sorry, I missed the planets part and was thinking dust. For planets the gravitational interactions will make the orbits chaotic over time, unless they are far apart. Get too far and you're unlikely to align on a plane. The Oort cloud is spherical rather than planar, for example. Closer to the star and the angular momentum, collisions, and interactions during the system's beginning stages will flatten things to nearly coplanar. $\endgroup$ Oct 24, 2014 at 15:23

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