Take the 2-minute tour ×  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 add comment  2 Answers  up vote 28
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