system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do orbits tend to be on the same plane? Is there some reason that perpendicular orbits do not exist?

As far as I know, planetary orbits around our solar system and other planetary systems all tend to be along a flat plane, and Pluto's odd, highly elliptical and slanted orbit is quite an anomaly. Given that space is in 3D, what makes the orbits stay in this coplanar configuration?

EDIT: Thanks for all the great answers, especially that video. Once I figured out that angular momentum essentially defines the plane, my question sounded really silly but I guess that's why ELI5 exists!
gpt
Planetary systems are believed to come about from big balls of dust/gas collapsing. The center forms the star, then rest forms the planet. If the original dust/gas ball has any spin at all, that angular momentum must be conserved as everything collapses, causing the whole mess to form a disk-shape before we managed to get any planets. Once you're in the disk shape everything is basically almost in one plane, so planets that form from there will all have very similar orbital planes unless something wierd happens.