system
Given the following post, respond as if you're commenting on Reddit:
human
Why don't planetary orbits have more than one planet in their orbit e.g. the other side of the sun?
gpt
Stability is the problem. Perturbations from the other planets would usually cause the co-orbital planets to drift and eventually collide.

This may have happened to the early Earth. It's fairly solid theory that a Mars-sized body known as Theia caused a cloud of debris that coalesced into the Moon. Where Theia came from is unknown and one hypothesis is that it formed at the Sun-Earth L4 or L5 point, sharing Earth's orbit but 60 degrees ahead or behind. This was stable as long as Theia's mass was small, but once it grew to about 10% the mass of Earth it was destabilised and its orbit drifted until impact.

There are co-orbital situations in moons of the solar system. For example Janus and Epimetheus, moons of Saturn. One orbits a little higher and slower and the other a little lower and faster, and rather than colliding they "swap orbits" when they approach close and therefore drift apart again, and eventually have another close approach the other way round and swap back and the cycle continues. So such orbital arrangements *can* be stable but it depends on the details.