Question

Explain why there is only one planet in each orbit around the sun?

Answer 1

It turns out that you can have more than one planet in the same orbit. Some studies indicate that this is actually what you would expect to have happened in the early solar system.

But then, over time, due to perturbations from other planets, the orbit would become chaotic and the inevitable collision between the two planets in the sameorbit would have happen. See e.g. this article:

http://arxiv.org/abs/astro-ph/0405372