Early theories of planetary formation:


The worlds come into being as follows:
many bodies of all sorts and shapes
move from the infinite into a great void;
they come together there
and produce a single whirl,
in which, colliding with one another
and revolving in all manner of ways,
they begin to separate like to like.

- Leucippus (~480-420 B.C.)

There cannot be more worlds than one. 

- Aristotle (384-322 B.C.)