Popular Celestial Mechanics

The discussions that all the planets in Whedons Firefly universe are
in
a single system brings up an interesting question.
Has NASA or someone worked out what different kind of planetary
systems could exist and/or sup****t life?
and address some of the examples in SF
Krypton
The twin planet in the same but opposite end of the orbit
such as Marvels Counter-Earth, and Journey to the Far Side of the Sun
can that remain stable?
how many planets can you pack into a system?
the two sun system of 2010
certainly the math and physics is more predictable
than the biology of alien life.

tphile