I just treated the unless as a rule instead of another sufficient condition.

Still arrived at the right answer, but probably still need to understand embedded conditionals better.

I'm kind of oblivious to astronomy, so I was wary of making the assumption that comets can only come within a planetary system.

If there are none inside the planetary system, that doesn't necessarily mean they won't come bombard you from the outside (in my books).

I chose (C), because if there are low chances that any one large planet can arise, then those chances are even lower for TWO of them. Not a slight on LSAT writers, I need to do better!