For the Universe I'm creating Humanity has devolved into groups of scattered tribes. This is due to the fact we ran afoul of a really nasty alien race, who decided to leave us homeless. A coalition of races had banded together to fight this menace, but mankind ended up with the sort end of the stick. It is the Great Diaspora, and we are only hanging on by our fingernails.
The problem I'm having is how to so thoroughly destroy, or at the very least render uninhabitable, the places Man once called home. If it were only Earth it would be a simple enough matter, but by the time the attack takes place he also has colonies on Luna, Mars, Ceres, Callisto, and Titan. How can I muck them up badly enough that we have to now seek a Promised Land elsewhere?
Ceres I figure is small enough it could just be reduced to rubble, but what about Luna? If you destroy the colony there, what's to prevent them from setting up new ones a few hundred miles away?
At the moment all I can think of is a "Gray Goo" weapon, short of blasting multiple planets and moons to smithereens. I'm hoping there's another option out there I haven't considered.
EDIT: I'd like to hold onto the Sun. You know, sentimental reasons. :)