Humanity had colonized all 9 (at the time of the graphic novel's writing, Pluto was a planet) planets thousands of years previously. It's likely that we had left our solar system long after, and we'd already gone into the afterlife (why someone didn't start having Luthor help is beyond this troper). So yes, humanity's homeworld was gone, but in it's place is a human empire. * All the same, it's a pretty big loss of life.