I'm betting either on it all being a sim, or in a follow-up game/DLC we pull the ol' switcheroo with Earth, using time travel to snatch some other equal-sized planet and zapping it into Earth's place just before Zinyak atomizes it (his homeworld, perhaps?) Future SR games would lack any real oomph if there's no more Earth to take over.
Actually, looking back at the Earth Scene, it doesn't even look like Earth at all, there are absolutely no distinguishable features on the surface and in all honesty it's much too bright for earth's atmosphere, even if you happened to be viewing the most ocean covered part of it there should always be at least one continent visible.
It might not have been Earth at all that got blown up there?