>From an hostile space alien point of view, ideally you want to get rid of humans, now the planet itself... Why not just keep it instead? It would be a waste of everything, starting with a waste of energy/doomsday device, to blow it up while the human menace is gone or the few survivors thrown back to stone age on a toxic wasteland Aside from *why* they would have such a need, dropping a handful of rocks of a certain mass down the gravity well at particular orbits would effectively destroy all of humanity and leave the planet perfectly fine after a few months/years. All they would need to do is hang out near Jupiter or something and claim to be our greatest allies until the rocks hit. Then wait for the dust to clear, any leftovers are easy-cleanup.