Are you an alien civilization with a go-getter attitude, looking for some ripe planet to invade and loot? To begin with, put some thought into why you're invading in the first place. Suppose you're looking for some prevalent substance like helium, water, oxygen, etc. In that case, there are millions of lifeless rocks where you can get the stuff essentially for free, without the need to fight off pesky hostile natives. Aluminum? Uranium? Mine an asteroid and leave the puny earthlings alone. If it's a life-bearing planet you're after, keep in mind that ours didn't evolve a sapient species until the most recent 0.004% of its existence. Given those odds, chances are pretty good that there's another life-bearing planet a similar distance away that you could quickly colonize without conquering. I'm with Kuru on this one; Earth seems more trouble than it's worth.