Thanks to the amazing scientific principle known as "low budget," the universe is utterly littered with earth-like worlds, all of which seem to support life pretty much exactly like here, only with goofier foreheads. It has long been a source of annoyance to me that few in visual SF seem the least bit interested in the fact that the universe *isn't* much like earth, and in fact that's what makes it interesting in the first place.
You tell me there's another earth out there, and, yeah, that's interesting, but frankly, it just means another planet full of mildew and allergies and some invariably awful ailment never before seen by humanity. Probably something involving the bowels in some traumatic fashion. It always seems to be the bowels.
(Wanna' know why I can't take Star Trek seriously? Reason # 426: No one ever gets the Hershey Squirts. Think about that. Think about the time you went to Mexico and drank the water. Think about the time you got bad clams at Red Lobster. Think about that one time you just kind of inexplicably came down with the cruds for no good reason, and spent three days with the sluices open at both ends. Think of how EASY it is to have something awful happen in your down-below areas, and then ponder the fact that no one in the entire 40 year history of Trek has *EVER* gotten the trots.)
Realistically, if I wanted to deal with animals and bad smells and weather and earthquakes and crap, I really wouldn't want to leave earth in the first place, now would I? Thus the idea of leaving our fairly miserable planet, which at least has some nice theme parks, and heading to some other fairly miserable planet just like ours, but *without* theme parks, doesn't have much appeal. Planets are for chumps. When people say, "Well, we need a planet to live on," you just look down your nose at them and say "You haven't really thought this through, have you, Picard. Planets are what we're here to rise above." It's a shame the way budgetary concessions end up shaping people's views of science and reality.
Anyway: As yet, Life has not been discovered anywhere else in the universe, apart from our little world. Most people assume life elsewhere is probably, however, and that proof will turn up any day. Public opinion (Post-Star Wars) is that life is everywhere, and aliens are real. Public opinion pre-Star Wars was just the opposite. "Aliens? Are you crazy? You need a good beating, my friend!"
As it happens, life elsewhere in the universe may be *much* rarer than previously thought. Life is, in large part, based on the interactions of Carbon and Oxygen. Well, we've been finding lots and lots of Carbon in space, but surprisingly little Oxygen.
This is actually kind of interesting. WHY is there so little O2 out there? According to our (Somewhat theoretical) models on such things, there should be plenty, but if so, where is it going? The answer may surprise you:
It appears that the Carbon is gobbling it up and turning it into Carbon Monoxide. That's right: poisonous exhaust like out of your car. Evidently once the cycle starts, the Carbon starts grabbing the Oxygen, and it doesn't stop until all the Oxygen has been used up. If this is the case, and it appears to be, then the likelihood of worlds that can support life just became vastly, vastly smaller.
It doesn't rule it out entirely - we're here, after all - but what it basically means is that even if you had a planet that had the same exact conditions as earth over the last couple billion years, you'd likely only end up with a planet of smog.