As of October 13, 2013, humans have discovered almost one thousand exo-planets, worlds outside our solar system. 998 planets in 759 planetary systems, most of them giant gas worlds. It seems like a lot, but it's nothing. Just look at the tiny space we have covered so far.
That space has a lot more planets in there. There's an estimated 17 billion Earth-sized worlds in our galaxy alone. Just in our galaxy. That doesn't even count worlds like Mercury or Jupiter.
And there are 500 billion galaxies. You do the math.
It's a humbling feeling mixed with hope (of finding someone else) and excitement (of exploration) and sadness (of knowing that you and I will never see any of these worlds.)