But here's why:
The Milky Way (the galaxy in which our Solar system is located) has around 400,000,000,000 stars. If only a 0.5% of those have planets, then there are 2,000,000,000 Solar systems out there. If out of those solar systems only 0.1% have any planets within an inhabitable region, there are 2 million planets suitable for life out there. Even if one out of a thousand actually harbors life, then there are 2 thousand living planets floating around. And this is being really pessimistic with the numbers.
Then we look beyond our galaxy. The Hubble telescope was pointed a tiny piece of the sky for a month taking in all the light it could get. The result was this famous picture:
Doesn't stop scientists from trying though, and last Monday they announced that one of the planets found by the Kepler space observatory, launched in March 2009, was determined to be within the inhabitable region of the solar system it belongs to. What this means is that the nature of the star combined with the distance at which the planet orbits it results in a viable environment for the existance of liquid water, a key ingredient for life as we know it.
|Being in the inhabitable zone is not a guarantee of life, as you see Mars inside ours, we have yet to find any signs of life in the red planet.|
This world, dubbed Kepler 22b, is 2.4 times larger than the earth, but it's current composition is yet unknown. It could be an Earth-like planet as well as a giant ball of liquid (such as Neptune). Astronomers will certainly keep tracking it and make calculations to estimate the density of the planet, it's gravity, and SETI scientists are monitoring it for any sign of intelligent life. It being 600 light years away though, has no clue we are here as we only started transmitting things around 100 years ago and those radio waves haven't reached that far yet.
In conclusion, don't make your "welcome to Earth" signs yet.