Quoted from Frippertron:There is a pretty interesting theory I read a while back, I have to find a link. It's a mathematical equation that basically says with the amount of planets that are revolving around all the stars in our galaxy that meet a distance and temperature threshold similar to ours and if just 1% of those had life, there would be a minimum of 40,000 planets with life in our galaxy. Interesting stuff.
The Drake Equation.