In 1961, astronomer Frank Drake formalized this idea into an equation, known as the Drake equation:
There are some very good websites that examine the math in detail, and even let you plug in your own numbers, but the gist is as follows. There are around 100 billion stars in our galaxy. We don’t know for sure, but scientists “guesstimate” that the average number of planets per star may be anywhere from one-half (i.e., one planet for every two stars) to as many as two or three. But of course only a fraction of those planets will be habitable.
To read more and see the great infographic flow chart go visit the Second Nexus at the link below.