But perhaps the most interesting and exciting aspect of all this is what it implies. The Milky Way galaxy is composed of about 200 billion stars, and is 100,000 light years across. The fact that we found a planet that is even anything like the Earth at all orbiting another star only 20 light years away makes me extremely optimistic that earthlike planets are everywhere in our galaxy. 20 light years is practically in our lap compared to the vast size of our galaxy, so statistically speaking, it seems very likely it’s not unique. I don’t want to extrapolate from a data set of two (us and them), but if this is typical, there could be millions of such planets in the galaxy. Millions. So we don’t know if this planet is all that much like Earth — the surface gravity may be quite high if it’s dense and small, for example, or it may not have any air, or it may have a thick atmosphere like Venus — but what it’s telling us is that smaller, lower mass planets at the right distance from their star for liquid water are almost certainly common in the galaxy.
I assume this means we can play around with the Drake equation? In any case, I am now reminded of Poul Anderson's essay "The creation of imaginary worlds: the world builder's handbook and pocket companion." You can read most of the essay online at Google Books. Or, find it in Writing Science Fiction & Fantasy. For us "squishy science" lovers the biochemist Hal Clament has an essay which follow's Anderson's which outlines how to create imaginary life.