Few and Far - Very Far - Between
NASA's announcement about the newest exoplanets - Discovery Channel has a great article about them here - includes not one but two planets that fall into the new "super terrestrial" (my term) category. These are rocky worlds about 2 or 3 times larger (and 14 to 18 times heavier) than Earth. That might sound big, but they are by far the smallest planets we've discovered orbiting stars other than our sun; until now, all we've found are gas giants like Jupiter and Saturn.
Hypothetical Rocky Planet
In the comments to yesterday's post, Krakatoa remarked that planets like Earth are probably rare ... really rare. This is called the Rare Earth Hypothesis - first advanced by Peter Ward and Don Brownlee - and essentially states that the huge number of fortunate coincidences required to get a mild, temperate, stable planet like the Earth mean such planets will be uncommon in the extreme. There are definitely lots and lots of planets, and a small but significant number of them probably develop critters like bacteria and blue-green algae, but complex living things like mice and trees and dragonflies will not have many places to live (the article linked above details all of the myriad conditions necessary for advanced life to develop; I heartily recommend reading it). Intelligent beings like humans will be even rarer.
There is, however, something I think Ward and Brownlee are leaving out. Earths - single planets with big moons just the right distance from a stable yellow star - are indeed going to be rare. But perhaps that is not the only configuration that can lead to planets with advanced life; perhaps we are being a bit "terracentric". For instance, a stable, habitable world might form as a moon of a gas giant (the gas giant itself in a warm, stable orbit around its parent star). Such an arrangement would be drastically different from what we have here in our solar system, but it still meets all of the criteria for producing a habitable planet; it achieves the necessary environment by different means. There are other arrangements of stars and planets I can think of that are similarly different from what we know in our solar system but could still lead to habitability: Two large terrestrial planets (like those just recently discovered, in fact) orbiting a common center of gravity while they also orbit close to a small, long-lived red dwarf. The point is that Earth's exact situation is probably not the only one that can lead to the development of advanced life.
Each particular arrangement of objects, circumstances, and events that leads to a stable, thriving biosphere is probably very rare. But there are probably many of them - some we can't even begin to imagine - each as rare as the one before it and different from all the rest. Intelligent beings that evolved on any one of them would tend to think "How fortunate for my species that all of these coincidences happened to create my nice, stable world. How rare it must be!" They'll be right - it is rare. But a couple hundred light years away is another world, with another - different - set of unlikely coincidences that has produced another warm water world teeming with life.
Now I'm not saying we live in a crowded, Star Trek-like galaxy. Life-bearing planets probably orbit less than one star in ten thousand, and the conditions necessary for complex life to develop will only be found on a small percentage of those (less than .01%, probably). But I'm still more optimistic than Ward and Brownlee; I think habitable planets with diverse, thriving biospheres are separated by mere hundreds of light years ... civilizations by perhaps a few thousand. For now no one knows for sure, but with all the recent discoveries I'm hopeful that we'll have our first glimmering, tantalizing hints of other habitable planets within my lifetime.
Time and technology will tell.