There are probably billions of Earthlike planets in our galaxy alone, predicts scientist Alan Boss. With NASA launching the Kepler satellite, seeking other Earths, you can expect the first discoveries in a few years.
The Kepler satellite will use the same planet-finding method that's already found a few hundred planets outside our solar system: looking for subtle dips in stellar brightness. But it'll use more sensitive methods, looking for smaller, cooler planets that are closer to Earth and more hospitable to life.
Boss, who's just written a new book called The Crowded Universe, argues that Earthlike planets should be quite common:
First, if you talk to astronomers who look at young stars, they will tell you that when stars form, they tend to have a little bit of angular momentum, which means that they can't accrete all of their matter and they end up having a disk around them. Such disks are what planetary systems form out of, basically the leftovers from the star-formation process. Essentially all young stars have these disks, so we expect that these young stars at least have the possibility of having planetary systems.
Second, those who worry about planet-formation processes find that it's very hard to stop Earth-like planets, or some sort of large, rocky object, from forming. Earths in some sense are easier to build than Jupiters, but we already know from our extrasolar planet census that Jupiters exist around at least 10 percent, and probably around 20 percent, of stars. So Earths should be even more common than that.
Finally, and even more directly, the planetary searches are already beginning to find a new class of planets called super-Earths with masses maybe five, 10 or 15 times the mass of Earth that orbit a little closer to their star than our planet does. These guys occur on roughly one third of nearby solar-type stars. And these are sort of the oddballs in some sense, which I think are very much just the tip of the iceberg of the spectrum of Earth-like planets. In any theoretical model of planet formation that people talk about, there should be a ton of Earths compared to these oddball super-Earths, so when we do a complete census we should find a lot of Earths. If these oddballs are there 30 percent of the time and the Jupiters are there 20 percent of the time, that means the ones we can't quite see should be there essentially all the time. So it's a very compelling story, and all the evidence from several different directions points toward Earths being quite common.
He also believes that life is quite tenacious and it's likely that many of these planets have water on them, and comets dumping amino acids and other prebiotic chemicals, making life pretty likely. So many of these Earthlike worlds could turn out to have our alien cousins on them. [Scientific American]