The Bad Astronomer writes "A recent astronomical report (abstract in Science) came out stating that as many as 1 in 4 sun-like stars have roughly earth-mass planets. But are they habitable? A simple bit of math based on some decent assumptions shows that there may be billions of potentially habitable worlds in the galaxy. '... astronomers studied 166 stars within 80 light years of Earth, and did a survey of the planets they found orbiting them. What they found is that about 1.5% of the stars have Jupiter-mass planets, 6% have Neptune-mass ones, and about 12% have planets from 3 – 10 times the Earth’s mass. This sample isn’t complete, and they cannot detect planets smaller than 3 times the Earth’s mass. But using some statistics, they can estimate from the trend that as many as 25% of sun-like stars have earth-mass planets orbiting them!' Getting to them, of course, is another problem altogether..."