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60 the solar system are quite common in our Galaxy, particularly in its inner regions. The fact that we have so far failed to find extraterrestrials cannot be explained in terms of stellar and galactic evolution." bottleneck. Planets There are certain determined definite centers, namely the suns, fiery bodies around which revolve all planets, earths, and waters, even as we see the seven wandering planets take their course around our sun. —Giordano Bruno, 1584!” Throughout this long debate, the plurality of intelligent life has been implicitly connected with the frequency of planets. Until recently, the existence of such worlds beyond our solar system rested on an argument from probability, without observational proof. We have had a breakthrough. As of mid-2006, almost 200 extrasolar planets had been discovered. Although our statistics still are limited, it appears that vast numbers of planets accompany other stars. Habitable Planets for Humans More than 40 years ago, Rand Corporation analyst Stephen Dole extrapolated from existing knowledge of our own solar system to calcu- late that our Galaxy might host as many as 640 million Earth-like planets with life. Dole postulated some basic limits for planets that would be habitable by an expanding human species: mass greater than 0.4 Earth, but less than 2.35 Earth; period of rotation less than 96 hours; age more than 3 billion years; illumination at low equatorial inclination between 0.65 and 1.35 Earth normal; orbital eccentricity less than 0.2; the mass of the star less than 1.43 Sun. Dole collaborated with Isaac Asimov on a popularized version of his study called Planets For Man. Inserting numbers into a Drake-like formula, including different estimates for different classes of stars, Dole and Asimov concluded that the average distance between a habitable planet and its closest neighbor in our region of the Galaxy is about 24 light-years. That distance may be less in more densely packed regions of the Milky Way. If these estimates are roughly accurate, there may be 600 million planets in our Galaxy that would be inhabitable by human beings. Dole and Asimov noted that the development of modified species of humans will inevitably broaden the concept of a habitable planet. “The Probabilities: The Astronomical Factors Bottom Line, Stars: Advantage, believers. Suitable stars are not the