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Planets 61 Galaxy might well . . . be inhabited by varieties of men who are not only of separate species but whose criteria of habitability in planets may not be the same.” They added that interstellar migration may become a new form of evolutionary pressure—both with respect to the new environ- ments to which Man will be exposed and to the new requirements made of his mind and character.'* These observations would apply to nonhuman intelligences as well. Extraterrestrials might be able to evolve and thrive under a wider range of conditions than those required by humans. If they expand beyond their biospheres of origin, they too might diversify, broadening their potential range. The new era of planetary discovery began in 1983 when astronomers detected a cloud surrounding the bright star Vega, possible evidence of dust and larger objects in orbit. A year later, astronomers obtained the first image of a circumstellar disk, around the star Beta Pictoris. Since 1990, observers have found that many young stars have protoplan- etary disks—potential solar systems in the making. The Hubble and Spitzer Space Telescopes have spotted such disks around mature, Sun-like stars known to have planets. They also have been detected around smaller, cooler M-class stars, which are much more numerous. At least 15% of nearby stars are surrounded by dusty disks." Dust disks around many stars are replenished from repeated collisions of large rocky objects, the way our solar system’s inner rocky planets were formed. Many disks have inner holes that may have been swept out by accreting planets. This may mean that Earth-like planets are fairly common around other stars.'° Researchers also have found that planets may form around young brown dwarfs. The dense, cool atmospheres of these stars are an ideal environ- ment for producing molecules, including water and carbon dioxide. These cool dwarfs may be as numerous as all the other stars in our region of the Galaxy.'® The first confirmed discovery of an extrasolar planet was made in 1991, when astronomers indirectly detected bodies orbiting a rapidly spinning neutron star known as a pulsar. Tiny variations in the regular pattern of signals from the pulsar indicated three planets, two of them weighing about the same as the Earth.’ Most experts regard the environment around a pulsar as extremely inhospitable to life. Four years later, astronomers drawing on computer analyses of small irregularities in the Sun-like star 51 Pegasi’s motion reported evidence of a planet. Observers also have detected dips in the luminosity of stars, which imply that a large planet has crossed their disks as seen from the Earth.'*