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62 A Mirror Image. Astronomer John MacVey suggested that alien scientists studying our Sun might regard it as planetless because of the lack of an appreciable wobble as it journeys through space.” As the search developed, astronomers found planets around some 10% of Sun-like stars; the real percentage may be much higher. Planetary sci- entist Stuart Ross Taylor proposed that we may eventually find that planets forming from disks rotating around young stars will occupy all available niches within the limits imposed by the cosmochemical abundances of the elements and the laws of physics and chemistry.”° In 2001, astronomers discovered the first planetary system resembling our own, with a Jupiter-like body in a circular orbit at the right distance. Two years later, searchers found a star—planet system even more like ours. Known as 18 Scorpii, this star has a mass, diameter, rotation, sunspot cycle, surface temperature, and age very similar to those of our own Sun. The system also has a Jupiter-like body in a nearly circular orbit far enough from its star to allow smaller, rockier planets to survive closer in. 18 Scorpii is only 46 light-years away; planets found in that system may be prime targets for instruments that search for signs of life.” Jupiter-like objects have ambiguous implications. Our own Jupiter may have played an important role in our evolution by diverting comets away from the inner solar system, where their impacts might have reduced the chances for life evolving on inner planets. According to one estimate, without Jupiter, the current impact rate of comets would be a thousand times higher, with catastrophic collisions occurring every 100,000 years. On the other hand, a system with only Jupiter-type planets might be unlikely to evolve intelligent life.” Astronomers have obtained the first images of planets orbiting other stars, including one about twice as massive as Jupiter but 20 times farther away from its sun.** The fact that we can see it at the astonishing distance of 450 light-years tells us how quickly our search for planets has progressed. In 2005, planet-hunters Geoffrey Marcy and Paul Butler detected the first rocky planet orbiting another star, a red dwarf 15 light-years away. This stony world, seven times as massive as the Earth, may be so close to its sun that life could not develop or survive on its surface. Astronomers who found another planet of comparable mass concluded that worlds smaller than Neptune may be common in the inner parts of many solar systems.” The prevailing conventional wisdom among astronomers long assumed that planetary systems were unlikely to form around binary or multiple stars, an estimated 60% or more of stars in our Galaxy. Now we know that they can. “The neglected majority of double stars,” wrote two German astronomers, “could fill the galaxy with planets.” Probabilities: The Astronomical Factors