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Planets 63 Parallel Programs, Different Fates In 1988—the year that NASA formally endorsed the SETI program— the agency’s Solar System Exploration Division established a working group on strategies for finding other planetary systems. Like the SETI pioneers, scientists interested in detecting extrasolar planets held work- shops to identify priorities and to build consensus.** Although both programs were modest in scale, official support of the search for alien planets has survived, but official support of searching for alien technolo- gies has not. At this writing, Earth-sized planets are still beyond our ability to detect directly; once again, we are operating at the limits of our instruments. Some models of solar system evolution suggest that an Earth-like planet is likely to form near our world’s position; if correct, such results imply that Earth-like planets may be a common feature of other systems.”” Some scientists challenge the assumption that our Sun and its family are typical. Having many worlds and moons in near circular orbits may be more the exception than the rule. Earth-like planets may not form in many systems; if they do, their orbits may be perturbed by Jupiter-sized bodies, ending up far out of the system’s original plane.”* On the other hand, we may be misled by selection effects. The more massive a planet is and the more closely it orbits around its star, the bigger the wobble, making hot Jupiters the easiest planets to find.” There may be many smaller planets whose effects are beyond our present ability to detect. If there are Earth-like planets, some may be very old. One study showed that such planets began forming in the Milky Way about 9.3 billion years ago and that their average age may be about 6.5 billion years—2 billion years older than the Earth. Charles Lineweaver of the Australian National University calculated that three-quarters of all “Earths” are older than ours.*° Some may have had plenty of time to evolve life, and possibly reeatt: intelligence. What does Earth-like mean? Artists who visualized the surfaces of other planets in our own solar system before the age of planetary exploration drew on analogies with Earth to produce recognizable, attractive environ- ments. The realities that our spacecraft discovered were very different from what we had imagined—and hoped for. We underestimated how alien other planets would be, even around our own star.” We also may be underestimating the variety of planetary environments that can support life, particularly by assuming that they must have liquid water on their surfaces. Our definition of “Earth-like” may need to be a broad one.