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The detection of many extrasolar planets constitutes a momentous advance for astronomy. But these discoveries, most of which were made after 1995, were not unanticipated—for centuries, people speculated about the existence of planetary systems around stars other than the Sun. Many of these systems caught astronomers by surprise. They revealed properties that would not fit the theoretical models of planetary system formation. For instance, recently, astronomers have been surprised to learn that some modestly spaced binary stars can also harbor planets. Theoretical work had suggested that planets could not form in such systems. Yet it is now clear that Jupiter-sized planets do indeed exist in at least a few multiple-star systems. And computational modeling suggests that smaller, terrestrial planets will one day be found in such systems, some of which are likely to contain water and orbit in their parent stars' habitable zones.
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