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Needless to say, the observation opened a whole new field for cosmic inquiry, and added force to the question that has intrigued scientists, philosophers, poets, religious believers, science-fiction devotees and every child who has ever gazed upward on a clear, moonless night: Are we alone?
As if on cue, NASA then announced that its Kepler spacecraft, two years into a three-and-a-half year mission to find Earth-size planets around nearby stars, had found a totally unexpected profusion of candidates. Of the 1,235 suspected planets spotted so far, moreover, about a third were in multiplanet solar systems like ours. Judging from these discoveries, it would appear that planets out there are as numerous as grains of sand. Twenty-five years ago, when I was a student in high school, only nine planets were known, all in our solar system. We learned their names and sequence from the sun, from the fleet-footed Mercury to icy Pluto. We learned of the runaway greenhouse effect that had stoked Venus to blistering temperatures and read about the giant storm that is Jupiter's red spot, and we gazed at pictures of the rings of Saturn that the Voyager spacecraft had sent back.
