Until we started getting a better handle on what's beyond our solar system, the search for extraterrestrial intelligence (SETI) required a bit more credulity. Martians, for instance, really haven't seemed likely for a long time -- despite the fear Orson Welles could arouse with his haunting, booming voice. The SETI notion -- always attractive to humans -- got a big boost when we finally moved past our provincial view of space and started learning more about our galaxy -- let alone beyond.
SETI must exist on planets, which aren't, of course, visible like stars. But there is evidence, as recounted in Lawrence Squeri's Waiting for Contact: The Search for Extraterrestrial Intelligence, that supports the idea that many -- maybe even most -- stars in the galaxy probably host a planet or more. And with the estimated 400 billion stars (some say the estimate is low) in our galaxy, that means the SETI search won't be exhausted for a long, long time.
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