Here’s what happens if astronomers make contact with a civilization on another planet.The false alarm happened in 1997. The Green Bank Radio Observatory in Green Bank, West Virginia, was picking up some unusual signals—and Seth Shostak, then the head of the Center for Search for Extraterrestrial Intelligence (SETI) Research in Mountain View, Caifornia, was convinced that they had come from intelligent life somewhere in the universe. “It looked like it might be the real deal,” Shostak recalled. Within a few hours, he had a call from The New York Times. But within a day, it became clear that the source...