t’s something people tell me all the time, and usually in hushed tones: “With a trillion planets out there, we really can’t be the only intelligent beings in the galaxy.” In other words, given the enormous amount of real estate in space, aliens are sure to exist. So why haven’t we found any? I don’t dispute this straightforward idea because, after all, it underpins the search for extraterrestrial intelligence (SETI). But not everyone agrees. A recent paper by three researchers at the University of Oxford is throwing shade on those who feel confident that the cosmos is thick with extraterrestrials....