In the thousands of years that people have been arguing about whether life exists elsewhere in the universe, one thing has been constant: No one really has had a clue. But not anymore. That’s because we finally know exactly where to look for aliens. Thanks to spectacular advances in science, we’ve identified many stars that have planets in the habitable zone where life can form. We are learning which of those planets are Earthlike enough to be worth pointing our telescopes at. We have giant telescopes equipped with spectrographs that can analyze light from distant stars, and powerful computers to simulate far-flung worlds. If we want to find aliens, we don’t need them to announce their presence to the cosmos. Instead, like detectives on a stakeout, we can just hang out with our doughnuts and cold coffee, watching and waiting. One form of evidence that astronomers are seeking on their great cosmic stakeout is “biosignatures”—features in a planet’s atmosphere that can come only from life. Scientists have learned from studying our own planet’s history that …