Are humans alone in the universe? The answer is more complicated than science fiction might suggest. Scientists have discovered billions upon billions of potentially habitable planets. But if life is possible elsewhere, why haven’t we found it? Host Megan McArdle talks with astrophysicist Adam Frank, author of “The Little Book of Aliens.” They discuss the search for alien life, why intelligent civilizations may be harder to find than microbes and the limits of what UFOs can reveal.