Thomas Mulligan examines the scientific challenge of defining and searching for non-Earth-like alien life forms that may not rely on carbon, water, or DNA.

And when people are convinced that they know a secret, world-shaking truth, they are willing to wait a long time for vindication. This is another way in which UFO disclosure resembles the coming of the Messiah: Both are constantly running behind schedule.

Neil deGrasse Tyson is an astrophysicist who is the director of the Hayden Planetarium at the American Museum of Natural History in New York City. He is the author of “Take Me To Your Leader: Perspectives on Your First Alien Encounter.”