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.

We have no idea what kind of life may exist on other planets, so it’s hard to know what to look for. Proxima Kósmos imagines a very different kind of solar system that could harbor life-forms we haven’t yet thought of.

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.”