Extraterrestrial: The First Sign of Intelligent Life Beyond Earth

by Avi Loeb

I take issue with the suspicion often visited on SETI. Compared to some flights of theoretical physics, the search elsewhere in the universe for something that is known to exist on Earth, the phenomenon of life, is a conservative line of inquiry. The Milky Way hosts tens of billions of Earth-size planets with surface temperatures similar to our own. Overall, about a quarter of our galaxy’s two hundred billion stars are orbited by planets that are habitable in the way Earth is, with surface conditions that allow liquid water and the chemistry of life as we know it. Given so many worlds—fifty billion in our own galaxy!—with similar life-friendly conditions, it’s very likely that intelligent organisms have evolved elsewhere. And that’s counting only habitable planets within the Milky Way. Adding all other galaxies in the observable volume of the universe increases the number of habitable planets to a zetta, or 1021—a figure greater than the number of grains of sand on all of the beaches on Earth. Some of the resistance to the search for extraterrestrial intelligence boils down to conservatism, which many scientists adopt in order to minimize the number of mistakes they make during their careers.