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
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