Night by night, star by star, astronomers are edging ever closer to learning just how crowded our universe really is—or at least our galaxy, anyway. A quarter century after the first exoplanets were found orbiting other stars, statistics from the thousands now known have revealed that, on average, each and every stellar denizen of the Milky Way must be accompanied by at least one world. Look long and hard enough for a planet around any given star in our galaxy and you are practically guaranteed to find something sooner or later.

But even a crowded universe can be a lonely place. Our planet-rich Milky Way may prove to be life-poor. Of all the galaxy’s known worlds, only a figurative handful resemble Earth in size and orbit—each occupying a nebulous “Goldilocks” region of just-rightness—a fairy-tale-simple ideal in which a world is neither too big nor too small, neither too hot nor too cold, to sustain liquid water and life on its surface. Instead, most of the Milky Way’s planets are worlds theorists failed to predict and have yet to fit comfortably in any conception of habitability: “super-Earths” bigger than our familiar planet but smaller than Neptune. No super-Earths twirl around our sun for solar system–bound scientists to directly study, making it that much harder to know whether any elsewhere are Goldilocks worlds—or, for that matter, whether any one-size-fits-all metric of habitability is hopelessly naive.

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