One: They’re nowhere—and no-when. Aliens don’t exist, and they never have. This scenario might have seemed more likely in the universe imagined by Aristotle and Ptolemy—a small assortment of celestial orbs spinning around a singular Earth. But that isn’t the universe anybody lives in. After searching the skies for Earthlike planets for centuries, cosmologists have, in the last two decades, broken open the cosmic piñata. Today they estimate as many as 500 billion billion sunlike stars, with 100 billion billion Earthlike planets. The more we learn about the universe, the more absurd it would seem if all but one of those bodies were bereft of life. To my mind, this is both the least likely answer to Fermi’s Paradox and the only one that fits all the evidence currently available to astrophysicists.

Two: Life is out there—but intelligence isn’t. Ellen Stofan predicts that we’ll find evidence of simple life on Mars or a faraway moon within the next 10 to 30 years. But she’s imagining something more like microbes or algae, not underwater cities in the liquid-methane lakes of Titan. This shifts the question from “Where is everybody?” to a more sophisticated query: What precisely is keeping an infinitude of dumb molecules from assembling to form an abundance of intelligent life?

Think about all the factors that add up to the creation of a human.