nasa’s ultimate goal for the telescope was to work out a figure known as eta-Earth, or η⊕. This is the average number of rocky, roughly Earth-size planets that can be found orbiting an average sunlike star at a distance that might, conceivably, render them habitable. After spending two years analyzing the data from Kepler, researchers recently concluded that η⊕ has a value somewhere between .37 and .6. Since there are at least four billion sunlike stars in the Milky Way, this means that somewhere between 1.5 billion and 2.4 billion planets in our galaxy could, in theory, harbor life. No one knows what fraction of potentially habitable planets are, in fact, inhabited, but, even if the proportion is trivial, we’re still talking about millions—perhaps tens of millions—of planets in the galaxy that might be teeming with living things. At a public event a few years ago, Ellen Stofan, who at the time was nasa’s chief scientist and is now the director of the National Air and Space Museum, said that she believed “definitive evidence” of “life beyond earth” would be found sometime in the next two decades.