The grid of squares to the right represents the patch of sky that Kepler stared at for nearly four years. So far, the space telescope – nicknamed the Planet Hunter – has confirmed the existence of 151 exoplanets and identified more than 3500 strong candidates.

Now, using what we know from Kepler, and simulations from its data by Courtney Dressing and David Charbonneau of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, New Scientist has estimated and mapped the density of habitable worlds across the whole sky. Given that the Milky Way is thought to contain between 100 and 200 billion stars, our best estimate of the total number of such planets in our galaxy is 15 to 30 billion.