Earth-size planets may be common in our galaxy according to extrapolations from new data taken by the Kepler spacecraft. Current models suggest that at least one in ten stars have an Earth-sized planet orbiting them. That would make the Milky Way Galaxy home to over ten billion Earth-size planets. However, this estimate applies only to planets effectively inside the orbit of Mercury, making these hot-Earths poor candidates for supporting life.
The graph above depicts the estimated fraction of stars that have close orbiting planets of various sizes. The number of Sun-like stars with Earth-like planets in Earth-like orbits is certainly smaller. On the other hand, the Kepler science team has just announced the discovery of four more of those.
Image Credit: NASA