According to astronomers reporting in the journal Nature, Kepler-78b is the first Earth-sized exoplanet found to have an Earth-like density. This image shows Kepler-78b and Earth. Kepler-78b is about 20 percent larger than Earth and weighs 1.8 times as much as our planet. Image credit: David A. Aguilar / Harvard-Smithsonian Center for Astrophysics. Kepler-78b orbits the Sun-like G-type star Kepler 78 in just 8.5 hours at a distance of less than 1...