About the cover
Hundreds of extrasolar planets have been found since the first confirmed discoveries in the mid-1990s. Many of them are large 'hot Jupiters', but steady improvements in observational capabilities have brought physicists closer to detecting smaller Earth-sized bodies, and now, the planet-hunting space observatory Kepler has discovered two in orbit around the Sun-like star Kepler-20. Planet Kepler-20 f has a radius almost identical to that of Earth, and Kepler-20 e is slightly smaller, at 0.87 times Earth's radius. The radii of the planets and the architecture of the system — at least three larger planets orbit the host star — suggest that the Earth-sized planets are rocky, and that the outer planet (Kepler-20 f) may have developed a thick atmosphere of water vapour.