Illustration of the Kepler-51 planetary system. Kepler-51 (top right) is a star that is orbited by three planets, Kepler-51b, Kepler-51c and Kepler-51d. These planets are known as super-puff planets because they are the size of Jupiter with masses around one hundredth that of Jupiter. This means the planets have very low densities with atmospheres of hydrogen and helium.