Illustration of the comparative sizes of Kepler 51 exoplanets. Kepler-51 is a Sun-like star, about 500 million years old, which is orbited by three gas giant exoplanets Kepler-51b, Kepler-51c and Kepler-51d. The colours of the exoplanets is illustrative. The Kepler-51 planets are all roughly the size of Jupiter with about one hundredth of Jupiter's mass. This means the planets have very low densities with atmospheres made up of hydrogen and helium, thought to be escaping into space.