Cutaway diagram showing the proposed interior of the extrasolar planet Kepler-11c. This is most likely a water-rich planet with an atmosphere of hydrogen and helium. It orbits its host star, Kepler-11 - a Sun-like star some 2150 light-years distant in Cygnus - with a period of 10 days. Kepler-11c is a large planet three times to diameter of Earth and thirteen times its mass, similar to Neptune and Uranus. Astronomers suspect that Kepler-11c has a solid icy core, similar in size to the entire Earth, topped with a thick ocean of water.