sentences:  GJ 1214 b was the second exoplanet (after CoRoT-7b) to have an established mass and radius less than those of the giant Solar System planets.It is three times the size of Earth and about 6.5 times as massive.Its low density indicated that it is likely a mix of rock and water, and follow-up observations using the Hubble telescope now seem to confirm that a large fraction of its mass is water, so it is a large waterworld.