Mercury (planet)
Mercury is one of four terrestrial planets in the Solar System, and is a rocky body like Earth. It is the smallest planet in the Solar System, with an equatorial radius of . Mercury is also smaller—albeit more massive—than the largest natural satellites in the Solar System, Ganymede and Titan. Mercury consists of approximately 70% metallic and 30% silicate material. Mercury's density is the second highest in the Solar System at 5.427 g/cm, only slightly less than Earth's density of 5.515 g/cm. If the effect of gravitational compression were to be factored out from both planets, the materials of which Mercury is made would be denser than those of Earth, with an uncompressed density of 5.3 g/cm versus Earth's 4.4 g/cm.
Kepler-22b
Kepler-22b, dubbed by scientists as a 'waterworld', might be an 'ocean-like' planet. It might also be comparable to the water-rich planet GJ 1214 b although Kepler-22b, unlike GJ 1214 b, is in the habitable zone. An Earth-like composition is ruled out to at least 1-sigma uncertainty by radial velocity measurements of the system. It is thus likely to have a more volatile-rich composition with a liquid or gaseous outer shell; this would make it similar to Kepler-11f, the smallest known gas planet. Natalie Batalha, one of the scientists on the Kepler Space Telescope project, has speculated, "If it is mostly ocean with a small rocky core, it's not beyond the realm of possibility that life could exist in such an ocean". This possibility has spurred SETI to perform research on top candidates for extraterrestrial life.