Most planets known to humans are either gaseous Jovian planets or solid terrestrial planets. Terrestrial planets include Mercury, Venus, Earth, and Mars. These inner planets have a rocky surface with metal interiors. The Jovian planets consist of Jupiter, Saturn, Uranus, and Neptune. While these planets are larger, their only land surface is a small rocky core surrounded by a large, thick atmosphere The gas giants, Jupiter and Saturn, are thought to have surface layers composed of liquid hydrogen rather than solid land; however, their planetary geology is not well understood. The possibility of Uranus and Neptune possessing hot, highly compressed, supercritical water under their thick atmospheres has been hypothesised. While their composition is still not fully understood, a 2006 study by Wiktorowicz et al. ruled out the possibility of such a water “ocean” existing on Neptune,though some studies have suggested that exotic oceans of liquid diamond are possible. The entire surface of a rocky planet or moon is considered land, even with a lack of seas or oceans for contrast. Planetary bodies that have a thin atmosphere often have land that is marked by impact craters since atmospheric conditions would normally break-down incoming objects and erode rough impact sites. Land on planetary bodies other than Earth can also be bought and sold although ownership of extraterrestrial real estate is not recognized by any authority.