Extraterrestrial liquid water, the presence of water in its liquid state, is a subject of wide interest because it is commonly believed to be a prerequisite for extraterrestrial life.
Earth, with oceanic water covering 71% of its surface, is the only planet known to have stable bodies of liquid water on its surface and liquid water is essential to all life on Earth. Earth orbits the center of the Sun's habitable zone, a region in which liquid water can exist on the surface of a planet with sufficient atmospheric pressure.
There is growing evidence of liquids below the surface of several natural satellites and dwarf planets elsewhere in the Solar System that may consist of liquid water, some are speculated to be large "oceans"; however, none have been confirmed. Oceans and water may be common in other planetary systems; for example, water vapour was found in 2007 in the proplanetary disc of a young star MWC 480, within 1 AU of the star.
Surface water was once thought to cover large areas of Venus and Mars and possible seasonal flows on warm Martian slopes have been observed since 2008.
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