Extraterrestrial liquid water, the presence of water in its liquid state, is a subject of wide interest because it is a commonly suggested prerequisite for the emergence of extraterrestrial life. Earth, with oceanic water covering 71% of its surface, is the only known planet with liquid water on its surface and is certainly the only one in the Solar System. Oceans and water may be common in other star systems and/or on their planets and other orbiting celestial bodies: for example, water vapour has recently been found at the right spot in a protoplanetary disc. Large bodies of water and extensive water systems were once thought to cover the Moon, Venus, and Mars. With increased telescopic resolution and enhanced observation techniques in modern times, these were ultimately disproven; however, the presence of water on Mars in the distant past remains a topic of contemporary debate.