Currently, Earth is the only known planet (or moon) to have consistent, stable bodies of liquid water on its surface. In our solar system, Earth orbits around the sun in an area called the habitable zone. The temperature, along with an ample amount of atmospheric pressure within this zone, allows water to be liquid for long periods of time. There is growing evidence suggesting that the planet Mars has subsurface liquid water, but this is yet to be confirmed.
Other than Earth, Europa (one of Jupiter’s 50 known moons) is the only celestial body in our solar system with evidence suggesting there is an ocean. It is hypothesized that Europa's subsurface ocean is salty, tidal, and causes its ice surface to move, resulting in large fractures which are clearly visible in the above image. Although Europa is thought to have the ingredients needed to support life—water, energy, organic compounds—it lies outside of our solar system's habitable zone.
Finding liquid water beyond our solar system may indicate the existence of extraterrestrial life. Using data collected by NASA's Kepler spacecraft and the W.M. Keck Observatory, scientists have determined that 22 percent of solar-type stars in our galaxy have Earth-sized planets in their habitable zone and could host life. Some of these planets are a mere 12 light years away and can be seen with the naked eye in the night's sky.
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NASA Hubble Space Telescope recently spotted what may be plumes of water vapor spouting from a possible subsurface ocean on Europa.