Earth, the third planet from the Sun, is covered by about 71 percent water, while the habitable zone is the region around a star where liquid water could exist on the surface of a planet.
Mars once had liquid water on its surface.
Venus may once have had oceans, which were lost as its atmosphere heated in a runaway greenhouse effect, and Venus's thick atmosphere traps heat in this way.
Mercury, the smallest planet and the planet closest to the Sun, has ice at its poles, hidden inside craters that never see sunlight.
The ROSINA instrument on ESA's Rosetta spacecraft measured the deuterium-to-hydrogen (D/H) ratio in the water of comet 67P and found it more than three times the ratio in Earth's oceans.
Galileo Galilei discovered Jupiter's four largest moons, and one of them, Europa, is thought to have a liquid-water ocean beneath its icy crust.
Saturn is the only planet in our solar system that is less dense than water, so it would float in a bathtub big enough to hold it. Saturn's rings are made of millions of separate pieces, mostly small, from tiny dust-sized icy grains to chunks as big as a house.
Saturn's moon Enceladus has an ocean hidden beneath its icy surface. NASA's Cassini became the first spacecraft to orbit Saturn, and in 2005 it found that Enceladus sends plumes of icy particles out into space.
Neptune's moon Triton has geysers that shoot icy material upward from its surface. Triton may once have been an independent object that Neptune captured.
Words to know
- habitable: fit for life
- plumes: rising streams
- deuterium: heavy hydrogen
- crust: outer shell
- craters: bowl-shaped pits
