Earth is the only planet that proves to have substantial amounts of liquid water on the surface. There are other planets with some liquid_water. Mars has small quantities of brine, basically water so saturated with salt that it won’t freeze even at extremely cold temperatures. But it can’t support life. Mars also has polar ice_caps. There are also other worlds known to have water, such as moons like Europa, Enceladus, and Ganymede with subsurface oceans. But they aren’t planets. It is overwhelmingly likely that many exoplanets have water on them as well, but it is tough to prove. The new James Webb telescope might be able to establish better evidence for exoplanets with liquid water.
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In a bit more detail, Earth is not the only planet to have water. It is the only planet to have oceans, which is a different matter.
Venus has a lot of water in its atmosphere, but the high surface temperatures keep it there, in the form of vapor and thick cloud layers. There is a tremendous amount of evidence that Venus experiences virga, which is rain that evaporates before it reaches the surface.
On Mars, you need to go down, not up. There is evidence that the Martian water is primarily locked up in a layer of permafrost under the surface, but at one time, liquid water did flow freely on the surface. This indicates that the atmosphere on the planet must have been thicker and warmer than it is now.
Jupiter has water and ice in its cloud layers, and one of its moons, Europa, is pretty much a ball of ice with a liquid water center, kept that way by the tidal energy due to Jupiter’s extreme gravitational pull. Saturn, Uranus, and Neptune seem to have water vapor and ice in their atmospheres, and Pluto and Charon are primarily a mix of ices, including water ice.
Well, to be pedantic, every planet has some water in the sense that there is some H2O present, even if it is sporadic and not in liquid form – may be gas, maybe ice, and perhaps bound up in rocks.
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As for liquid water, as far as we know, Earth is the only planet in our solar system that has liquid water on its surface all the time.
Europa probably has an ocean of liquid water under an icy crust. Perhaps Enceladus, a moon of Saturn, also has liquid water in crevices within its icy crust. There may be other places in the outer solar system like that.
Mars certainly has frozen water below the surface. The Phoenix lander found ice a few inches (cm) below the surface. There is evidence of more water below the surface elsewhere, as well.
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Jupiter and Saturn have heaps of water in their atmospheres, although it is vapor in the outer layers.
In general, most solar systems will be rich in water. Besides. Hydrogen and Oxygen are also some of the most common features. Around a few individual stars, carbon is more abundant than oxygen, and there one would expect the hydrogen to bind with the more abundant carbon to make methane (CH4) and other hydrocarbons. Titan, Saturn’s largest moon, has chemistry like that.