Earth is a watery planet, the only one in the Solar System with bodies of liquid water on the surface. Water is essential to life as we know it, so its existence elsewhere interests us. Yet although there is a lot of water here, there seemed to be little or none elsewhere. Fortunately, space telescopes and space probes have detected what we couldn't see before, and now we're finding water everywhere.

Comets, meteoroids, asteroids, Kuiper belt objects
A comet's nucleus contains frozen water, as do many asteroids and meteoroids. (A meteoroid is a chunk of space rock smaller than an asteroid.) They enriched other bodies with water through collisions in the early Solar System. And out beyond Neptune is a region called the Kuiper belt. It's full of icy objects, most of which are made of frozen methane, ammonia and water. (They're all called ices.)

Pluto was the first Kuiper belt object discovered. Its bedrock is water ice which is as strong as rock at low temperatures. Astronomers also think that Pluto has a liquid ocean deep in its interior. Ammonia lowers the temperature at which water freezes, so its presence is probably what keeps the ocean liquid. Pluto's moon Charon had an ocean, but it froze about two billion years ago.

Ceres is the biggest asteroid and smallest dwarf planet. Even before the spacecraft Dawn went to visit, a space telescope had detected water vapor on Ceres. Data from Dawn supports the idea that Ceres has internal layers which include a rocky core, icy mantle, and a liquid ocean under the ice. The mantle may contain more fresh water than Earth's oceans do.

Rocky planets
Earth wasn't alone in having lots of water in the early days of the Solar System. Our neighbors Mars and Venus did too.

There is still some water vapor in the Martian atmosphere, but most of the remaining water is frozen. Sometimes flows of briny water occur on the surface, but nothing comparable to the days when Mars had lots of surface water, possibly including a large ocean. The atmosphere was thicker, and its climate quite different to the cold arid planet we know now. However when Mars's magnetic field shut down, the planet had no protection from solar eruptions. Energetic particles from the Sun stripped most of the atmosphere away, and once the atmosphere was gone, the surface water followed.

Cloudy Venus used to be imagined as a rainy tropical paradise. But then we found out that the clouds are sulfuric acid, and the surface is a desert with temperatures high enough to melt lead. It's hard to believe now that Venus once had enough water to cover the planet 25 meters (80 ft) deep.

As the planet closest to the Sun, Mercury isn't a likely place to find water. The part of the planet facing the Sun may heat up to 427 °C (800 °F). Yet it doesn't stay that way as Mercury rotates on its axis, because it has no atmosphere to hold in the heat. And unlike Earth's tilted axis, Mercury's axis is straight up and down, so the Sun never shines on either pole. The temperature at Mercury's poles is always below -83 °C (-136 °F). In 2012 the spacecraft MESSENGER discovered ice in permanently shadowed craters. There may be as much as a trillion tons of it.

The Moon
For a long time everybody knew there was no water on the Moon, it was bone dry. Since the Moon has no atmosphere, it couldn't have bodies of water. Water would either freeze or sublime, i.e., go from ice to vapor. But that's changed. Based on data collected by several spacecraft, we can see that there is water on the Moon. Furthermore it's not just the abundant water ice in permanently shadowed craters. There seems to be water – admittedly, in very low concentrations – all across the Moon. Although the Moon is still a desert, it's not completely dry.

The moons of the giants
In order to find great oceans of water, we must turn to the larger moons of the outer planets. Except for Saturn's largest moon Titan, they don't have atmospheres, and they're billions of kilometers from the Sun. Nor will we find watery oceans on the surface, as they're safely tucked away in the interiors.

Titan and Jupiter's moon Ganymede are the two largest moons in the Solar System. Titan has a thick icy crust sitting on top of a liquid ocean that is probably water and ammonia. Ganymede, which is slightly bigger than Titan, has a global salty ocean trapped between layers of ice. It's about ten times as deep as Earth's oceans.

Saturn's moon Enceladus must have water under the surface in order to supply the geysers of water that NASA's Cassini spacecraft has seen and sampled. An analysis of the moon's gravitational field also supports the existence of a liquid ocean.

Even more interesting than Enceladus is Jupiter's moon Europa. It's one of the likeliest places in the Solar System to have extraterrestrial life. Its deep subsurface ocean borders the moon's rocky mantle, and tidal heating and maybe volcanic activity keep it liquid. This could create conditions similar to Earth's hydrothermal vents. These structures on the ocean floor, well beyond the Sun's light, have their own families of living organisms.

The interiors of Europa and Enceladus, as well as those of other distant moons, are heated by tidal flexing. The gravitational influence of a planet – and in some cases, other moons – causes land tides in which a moon is squeezed and stretched. This process releases considerable heat, almost certainly enough to keep the internal oceans liquid.

It looks as though Earth is the odd one out with so much water on the surface.