Earth is the only known planet with liquid water on its surface and a thick, breathable atmosphere. About 71% of Earth’s surface is covered in water, with oceans holding roughly 96.5% of the total supply. But Earth isn’t the only world with water and an atmosphere. Several planets, moons, and even distant exoplanets have some combination of the two, just not in the life-friendly form we enjoy here.
Earth: The Only Planet With Liquid Surface Water
Earth sits in what scientists call the habitable zone, sometimes nicknamed the “Goldilocks zone,” where a planet receives just enough energy from its star for liquid water to persist on the surface. Our atmosphere is roughly 78% nitrogen and 21% oxygen, with trace amounts of water vapor, carbon dioxide, and other gases. That blanket of air does two critical things: it traps enough heat to keep water from freezing solid, and it creates enough surface pressure to keep water from boiling away into space.
Water also exists far beyond the oceans. It fills rivers, lakes, and underground aquifers, sits frozen in glaciers and ice caps, and floats as vapor in the atmosphere. No other planet in our solar system comes close to this kind of water cycle.
Mars: A Dry World That Once Had Rivers
Mars has a thin atmosphere, almost entirely carbon dioxide, at a surface pressure roughly 1/100th of Earth’s. That’s too thin and too cold for liquid water to exist on the surface today. What water remains is locked in polar ice caps and buried as subsurface ice. The atmosphere holds only a trace of water vapor.
It wasn’t always this way. Riverbeds, lake basins, and mineral deposits all point to a warmer, wetter past. So what happened? NASA’s MAVEN mission found that the solar wind, a constant stream of charged particles from the Sun, has been stripping gas from Mars at a rate of about 100 grams per second. Mars lacks the strong global magnetic field that shields Earth, so over billions of years the solar wind gradually peeled away most of the atmosphere. With less atmospheric pressure, surface water evaporated and escaped to space. During solar storms, the rate of atmospheric loss increases significantly, and the young Sun was far more active, meaning the damage was much worse early on. About 75% of the escaping gas leaves through a “tail” region behind Mars, with another 25% escaping over the poles.
Venus: Thick Atmosphere, Almost No Water
Venus has an incredibly dense atmosphere, about 90 times the surface pressure of Earth’s, made almost entirely of carbon dioxide. But water is nearly absent. The total water abundance in the Venusian atmosphere averages around 50 parts per million, and near the surface, free water vapor drops to roughly 10 parts per million. For comparison, Earth’s atmosphere can hold several percent water vapor in humid conditions.
Venus likely had more water early in its history, but a runaway greenhouse effect boiled it away. The little water vapor that remained high in the atmosphere was broken apart by ultraviolet light, and the hydrogen escaped to space. Today, the surface temperature sits around 465°C (870°F), hot enough to melt lead. Venus is a cautionary example of what happens when a planet’s atmosphere traps too much heat.
Jupiter and Saturn: Water Hidden in Cloud Layers
Gas giants don’t have solid surfaces, so “water on the planet” means something different here. Both Jupiter and Saturn have layered atmospheres with ammonia clouds near the top, ammonium hydrosulfide clouds in the middle, and water clouds deeper down, where temperatures reach about 270 Kelvin (just below freezing). These water clouds exist under enormous pressure and are buried beneath hundreds of kilometers of other gases, making them impossible to access or observe easily. Neither planet is remotely habitable, but water is definitely part of their atmospheric chemistry.
Ocean Moons: Water Beneath the Ice
Some of the most promising places to find liquid water aren’t planets at all. Several moons in the outer solar system have confirmed or strongly suspected subsurface oceans beneath shells of ice. The list includes Europa and Ganymede (orbiting Jupiter), Enceladus and Titan (orbiting Saturn), and possibly Callisto, Triton, and even the dwarf planet Ceres.
Europa and Enceladus get the most attention. Enceladus actively shoots plumes of water vapor and ice particles into space from cracks near its south pole, giving spacecraft a way to sample the ocean without landing. Europa also shows evidence of water plumes, and NASA’s Europa Clipper mission carries instruments designed to analyze the chemical makeup of any material ejected from the surface. These moons don’t have true atmospheres in the way Earth does, but they have thin exospheres, wisps of gas generated partly by the water escaping from below.
The big question is whether these oceans have the right chemistry for life. Both Europa and Enceladus are thought to have rocky seafloors in contact with saltwater, which could drive the kind of chemical reactions that support microbial life on Earth.
Exoplanets: Water Signs Beyond Our Solar System
The James Webb Space Telescope has started finding water-related molecules in the atmospheres of planets orbiting other stars. One standout is K2-18 b, an exoplanet about 8.6 times Earth’s mass. Webb detected methane and carbon dioxide in its atmosphere, along with a notable shortage of ammonia. That combination supports the idea that K2-18 b could be a “Hycean” world: a planet with a hydrogen-rich atmosphere sitting above a global water ocean. There was even a tentative detection of dimethyl sulfide, a molecule that on Earth is produced only by living organisms. That signal needs further confirmation, but it’s tantalizing.
Another candidate is TOI-715 b, a super-Earth about 1.55 times Earth’s radius orbiting inside the conservative habitable zone of a red dwarf star. Its tight 19-day orbit might sound extreme, but red dwarfs are much dimmer than the Sun, so the planet receives a moderate amount of energy. Whether TOI-715 b actually has water or an atmosphere is still unknown, but it ranks high on the list of targets for Webb’s spectroscopic observations, especially if it turns out to be a water world rather than a bare rocky planet.
The habitable zone itself is an imperfect guide. A planet’s actual ability to hold liquid water depends on its mass, atmospheric composition, cloud cover, and albedo (how much light it reflects). A planet outside the traditional habitable zone could still have liquid water if its atmosphere traps enough heat, while a planet inside the zone could be bone dry if it never had an atmosphere to begin with. Red dwarf stars pose an extra challenge: their habitable zones are so close that orbiting planets get blasted with intense X-ray and ultraviolet radiation, along with frequent stellar flares, both of which can strip away atmospheres over time.
Why Atmosphere and Water Go Together
A planet needs sufficient atmospheric pressure to keep water in liquid form. Below a certain pressure threshold, water can only exist as ice or vapor, skipping the liquid phase entirely. This is exactly the situation on Mars today. Conversely, too much atmosphere and too much greenhouse warming, as on Venus, can push temperatures so high that water boils off and eventually escapes to space permanently.
Holding onto an atmosphere over billions of years requires either strong gravity (as with gas giants), a protective magnetic field (as with Earth), or both. Mars, with its weak gravity and absent magnetic field, lost the atmospheric battle. Understanding this interplay between magnetic fields, atmospheric pressure, and stellar activity is central to predicting which worlds beyond our solar system might hold onto water long enough for anything interesting to happen in it.