Neptune and Uranus, the two ice giants, contain far more water as a fraction of their mass than any other planet, with water, ammonia, and related ices making up the bulk of their interiors. In absolute terms, though, Jupiter’s sheer size means it could rival or exceed them in total water mass despite water being a much smaller slice of its composition. The answer shifts depending on whether you care about concentration, total volume, or liquid water you could swim in, and each framing points to a different winner.
Why the Ice Giants Dominate by Proportion
Uranus and Neptune earned the label “ice giants” because water and other volatile ices are thought to account for roughly 60 to 70 percent of their total mass. That is a staggering concentration compared to the gas giants Jupiter and Saturn, where hydrogen and helium overwhelmingly dominate. Interior models of Uranus and Neptune suggest that beneath a relatively thin hydrogen-helium envelope lies a deep, hot mantle composed largely of water, methane, and ammonia compressed to extreme densities. Thermodynamic models fitting the planets’ measured gravity fields find that Neptune requires a much higher water fraction in its outer envelope than Uranus does, with Neptune needing at least ten times the water concentration relative to hydrogen compared to Uranus in that region.1The Planetary Science Journal. Thermodynamically Governed Interior Models of Uranus and Neptune
That difference matters. Even though the two planets are often lumped together as near-twins, Neptune appears to be considerably more water-rich relative to its size. Because Neptune is also slightly more massive than Uranus (about 17 Earth masses versus about 14.5), it carries a larger absolute quantity of water as well. If you ask which single planet has the highest water-to-everything-else ratio, Neptune is the strongest candidate by a wide margin.
What About Jupiter’s Enormous Bulk?
Jupiter is more than 300 times the mass of Earth, which means even a modest water fraction adds up to a colossal quantity. Measurements from the Galileo probe in 1995 found less water than expected, but that probe dropped into an unusually dry atmospheric hotspot. NASA’s Juno mission, orbiting Jupiter since 2016, has returned microwave radiometer data suggesting that water is more abundant in the planet’s equatorial atmosphere than Galileo indicated, though the global average is still being refined. Estimates from various models place Jupiter’s bulk water content somewhere between a few and perhaps ten or more Earth masses of water, but the uncertainty is large.
Here is the tension: Neptune may contain the equivalent of a few Earth masses of water packed into a body only 17 times Earth’s mass, while Jupiter may contain a comparable or even larger absolute amount spread through a body more than 300 times Earth’s mass. Which one “has more water” depends on whether you are asking about the fraction or the total. For sheer tonnage, Jupiter could win. For the planet most defined by water, Neptune is the clear answer. Saturn falls somewhere in between, with much more mass than the ice giants but a similarly hydrogen-dominated composition that keeps its water fraction low.
Water That Does Not Behave Like Water
One reason the question is tricky is that the “water” inside these planets bears almost no resemblance to anything you would recognize. At the pressures found deep inside Neptune and Uranus, water enters exotic states that do not exist on Earth’s surface. One well-studied phase is called superionic water, where oxygen atoms lock into a solid crystal lattice while hydrogen atoms (as bare protons) flow freely through that lattice like a liquid. Simulations confirm that an extended superionic water layer likely exists deep in Neptune-like planets.2Icarus. The phase diagram of water and the magnetic fields of Uranus and Neptune Further modeling shows that at the temperatures and pressures inside these worlds, even mixtures of silica and water take on superionic behavior, with protons diffusing freely while silicon and oxygen frameworks stay rigid.3PubMed. Superionic Silica-Water and Silica-Hydrogen Compounds in the Deep Interiors of Uranus and Neptune
At still higher pressures, laboratory experiments and simulations have found that water ice can enter a partially ionic solid phase where the familiar tetrahedral bonding of ice breaks down entirely, replaced by alternating layers of hydroxide and hydronium-like units carrying partial charges.4Nature Communications. High pressure partially ionic phase of water ice These exotic phases are electrically conductive, and planetary scientists think they help explain the unusual, tilted magnetic fields of Uranus and Neptune. So when we say these planets “contain water,” what we really mean is that they contain enormous quantities of H₂O molecules, but those molecules exist in states that look nothing like ice cubes or ocean waves. This makes comparisons with Earth’s oceans a bit apples-to-oranges.
Europa, Ganymede, and the Ocean Moons
Some of the most exciting water in the solar system is not inside a planet at all. Several moons harbor subsurface oceans of actual liquid water, kept warm by tidal heating from their parent planets. If you care specifically about liquid water rather than exotic compressed states, these moons deserve attention.
Europa, orbiting Jupiter, has an ice shell estimated at roughly 30 kilometers thick sitting above a liquid ocean about 100 kilometers deep.5Icarus. Thermal Equilibrium States of Europa’s Ice Shell: Implications for Internal Ocean Thickness and Surface Heat Flow That ocean alone contains roughly two to three times the volume of all Earth’s oceans combined. Ganymede, Jupiter’s largest moon and the largest moon in the solar system, may have an even deeper water layer. Its water-rock interface sits at about 800 kilometers below the surface, and its ocean could include multiple layers of high-pressure ice sandwiched between liquid water zones, especially if the ocean is salty enough to make those ice layers buoyant.6Journal of Geophysical Research: Planets. Geophysical Investigations of Habitability in Ice‐Covered Ocean Worlds In raw volume of H₂O (ice plus liquid), Ganymede likely holds more water than Earth does.
Saturn’s moon Titan also hosts a subsurface ocean of water mixed with ammonia beneath its icy crust.7PubMed Central. Protein Stability in Titan’s Subsurface Water Ocean Early in Titan’s history, ammonia-enriched water fluids interacted with silicate rock and were brought to the surface through cryovolcanism.8Journal of Geophysical Research: Planets. Silicate interactions with ammonia‐water fluids on early Titan Enceladus, another of Saturn’s moons, famously shoots geysers of salty water into space from cracks near its south pole, confirming a subsurface reservoir there as well, though Enceladus is tiny and its ocean is correspondingly smaller.
None of these moons count as “planets,” of course, so they do not technically answer the title question. But they reshape the picture. If you zoom out and ask which body in the solar system has the most liquid water, Europa or Ganymede could plausibly beat Earth. The moons remind us that liquid water is surprisingly common in the outer solar system, it is just hidden under ice.
Earth’s Underappreciated Water Budget
Earth is the only planet with stable liquid water on its surface, which is why it looks so blue from space. Its oceans hold about 1.335 billion cubic kilometers of water, covering roughly 71 percent of the surface. That is a lot by human standards, but it is a thin film on a planetary scale. All of Earth’s water, surface and subsurface combined, makes up less than about 0.1 percent of the planet’s total mass.
What many people do not realize is that Earth stores a significant amount of water deep inside. The mantle transition zone, a layer roughly 410 to 660 kilometers below the surface, contains minerals like ringwoodite that can hold water within their crystal structure. Research fitting observed mantle viscosity profiles suggests that ringwoodite in this zone contains around 1 to 2 weight percent water, making the transition zone nearly water-saturated on a global scale.9PubMed Central. A nearly water-saturated mantle transition zone inferred from mineral viscosity If those estimates are correct, the mantle transition zone alone could hold as much water as all the surface oceans or more. This water is not sloshing around as liquid; it is locked inside mineral structures at extreme pressure. But it is still H₂O, and it affects how the mantle flows, how volcanoes work, and how plate tectonics operate.
Even counting all of this hidden water, Earth’s total water inventory remains modest compared to an ice giant. The distinction is that Earth has the most accessible, surface-stable liquid water of any planet, which is why it supports the only biosphere we know of.
What Happened to Water on Mars and Venus
Mars and Venus are cautionary tales about how easily a planet can lose its water. Venus today is searingly hot and bone-dry at the surface, but measurements from the Pioneer Venus probe found that deuterium (heavy hydrogen) is about a hundred times more enriched relative to normal hydrogen in Venus’s atmosphere compared to Earth. That enrichment is a chemical fingerprint of massive water loss: lighter hydrogen escapes to space more easily, leaving the heavier isotope behind. The measurement implies Venus once outgassed at least 0.3 percent of an Earth ocean’s worth of water, and the true original amount could have been much greater.10PubMed. Venus was wet: a measurement of the ratio of deuterium to hydrogen Whether Venus ever had standing oceans or merely a thick steam atmosphere remains debated, but it clearly started with far more water than it has now.
Mars tells a different version of the same story. Its thin atmosphere cannot support liquid water on the surface today, but orbital radar has revealed water ice buried in the subsurface and locked in both polar caps. The MARSIS radar instrument probed the widespread Medusae Fossae Formation and found it could contain large amounts of water ice, and it also provided evidence of a stable body of liquid brine beneath the south polar layered deposits.11Space Science Reviews. Water Ice in the Subsurface and Polar Caps of Mars Carved river valleys and lake beds across the Martian surface confirm that liquid water was abundant billions of years ago. Mars lost most of it to space after its magnetic field weakened, though exactly how much remains underground is still an open question.
Pluto and the Outer Reaches
Water turns up in places you might not expect. Pluto, demoted from planet status in 2006, appears to have a subsurface ocean of its own. Analysis of the Sputnik Planitia basin, the large heart-shaped feature visible in New Horizons images, indicates that the basin can only produce the observed gravitational anomaly if Pluto has a salty ocean more than 100 kilometers thick beneath its icy crust.12Geophysical Research Letters. Formation of the Sputnik Planum basin and the thickness of Pluto’s subsurface ocean A world smaller than Earth’s Moon, orbiting billions of kilometers from the Sun, maintaining a liquid ocean for billions of years is one of the more surprising discoveries in recent planetary science. It suggests that subsurface oceans may be common among icy bodies throughout the outer solar system, not just around the giant planets.
Comets and asteroids also carry water, though in much smaller total quantities. The broader relevance is that these small bodies are thought to have delivered much of Earth’s water in the first place. Isotopic evidence from meteorites, particularly the CI and CM groups that are probably related to the most common carbon-rich asteroids, suggests that asteroids rather than comets were the main source of hydrogen (and by extension water) accreted by the terrestrial planets.13PubMed Central. Water Reservoirs in Small Planetary Bodies: Meteorites, Asteroids, and Comets Modeling of how deuterium fractionation works in the early solar nebula supports this picture: the hydrogen isotope ratio matching Earth’s ocean water is reproduced at asteroid-belt distances from the Sun, while cometary water tends to carry a heavier isotopic signature.14The Astrophysical Journal. Chemodynamical Deuterium Fractionation in the Early Solar Nebula: The Origin of Water on Earth and in Asteroids and Comets
How the Snow Line Shaped Everything
The distribution of water across the solar system is not random. It traces back to a feature of the early solar nebula called the snow line (or frost line), the distance from the young Sun beyond which temperatures were low enough for water vapor to condense into solid ice. Inside the snow line, water stayed as vapor and was largely swept away by solar radiation, leaving the inner planets relatively dry. Beyond it, water ice was abundant and could be incorporated into growing planetary bodies.
This is why the giant planets and their moons are so water-rich. They formed in the zone where water ice was a major building block. Research into how the snow line moved during energetic outbursts from the young Sun shows that cooling after these flare-ups could rapidly create pebble-sized ice grains in a band stretching from roughly Jupiter’s orbit outward to Saturn’s distance and beyond.15Monthly Notices of the Royal Astronomical Society. FU Orionis outbursts, preferential recondensation of water ice, and the formation of giant planets Those icy pebbles accumulated into the cores that eventually became Jupiter, Saturn, Uranus, and Neptune. The terrestrial planets, forming inside the snow line, had to get their water delivered later by impacts from water-bearing asteroids and comets that had formed farther out.
This origin story explains the paradox at the heart of the question. Earth feels like a water world to us because we live on its surface, but cosmically speaking it is a dry rock that received a thin veneer of water secondhand. The real water worlds are the ice giants, built from water ice from the start.
Detecting Water Beyond Our Solar System
The same question, which planet has the most water, is starting to be asked about other star systems too. Advances in direct imaging spectroscopy have made it possible to detect water absorption lines in the atmospheres of exoplanets. Researchers have resolved individual molecular lines from both water and carbon monoxide in the atmosphere of a massive planet orbiting the star HR 8799, less than 40 astronomical units from its host star.16PubMed. Detection of carbon monoxide and water absorption lines in an exoplanet atmosphere This technique works best for large, hot, self-luminous planets in wide orbits, but it opens a window onto how common water-rich worlds might be across the galaxy.
The James Webb Space Telescope has expanded this capability enormously since its launch, detecting water vapor in the atmospheres of smaller and cooler exoplanets than were previously accessible. Early results suggest that water is common in exoplanet atmospheres, which aligns with what we see in our own solar system: water is one of the most abundant molecules in the universe, formed from the two most common reactive elements, hydrogen and oxygen. The real question is not whether distant planets have water, but how much, in what state, and whether any of it sits on a rocky surface the way Earth’s does. For now, Neptune-like worlds orbiting other stars are probably the best candidates for the highest water content, just as Neptune leads the pack here at home.