The Surprisingly High Abundance of Water Worlds

For years, water worlds were treated as an exotic possibility — scientifically plausible, but statistically rare. Planets dominated by deep global oceans, wrapped in thick atmospheres and layered with high-pressure ice, seemed like outliers in the cosmic inventory. The search for exoplanets focused primarily on “Earth-like” rocky worlds with thin atmospheres and moderate climates. However, as observational data have accumulated, a different picture has emerged. Water-rich planets may not be exceptional at all. They could be one of the most common planetary types in our galaxy.

Defining a Water World

In planetary science, a “water world” refers to a planet where water — in the form of liquid, ice, or supercritical fluid — makes up a substantial fraction of the planet’s total mass. In extreme cases, this fraction could range from 10% to 50%. For comparison, all of Earth’s oceans, glaciers, and groundwater combined account for less than 0.1% of our planet’s mass. Despite its vast seas, Earth is not technically a water world.

These planets are believed to form beyond the “snow line” in a protoplanetary disk — the region far enough from a young star where temperatures are low enough for water to condense into ice. In such regions, ice becomes a primary building material. Growing planetary embryos accumulate large quantities of frozen volatiles. If gravitational interactions later cause these bodies to migrate inward, closer to their host star, the ice can melt, potentially forming global oceans hundreds of kilometers deep.

What the Data Are Telling Us

The shift in perspective is largely driven by exoplanet surveys conducted by missions such as Kepler and TESS, along with increasingly precise measurements of planetary mass and radius. These two parameters allow scientists to estimate average density, which in turn provides clues about internal composition.

A striking pattern has appeared among planets with radii between about 1.5 and 2.5 times that of Earth — often referred to as super-Earths or sub-Neptunes. Many of these planets have densities too low to be composed purely of rock and iron, yet not low enough to require thick hydrogen-helium envelopes like those of gas giants. One consistent explanation is that they contain large amounts of water.

Consider a hypothetical planet with twice Earth’s radius but only three to four times Earth’s mass. A purely rocky composition would predict a higher mass for that size. If it lacks a massive gaseous envelope, the missing density must be explained by lighter materials — most plausibly water. Interior models suggest such planets may consist of a rocky core, overlaid by a thick mantle of water, transitioning from liquid ocean layers to high-pressure ice phases such as Ice VII or Ice X at extreme depths.

Oceans Without Shores

A water world is not simply “Earth with more ocean.” On some of these planets, the ocean depth could reach hundreds of kilometers. At such pressures, the water at the bottom would not remain liquid. Instead, it would transform into exotic crystalline ice phases that exist only under extreme compression. These high-pressure ice layers could separate the liquid ocean from the rocky mantle below.

This structural difference has major implications. On Earth, the interaction between ocean water and silicate rock drives key geochemical cycles, including the carbon-silicate cycle that stabilizes long-term climate. Hydrothermal vents at mid-ocean ridges circulate minerals and may have played a crucial role in the origin of life. If a high-pressure ice layer isolates the ocean from the mantle, those interactions might be limited or absent.

However, this does not automatically eliminate the possibility of habitability. Some models suggest that convection within the ice layer or localized melting could still enable material exchange. Additionally, life — if it exists — might adapt to very different chemical regimes than those found on Earth.

Atmospheric Effects and Climate

The atmospheric properties of water worlds add another layer of complexity. Planets located closer to their stars could develop thick steam atmospheres, producing strong greenhouse effects. Under certain temperature and pressure conditions, water may enter a supercritical state — neither a conventional liquid nor a gas, but something in between. In such cases, the boundary between ocean and atmosphere may blur entirely.

Observations of exoplanet atmospheres using transmission spectroscopy have already detected water vapor in several cases. As next-generation telescopes refine these measurements, scientists may be able to quantify water abundance more precisely and distinguish between steam-dominated atmospheres and those containing lighter gases.

A Statistical Shift

Perhaps the most significant realization is statistical. Early in the exoplanet era, astronomers naturally focused on finding close analogs to Earth. Yet the data now indicate that the most common planets in the Milky Way are not exact Earth-sized rocky worlds. Instead, planets in the super-Earth and sub-Neptune range appear to dominate.

If even a fraction of these planets are water-rich, then water worlds could number in the billions within our galaxy alone. That possibility challenges long-standing assumptions about what constitutes a “typical” planet.

In fact, Earth’s balance of oceans and continents may be relatively unusual. Our planet’s moderate water inventory, combined with exposed landmasses and active plate tectonics, might represent a specific and perhaps rare evolutionary pathway.

Rethinking Habitability

The growing evidence for the widespread presence of water worlds forces a reconsideration of habitability criteria. Traditionally, the search for life has centered on rocky planets within the habitable zone — the region where liquid water can exist on the surface. But if vast ocean planets are common, life could potentially emerge in environments very different from Earth’s.

Rather than narrow coastal ecosystems and shallow seas, entire biospheres — if they exist — might thrive beneath global oceans with no continents at all. Alternatively, some water worlds may prove too isolated internally to sustain long-term biological processes.

Either way, the implication is clear: the cosmic distribution of water may be far more generous than once assumed. Oceans could be among the most widespread environments in the galaxy.

Water worlds are no longer speculative curiosities. They are emerging as a dominant planetary class. And as our detection methods improve, we may find that deep, planet-wide oceans are not exceptions — but one of the universe’s standard designs.