Earth May Not Be So Special After All: New Study Reveals Most Exoplanets Are Drier Than We Thought
A new study from researchers at ETH Zurich, the Max Planck Institute for Astronomy, and the University of California, Los Angeles (UCLA) is reshaping how scientists think about water-rich worlds. The study suggests that planets outside our solar system, especially those larger than Earth but smaller than Neptune, likely contain far less surface water than previously believed.
For years, many scientists imagined that a certain type of exoplanet—known as sub-Neptunes—could host deep global oceans beneath hydrogen-rich atmospheres. These hypothetical planets, called Hycean worlds (from “hydrogen” and “ocean”), were once seen as promising candidates in the search for extraterrestrial life. But new findings show that such planets may not be the watery paradises we hoped for.
According to the study, chemical interactions between a planet’s atmosphere and interior drastically reduce the amount of water that remains near the surface. Instead, much of the water becomes locked away deep within the planet’s core. This revelation has major implications not only for how we understand these distant worlds but also for how we assess Earth’s own uniqueness in the cosmos.
What Makes Sub-Neptune Planets So Interesting
Sub-Neptunes occupy a curious middle ground. They’re larger than Earth but smaller than Neptune, typically ranging between 1.5 and 4 times Earth’s radius. Our solar system doesn’t have a planet of this size class, but telescopes like Kepler and TESS have found that sub-Neptunes are among the most common types of planets in the galaxy.
One particularly famous example is K2-18b, located about 124 light-years away in the constellation Leo. This planet orbits a small red dwarf star and has been studied extensively because its atmosphere contains water vapor, methane, and carbon dioxide—molecules that sparked speculation it could host a liquid water ocean.
However, the new ETH Zurich-led study challenges this idea. The researchers argue that K2-18b and other sub-Neptunes likely don’t have thick layers of water or vast global oceans. Instead, they are probably dominated by rocky interiors and dense hydrogen envelopes with only a small fraction of water.
What the Researchers Actually Did
The team, led by Aaron Werlen and Caroline Dorn, combined two advanced models—one that tracks planetary evolution and another that simulates chemical processes between a planet’s atmosphere and its molten interior.
Their analysis covered 248 model planets, each representing a variation of sub-Neptune composition, temperature, and distance from its star. The key innovation in their approach was including chemical coupling—the idea that gases in a planet’s atmosphere don’t just sit idly above the surface but actively react with the molten rock beneath.
In their simulations, these planets initially possessed deep, hot magma oceans covered by thick hydrogen-rich atmospheres. Over millions of years, elements in the magma and gases in the atmosphere interacted chemically. These reactions destroyed a significant portion of the water (H₂O) by splitting it into hydrogen and oxygen, which then bonded with metals and silicates in the magma.
The end result? Most of the water that these planets started with disappeared into their interiors. Only a small amount—typically a few percent at most—remained as surface water or atmospheric vapor.
The Surprising Chemistry Behind It
The researchers analyzed 26 different chemical components in equilibrium to understand how hydrogen, oxygen, and various metals interacted. They discovered that under the high pressures and temperatures of these young planets, hydrogen and oxygen prefer to combine with metallic compounds rather than form stable water molecules.
This process means that even if a planet begins with a lot of ice or water, chemical bonding over time drives most of that water deeper into the planet. The end result is a body that may appear to have water in its atmosphere but actually contains very little liquid water overall.
In other words, those spectacular illustrations of “water worlds” with endless global oceans might be more imagination than reality.
The Role of the “Snow Line”
A key insight from this study concerns where planets form in their star systems. The snow line is the distance from a star beyond which temperatures are cold enough for water to freeze into ice.
Scientists long assumed that planets forming beyond the snow line would naturally accumulate large ice inventories, leading to water-rich worlds. But the ETH Zurich study found the opposite might be true.
Planets that formed inside the snow line—where it’s too warm for ice—can actually develop more water-rich atmospheres than those that formed farther out. This happens because hydrogen in the atmosphere can chemically react with oxygen in the planet’s molten silicates to create new water molecules.
The paradox is that these water-rich atmospheres do not necessarily correspond to water-rich planets. The water detected is more likely produced chemically rather than inherited from icy origins.
Implications for the Search for Life
For astrobiologists, this research is both sobering and enlightening. It suggests that sub-Neptunes—previously thought to be promising candidates for habitability—are probably poor environments for life as we know it.
These planets likely have high pressures, dense hydrogen atmospheres, and limited surface water, making them far less hospitable than earlier models indicated. The dream of finding an ocean-covered exoplanet teeming with microbial life may have just become a little harder.
But there’s also an intriguing upside: Earth might not be the rare, water-rich jewel we thought it was. According to the study, Earth’s total water content is fairly typical compared to many other rocky planets. The big difference lies in how much of that water is accessible at the surface.
This finding subtly reframes our place in the universe. Instead of being an outlier, Earth might represent a common planetary balance—enough water to sustain life, but not so much that it becomes a global ocean world.
Why This Study Matters
This research highlights a major blind spot in earlier models: the lack of chemical realism in planet formation theories. Many previous studies treated a planet’s atmosphere and interior as separate layers, ignoring how materials mix and react under extreme conditions.
By integrating chemistry, the ETH Zurich team revealed that a planet’s composition evolves dynamically, with its surface and atmosphere deeply influencing one another.
This understanding also affects how scientists interpret data from telescopes like the James Webb Space Telescope (JWST). When JWST detects water vapor in an exoplanet’s atmosphere, that doesn’t automatically mean the planet has oceans. It could just mean that hydrogen and oxygen are reacting chemically within its upper layers.
The new findings will likely reshape how planetary scientists interpret atmospheric spectra, especially for sub-Neptunes and mini-Neptunes—the most common planet types discovered so far.
What About K2-18b?
K2-18b remains one of the most closely studied exoplanets because of its size, location, and atmospheric signals. Earlier data from JWST showed traces of methane, carbon dioxide, and possibly dimethyl sulfide (DMS), a gas associated with biological processes on Earth.
Those detections led to speculation that K2-18b might host a liquid water ocean beneath its atmosphere—a true Hycean world.
However, according to the ETH Zurich study, the chemistry of sub-Neptunes makes that scenario unlikely. K2-18b probably has a thick hydrogen atmosphere and a rocky or icy interior, with limited liquid water at the surface.
So while it may still be an exciting target for observation, it’s unlikely to resemble the kind of blue ocean planet we often imagine.
Earth in a New Light
One of the most intriguing conclusions of the research is how it puts Earth into perspective. Despite our planet’s abundant oceans, the new models suggest that Earth’s water inventory isn’t exceptional. Many other worlds may contain similar total water percentages—just distributed differently within their interiors.
The fact that Earth’s water remains accessible on the surface might be a rare but natural outcome of planetary evolution, rather than an anomaly.
This realization also helps refine our understanding of habitability. Instead of searching for planets swamped in water, scientists may now focus on those with moderate water content—a balance that allows for both continents and oceans, like Earth.
The Bigger Picture
This research doesn’t just apply to K2-18b—it reshapes the entire conversation about how planets form and evolve. By acknowledging the deep chemical ties between a planet’s surface and interior, scientists can build more realistic models of exoplanets’ compositions and histories.
It also hints that the origins of water—a key ingredient for life—might depend less on where a planet forms and more on how it chemically evolves during its early molten phase.
In short, water may be common across the universe, but surface oceans may not be.
And while that makes the search for habitable worlds more challenging, it also makes every discovery more meaningful.
