When astronomers announced in April 2025 that the distant planet K2-18b might be a marine world cloaked in a deep global ocean, the discovery captured imaginations worldwide. The idea was electrifying: an exoplanet 124 light-years away, circling a cool red dwarf star, perhaps teeming with alien life. For a moment, it felt as though humanity had taken one step closer to answering its oldest question: Are we alone?
But science is never static. Just months later, a new study has upended that vision of endless oceans and thriving ecosystems. Instead of watery wonderlands, planets like K2-18b—known as “sub-Neptunes”—may be far drier than previously believed, their oceans reduced to thin layers at best, or swallowed almost entirely into their interiors.
The research, led by ETH Zurich in collaboration with the Max Planck Institute for Astronomy in Heidelberg and the University of California, Los Angeles, suggests that the universe may not be overflowing with Hycean planets—worlds with hydrogen-rich atmospheres and vast oceans. Instead, the story of water on alien planets is far more complex, and perhaps far less generous to life.
The Mystery of Sub-Neptunes
K2-18b belongs to a class of exoplanets that lies between Earth and Neptune in size. Larger than our home world yet smaller than the ice giant, sub-Neptunes are alien to our solar system but common across the galaxy. Thousands have been identified, and astronomers once considered them prime candidates for habitability.
The reasoning seemed simple: many of these planets are thought to have formed beyond the snow line, the region of a star system where water freezes into ice. In this cold nursery, they could have accumulated vast amounts of frozen water before drifting inward toward their stars. With thick hydrogen atmospheres and warming starlight, those icy reserves might melt into deep oceans, turning sub-Neptunes into Hycean worlds—potential havens for life.
The idea was compelling, even poetic. Oceans beneath alien skies, waves rippling on distant shores, life emerging far from Earth’s cradle. But the new study warns that this vision may have been built on a shaky foundation.
The Chemistry That Changed Everything
The flaw in earlier studies, according to Caroline Dorn, professor of exoplanets at ETH Zurich, was the neglect of chemistry—the subtle but decisive interactions between a young planet’s atmosphere and its molten surface.
“When planets like K2-18b were forming, they were not calm ocean worlds,” explains Aaron Werlen, lead author of the study. “They were fiery spheres covered in magma oceans, wrapped in hydrogen atmospheres.”
This combination, as the researchers modeled, creates a dramatic outcome. At such extreme temperatures, water molecules don’t remain stable. Instead, hydrogen and oxygen are stripped apart, bonding with metals and silicates in the magma. The result is that much of the water vanishes—not into space, but into the planet’s rocky interior, locked away in minerals.
The team simulated 248 model planets, calculating the chemical equilibrium of 26 different compounds. The outcome was clear: the majority of water disappears, leaving only a few percent of a planet’s total mass as surface-accessible H2O.
In other words, the grand oceans of Hycean planets may never have existed.
The Disappearing Oceans
The implications of this chemistry are profound. Even if sub-Neptunes once accumulated enormous stores of water ice during their formation, most of it is either chemically destroyed or buried deep within the planetary mantle. Instead of vast seas covering alien worlds, what remains at the surface is meager.
“Hycean planets with 10 to 90 percent of their mass in water are very unlikely,” Dorn concludes. “Instead, we find that the actual water content at the surface is limited to a few percent.”
This finding overturns the once-optimistic picture of watery exoplanets scattered across the galaxy. It also narrows the scope of where life might emerge. If deep oceans are rare, then truly habitable planets may be far scarcer than we imagined.
Earth’s Place in the Universe
And yet, there is a twist of hope in this story. While K2-18b may not be a watery haven, the new results suggest something remarkable: Earth itself may be more typical than we thought.
“Earth may not be as extraordinary as we imagine,” Dorn reflects. “In our models, it appears to be a common kind of planet.”
This realization is profound. For years, we have wondered whether Earth is a cosmic outlier, blessed with oceans against all odds. But if sub-Neptunes are too dry, then perhaps smaller, rocky planets like Earth—modest in size, balanced in water—are the real norm. That makes the search for life beyond our world no less urgent, but it shifts the focus toward Earth-sized planets, the kind that telescopes of the future will be able to study in greater detail.
A Cosmic Paradox
Perhaps most surprising of all, the study revealed that the planets with the most water-rich atmospheres are not those that formed far from their stars, laden with ice, but those that formed closer in, within the snow line. Here, instead of inheriting water from frozen reservoirs, planets generated it chemically: hydrogen in their atmospheres reacted with oxygen from molten silicates, creating new water molecules.
This paradox turns conventional wisdom on its head. Ice-rich formation does not guarantee water-rich worlds. Instead, the balance between magma oceans and atmospheres appears to be the true architect of a planet’s destiny.
Rethinking the Search for Life
The dream of Hycean worlds may have dimmed, but the search for extraterrestrial life is far from over. If anything, this new study sharpens our vision. It tells us where not to look, and where our best chances may lie.
The James Webb Space Telescope has already begun probing the atmospheres of exoplanets, but the next generation of observatories—larger, sharper, more sensitive—will be needed to study Earth-sized worlds orbiting other stars. There, in smaller planets with balanced water budgets, may lie the best hope of finding a true twin to our home world.
The Story Continues
The tale of K2-18b reminds us of the beauty and the humility of science. One moment, we dream of alien oceans; the next, we discover the silent chemistry that erases them. But this is not a disappointment—it is progress. With each step, we refine our understanding of the universe and our place within it.
K2-18b may not host the marine paradise we once envisioned, but it has already given us something just as valuable: a clearer map of the cosmos, a sharper sense of where to seek life, and a renewed appreciation for the fragile balance of Earth’s oceans.
The oceans of alien worlds may vanish into stone, but the human desire to seek, to question, and to understand flows endlessly on.
More information: Sub-Neptunes Are Drier Than They Seem: Rethinking the Origins of Water-Rich Worlds, The Astrophysical Journal Letters (2025). DOI: 10.3847/2041-8213/adff73
