For years, astronomers have been excited about the idea that some planets beyond our solar system might be covered by vast, global oceans.
One such world, K2-18b, made headlines in April 2025 when researchers suggested it could be a marine planet with deep oceans and possibly even life.
But a new study led by ETH Zurich tells a very different story: planets like K2-18b, known as sub-Neptunes, are not likely to be ocean worlds at all. In fact, they probably contain far less water than once imagined.
Sub-Neptunes are a class of planets larger than Earth but smaller than Neptune, and while our solar system has none, they appear to be common in the galaxy.
Some of these planets likely formed far from their host stars, in regions where ice could build up, and then moved inward over time. Because of this, scientists once believed that they might hold onto huge amounts of water.
These worlds were nicknamed “Hycean planets,” a blend of “hydrogen” and “ocean,” and were thought to have deep water layers beneath a hydrogen-rich atmosphere.
The new study, published in The Astrophysical Journal Letters, challenges that view.
Professor Caroline Dorn and her team at ETH Zurich, working with colleagues in Germany and the United States, included something earlier models had ignored: the chemical interactions between a planet’s atmosphere and its interior.
By adding chemistry to the picture, they discovered that much of the water scientists expected on sub-Neptunes is destroyed or locked away inside the planet.
Early in their lives, these planets would have been covered in hot magma oceans and wrapped in a thick blanket of hydrogen gas.
This combination persisted for millions of years. The researchers built a model that showed how the hydrogen in the atmosphere reacted with oxygen in the molten rock below, forming water molecules that then sank into the planet’s interior. Instead of collecting on the surface, the water largely disappeared underground, bound into minerals and metallic compounds.
To test their theory, the team ran computer simulations of 248 different planets, tracking 26 chemical components. The results were clear: far less water survived on the surface than previous studies predicted.
At most, only a few percent of a planet’s mass could remain as water. That means the dream of sub-Neptunes with oceans making up half of their mass is highly unlikely. According to Dorn, Hycean worlds with 10–90 percent water are essentially ruled out.
This conclusion also makes the search for alien life more challenging. Liquid water on the surface is one of the key requirements for life as we know it. If sub-Neptunes are too dry, then smaller rocky planets may be our best hope—but those will be harder to study, even with advanced observatories like the James Webb Space Telescope.
Interestingly, the study also suggests that Earth might not be as unusual as once thought. The amount of water on our planet may actually be quite typical. “Earth may not be as extraordinary as we think,” Dorn explained. “In our study, it appears to be a normal planet.”
The researchers were surprised by another twist: planets with the wateriest atmospheres weren’t those that formed in icy regions far from their stars. Instead, the water came from chemical reactions within the planets themselves, as hydrogen combined with oxygen in molten rock. This finding challenges traditional ideas that tie water-rich planets to icy origins.
These new insights reshape our understanding of how planets form and what they’re made of. They also highlight the importance of chemistry in shaping alien worlds. As lead author Aaron Werlen put it, the balance between magma and atmosphere plays a far greater role in determining a planet’s water content than how much ice it gathered during its birth.
In other words, sub-Neptunes may not be ocean planets after all—and Earth’s modest supply of water might be just the cosmic norm.
