A new study challenges the idea that Hycean planets are ocean worlds. Chemical reactions between hydrogen-rich atmospheres and molten interiors may reduce surface water, making these sub-Neptunes less likely to support life than once thought.
In April 2025, the exoplanet K2-18b sparked a lot of excitement. This planet orbits a small star located approximately 124 light years from Earth. Scientists from Cambridge suggested that it might be a water world, a planet covered by a massive ocean and possibly capable of supporting life.
However, a new study suggests that planets similar to K2-18b, known as sub-Neptunes, are probably not dominated by water. The conditions on such planets are likely more hostile than previously thought.
What is a Sub-Neptune or Hycean Planet?
Sub-Neptunes are planets that are larger than Earth but smaller than Neptune. There is no planet exactly like them in our Solar System, although they appear to be common throughout the galaxy. Some of these planets formed far from their stars, beyond what astronomers refer to as the snow line, the region where water in its solid form can exist. Over time, these planets may have moved inward.
Scientists had previously thought that some of these planets could have accumulated significant amounts of water during their formation, resulting in deep global oceans beneath thick hydrogen-rich atmospheres.
Planets with such characteristics are sometimes called Hycean worlds, a term combining “hydrogen” and “ocean.”
The New Study: How Chemistry Changes the Picture
This recent research was led by ETH Zurich, with contributions from the Max Planck Institute in Heidelberg and UCLA. It challenges the idea that Hycean worlds are common. The key finding is that past studies often overlooked the chemical interactions within planets, how the atmosphere reacts with the planet’s hot interior, such as magma.
Early in a planet’s life, a sub-Neptune might have a magma ocean, a layer of molten material beneath its atmosphere. Over millions of years, hydrogen gas from the atmosphere and materials like metals and silicates within the magma interact chemically. Using computer simulations, the researchers modelled how these chemical reactions affect the amount of water that remains on or near the surface of such planets.
What the Simulations Showed
The team ran models for 248 different planets. Their calculations revealed that most of the water originally acquired during the planet’s formation does not remain as water on or near the surface. Instead, hydrogen and oxygen atoms often end up bound to metallic compounds, which then sink or are lost into the deep interior or core of the planet.
Although such models have limitations, the researchers are confident in the overall trends. The result is that only a few percent of the planet’s mass remains as surface water at most. The rest is hidden in forms that are not suitable for life as we know it. Therefore, worlds with 10 to 90 percent water by mass, as previously speculated, seem highly unlikely.
Implications for Life
If there is much less water on the surface, the likelihood of conditions favourable to life drops significantly. Habitable conditions are considered to require liquid surface water, moderate pressures and temperatures, and an atmosphere that supports favourable chemistry. These requirements may be more applicable to smaller planets, similar to Earth.
This study also suggests that our own planet might not be that unique. Earth’s water content could be more typical among rocky or sub-Neptune type planets than previously assumed.
Why This Changes How We Search for Life
These findings will influence how astronomers interpret data from telescopes like the James Webb Space Telescope. When we observe certain atmospheric signatures, we’ll need to determine whether they reflect true surface water or just chemical water produced during formation, or if most of the water has been lost or buried.
It also means that the search for life might be more challenging because surface oceans are less likely. Planets once thought to be ocean worlds could instead be largely rocky or metallic, with only a thin layer of water or water locked inside. The more we understand about planets’ chemical composition and formation history, the better we can assess whether they might be habitable.