An international team of researchers led by ETH Zurich has found that many sub-Neptune exoplanets—those larger than Earth but smaller than Neptune—are far drier than previously assumed. For years, scientists speculated that these planets could be ocean worlds, covered in deep layers of water and potentially capable of supporting life. However, new simulations suggest that such Hycean planets are highly unlikely to exist. Instead, the amount of surface water on these exoplanets is limited to only a few percent, placing them closer to Earth in terms of water content.
The study, conducted in collaboration with the Max Planck Institute for Astronomy and the University of California, Los Angeles, challenges the widely held view that sub-Neptunes formed beyond the snow line—the region in space where water freezes into ice—retained massive water reservoirs. While these planets may have initially accumulated significant amounts of ice, chemical interactions between their magma oceans and hydrogen-rich atmospheres caused most of that water to evaporate or be absorbed into their interiors.
According to Caroline Dorn, professor of exoplanets at ETH Zurich, the breakthrough came from considering the chemical coupling between planetary interiors and atmospheres, a factor ignored in earlier research. By modeling 248 different planets and analyzing 26 chemical components, researchers discovered that water molecules (H2O) were destroyed as hydrogen and oxygen bonded with metals and silicates, sinking into the planet’s core. This left only trace amounts of water at the surface.
The simulations show that Hycean worlds with 10–90% water mass are extremely unlikely, overturning earlier optimistic views that such planets could harbor life. Even though a few high-profile exoplanets such as K2-18b sparked excitement by appearing to show signs of water-rich atmospheres, the new study suggests their water content is far more limited than anticipated.
This finding complicates the ongoing search for extraterrestrial life, as it narrows the range of exoplanets likely to host stable, life-supporting oceans. The results indicate that only smaller, rocky planets—similar to Earth—may provide the right conditions for liquid water. Observing these worlds, however, will require more advanced telescopes than even the James Webb Space Telescope currently offers.
Interestingly, the research also revealed a paradox. Planets with the most water-rich atmospheres were not those formed far beyond the snow line but rather those formed inside it. On these planets, water was generated through chemical reactions between hydrogen in the atmosphere and oxygen in silicates from the magma ocean, rather than accumulated as frozen ice. This underscores the importance of magma-atmosphere equilibrium in determining planetary composition.
Conclusion: The new study fundamentally changes how we view exoplanets and their potential to support life. Instead of being exotic oceanic worlds, most sub-Neptunes appear to be relatively dry, with water making up only a tiny fraction of their composition. Far from being unique, Earth may represent a typical planetary model in terms of water content. These insights not only reshape theories of planetary formation but also guide future searches for habitable worlds beyond our solar system.
