In their scientific direction led by Quentin Williams, the scientists have produced a series of breakthroughs in elucidating the origins of these strange, water-heavy exoplanets. According to their new study, hydrogen-rich sub-Neptunes can act as forerunners of water-rich sub-Neptunes and super-Earths. The researchers published their findings in Nature on October 30, 2025. These discoveries shine new light on the evolution and makeup of fascinating worlds that fall in between sizes of Earth and Neptune-sized bodies.
The research team used state-of-the-art laser-heated diamond-anvil cell experiments on silicate melts in a hydrogen-rich environment. This method revealed a striking reaction: oxygen liberated from silicate melt interacts with hydrogen to yield water. The production-weighted results revealed an astounding increase almost quadrupling average water produced. This was significantly higher than previous predictions based off low-pressure ideal gas extrapolation.
The Role of Sub-Neptunes
Sub-Neptunes are the most common type of exoplanets we’ve found so far. They are bigger than our planet but smaller than gaseous Neptune. These planets often have a rocky core with a thick envelope of hydrogen (or water). The physical features and dynamic atmospheres of sub-Neptunes are key to unlocking the mysteries of life outside our home planet.
“The reaction we report here suggests that these planet types may be fundamentally related: hydrogen-rich sub-Neptunes could be the precursors of water-rich sub-Neptunes and super-Earths.” This link may change how scientists study the processes of planetary formation and evolution.
The research raises the possibility that sub-Neptunes, or hycean worlds, are more abundant than previously assumed. These planets might have a hydrogen-rich atmosphere that caps a deep layer of liquid water over rocky cores, if they are able to hold on to such an atmosphere. This unexpected discovery provides fresh paths by which to explore the habitability of such exoplanets.
Experimental Insights
The study combined state-of-the-art laser-heated diamond-anvil cell experiments to mimic conditions deep within these planets. By subjecting silicate melts to moderately high pressures in a hydrogen-rich environment, the team was able to observe chemical reactions that produced oxygen. It liberated a lot of water as it caused the reaction of oxygen and hydrogen along the way. This water accounted for up to about several tens of wt%.
We saw incredible reactions between warm, dense hydrogen fluid and silicate melt that shocked all of us,” said H. W. Horn, the HIREX ded study team. At high pressures, these reactions release silicon from the magma, and the silicate and alloy phases transform into alloys and hydrides, they reported. This key piece of evidence lays the groundwork for understanding the central role of high-pressure conditions in planetary formation.
These discoveries defy existing theories of planetary formation. They share that the processes that create water in these exoplanets are much more complicated than we had ever imagined.
Implications for Exoplanet Research
The impact of this research goes well beyond the ivory tower. The reality of water-rich sub-Neptunes and hycean worlds opens up an exciting new frontier for future exploration. Finding planets like these will help humanity take the next steps toward finding the right conditions for life out there among the stars.
For one thing, scientists are still figuring out how planetary formation works. Quentin Williams and his ASU team have certainly placed their study to be a key, early mover in this promising new space. This work goes a long way in clarifying how our own water-rich exoplanets may have gotten their start. It encourages further investigation into the wide range of planetary systems outside our own solar system.