A curious phenomenon has left astronomers puzzled over a strange category of planets orbiting distant stars, according to newly published research.
Scientists report the discovery of surprisingly water-rich exoplanets, despite forming in places once believed to be far too hot for water to exist. Now, a new study published in the journal Nature suggests these mysterious water worlds may be manufacturing their own oceans through chemical reactions deep within their interiors.
The research challenges fundamental assumptions about how planets form and where water can exist in the universe, potentially reshaping scientists’ understanding of planetary evolution and the search for habitable worlds beyond our solar system.
The study, led by researchers at Arizona State University in collaboration with teams from the Open University of Israel and the University of Chicago, focused on sub-Neptunes—planets larger than Earth but smaller than Neptune. These worlds are among the most common types of exoplanets discovered by NASA’s Kepler mission, with sizes ranging from one to four times Earth’s radius.
Scientists typically model these planets as having rocky cores surrounded by thick envelopes of either hydrogen gas or water. This composition makes sense for planets that form far from their host stars, beyond the “snow line” where temperatures drop low enough for water to freeze as ice. But observations have revealed numerous sub-Neptunes orbiting much closer to their stars, where conditions should be far too hot to retain surface water.
So where is this water coming from?
Previous theories couldn’t adequately explain the abundance of water on these exoplanets. The first theory is that comets and asteroids can deliver some water to planetary surfaces through surface impacts. However, the amounts observed on many sub-Neptunes far exceed what could plausibly arrive through such a bombardment. The second theory postulates that water-rich planets must form in the outer reaches of their solar systems and then migrate inward over time.
The new study offers a different explanation: the planets are producing water internally through reactions between their hydrogen atmospheres and molten rock cores.
Using diamond-anvil cells and pulsed laser heating at the University of Chicago’s Advanced Photon Source synchrotron facility, the research team recreated the extreme conditions found at the boundary between a sub-Neptune’s rocky core and its hydrogen envelope. These experiments subjected samples of olivine, fayalite, and silica—common rock-forming minerals—to pressures up to 10,000 times Earth’s atmospheric pressure and temperatures exceeding 3,000 degrees Kelvin.
Under these conditions, the silicate minerals melted into magma. Then, something interesting happened. The oxygen atoms liberated from the molten rock reacted with hydrogen from the surrounding atmosphere to produce substantial quantities of water.
“We found that oxygen liberated from the silicate melt reacts with hydrogen, producing an appreciable amount of water up to a few tens of weight per cent, which is much greater than previously predicted,” the researchers wrote.
The experiments also revealed that silicon from the rocks forms alloys with iron and creates silicon hydrides, further facilitating the water-producing reactions. The researchers argue that pressure plays a vital role in enabling these reactions. The extreme pressures found inside sub-Neptunes allow the chemical processes to proceed far more efficiently than previous low-pressure experiments had suggested.
According to the team’s calculations, these reactions could generate water concentrations high enough to transform hydrogen-rich worlds into water-rich ones over billions of years. The findings suggest that hydrogen-rich sub-Neptunes may actually be precursors to water-rich planets, representing different stages in a single evolutionary process rather than fundamentally different types of worlds.
The discovery has significant implications for understanding how common water-bearing planets might be throughout the galaxy. If sub-Neptunes can generate their own water regardless of where they form, water worlds could be far more abundant than previously thought. This expands the potential for finding planets with conditions suitable for life, since water is considered essential for biological development.
Looking ahead, the researchers note that future experiments could explore whether similar reactions occur with different planetary materials and compositions, further mapping the diversity of possible water-producing mechanisms. Observations from space telescopes like the James Webb Space Telescope may also help refine how scientists interpret atmospheric water detections on exoplanets, distinguishing between water that formed in place versus water delivered from elsewhere.
As researchers continue probing the mysteries of distant planets, they may find that water, and perhaps even life, exists in places previously thought impossible.
MJ Banias covers space, security, and technology with The Debrief. You can email him at mj@thedebrief.org or follow him on Twitter @mjbanias.
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