How Hydrogen and Magma Create Water Deep Inside Planets express
Sub-Neptunes — worlds slightly smaller than Neptune — are common among exoplanets. They are detected using the transit method (by the dimming of a star), pioneered by William Borucki. Previously, it was thought that such planets were either dry and surrounded by hydrogen, or wet, with large amounts of water. The wet ones supposedly formed far out, where ice gathers, and then migrated closer to their stars. A new experiment breaks this mold.
In the lab, a mixture of hydrogen and silicate melt (magma) was compressed at thousands of degrees. The planet’s depths acted like an alchemical furnace: hydrogen aggressively ripped oxygen from minerals, and from the scorching rock, water was born. This reaction runs to completion: silicon goes into a metallic alloy, and every kilogram of magma yields up to a hundred grams of water — nearly half a cup. The mass fraction of water reached tens of percent, far higher than old estimates based on ideal gas models.
This means a planet with a hydrogen atmosphere and a magma ocean can transform into a water world without leaving its orbit. Atmospheric analysis using spectroscopy (breaking down light) is now unreliable for determining a planet’s place of birth. Detecting water doesn’t prove migration — the planet could have flooded itself. Astronomers studying exoplanets will have to rewrite their biographies.
Original abstract
as written by the authors arXiv:2511.06507
Close-in transiting sub-Neptunes are abundant in our galaxy \cite{fulton2017california}. Planetary interior models based on their observed radius-mass relationship suggest that sub-Neptunes contain a discernible amount of either hydrogen (dry planets) or water (wet planets) blanketing a core composed of rocks and metal \cite{bean2021nature}. Water-rich sub-Neptunes have been believed to form farther from the star and then migrate inward to their present orbits \cite{bitsch2021Dry}. Here, we report experimental evidence of reactions between warm dense hydrogen fluid and silicate melt that releases silicon from the magma to form alloys and hydrides at high pressures. We found that oxygen liberated from the silicate melt reacts with hydrogen, producing a significant amount of water up to a few tens of weight percent, which is much greater than previously predicted based on low-pressure ideal gas extrapolation \cite{misener2023Atmospheresa,schlichting2022Chemical}. Consequently, these reactions can generate a spectrum of water contents in hydrogen-rich planets, with the potential to reach water-rich compositions for some sub-Neptunes, implying an evolutionary relationship between hydrogen-rich and water-rich planets. Therefore, detection of a large amount of water in exoplanet atmospheres may not be the optimal evidence for planet migration in the protoplanetary disk, calling into question the assumed link between composition and planet formation location.
One kilogram of silicate magma, after reacting with hydrogen, can yield up to 100 grams of water — that’s nearly half a cup of liquid born right in the planet’s interior.