For years, astronomers have believed that water-rich planets could only form beyond a system’s “snow line,” the region where icy materials condense during planet formation. Yet a recent Nature study challenges this foundation, showing that hydrogen-rich sub-Neptunes, planets between Earth and Neptune in size, can generate vast quantities of water internally through high-pressure chemical reactions. These findings redefine how scientists interpret planetary composition, habitability, and evolution. If hydrogen and molten rock can combine to produce water, then even close-in planets orbiting near their stars could harbour deep oceans beneath dense atmospheres, complicating the once-clear distinction between dry and wet worlds.
Can dry planets turn wet: The hydrogen paradox
Sub-Neptunes are among the most common exoplanets detected by NASA’s Kepler mission, typically measuring one to four times Earth’s radius. Their composition has long puzzled researchers, as many exhibit densities inconsistent with purely rocky or purely gaseous structures. Traditionally, two formation pathways were proposed: hydrogen-dominated dry planets formed close to the star, and water-rich wet planets formed farther away, migrating inward later.The recent Nature study reveals a third, more complex mechanism. At immense pressures, hydrogen does not remain chemically inert as previously assumed. Instead, when it interacts with molten silicate rock deep within a planet’s core-envelope boundary, it triggers reduction reactions that liberate oxygen from the rock. This oxygen then binds with hydrogen to form water, fundamentally transforming the planet’s internal chemistry.The researchers demonstrated that even a modest hydrogen envelope could yield water content comprising up to several tens of weight per cent, far exceeding previous theoretical predictions. This mechanism implies that water production is not limited to the cold, outer regions of solar systems but can occur naturally within hot, hydrogen-rich interiors.
How is water formed on dry planets
Using diamond-anvil cell experiments and pulsed laser heating, scientists simulated the extreme pressures and temperatures, up to 22 gigapascals and over 4,500 kelvin, found at the boundary between a rocky core and its hydrogen envelope. When silicate minerals such as olivine and fayalite were exposed to dense hydrogen, silicon was reduced from its oxidised state (Si⁴⁺) to metallic silicon, forming iron-silicon alloys and silicon hydrides (SiH₄). Oxygen released from these reactions combined with hydrogen to produce substantial quantities of water.The process was confirmed through X-ray diffraction and Raman spectroscopy, which detected both the characteristic Si–H and O–H bond vibrations in the samples. This evidence demonstrates that silicates can disappear entirely under these conditions, converting into new compounds while generating water in quantities previously thought impossible under planetary pressures.These reactions likely occur at the core-envelope boundary (CEB) of sub-Neptunes with masses between three and ten times that of Earth and hydrogen–helium atmospheres of two to twenty per cent by weight. Because hydrogen dissolves readily in molten rock at such pressures, it can permeate the silicate layers and sustain water-producing reactions for billions of years. Convective mixing within molten interiors further enhances these processes, maintaining equilibrium between the deep core and the upper envelope.
From hydrogen giants to ocean worlds
The implications of these findings extend beyond chemistry into planetary evolution. The Nature study suggests that hydrogen-rich sub-Neptunes can evolve naturally into water-rich planets as internal reactions gradually convert atmospheric hydrogen into water. Over time, as the hydrogen envelope erodes through stellar radiation or thermal escape, the remaining planet may resemble a super-Earth with a deep oceanic mantle or even a surface ocean.This theory provides a compelling explanation for the growing number of close-in water-rich exoplanets discovered within regions once considered too hot for water to exist. Rather than migrating inward from icy regions, these planets could have become water-bearing worlds through internal transformation.The study further proposes that Hycean planets, worlds with hydrogen atmospheres overlying vast water layers, could be more common than previously imagined. Their existence bridges the evolutionary pathway between hydrogen-dominated sub-Neptunes and ocean-covered super-Earths. If verified, this process could unify two previously distinct planetary categories under a single formation model governed by high-pressure chemistry.
Implications for habitability and future exoplanet research
This discovery profoundly affects how scientists assess exoplanet habitability. The presence of water has traditionally served as a proxy for a planet’s potential to support life, yet these findings suggest that water abundance does not necessarily indicate formation in cold, outer regions or migration from beyond the snow line. Instead, it may arise internally through hydrogen–rock interactions deep below the surface.Such endogenic water production challenges previous assumptions that linked a planet’s composition directly to its location of origin. Planets formed entirely from dry materials near their stars could still become rich in water, reshaping the search for habitable environments in other systems.The next generation of observatories, including the James Webb Space Telescope (JWST) and upcoming Ariel mission, will be able to probe the atmospheric spectra of sub-Neptunes for water vapour, hydrogen, and silicon hydrides. Distinguishing between endogenic and exogenic water signatures will help test whether this mechanism operates widely across exoplanetary systems.If water-rich atmospheres can indeed form through deep chemical processes, it will mark a turning point in planetary science, one where habitability depends less on a planet’s birthplace and more on its internal geochemistry. The Nature study’s findings therefore redefine one of astronomy’s central questions: not merely where water comes from, but how planets themselves can create it.Also Read | This 70-million-year-old fossil still glows with shifting colours; scientists finally know why
