According to Nature, new experimental research reveals that sub-Neptune planets between 3-10 Earth masses can generate enormous quantities of water internally through reactions between hydrogen atmospheres and molten silicate cores. At pressures exceeding 25 GPa and temperatures around 4,500-5,000 K, these hydrogen-magma reactions produce water with remarkable efficiency—creating 18.1% water by weight in experiments, which translates to potentially tens of weight percent water production at planetary scales. The research shows these reactions can continue for billions of years in planets with 2-20% hydrogen-helium atmospheres, fundamentally challenging the assumption that water-rich planets must form beyond their star’s snow line. This internal water production mechanism suggests hydrogen-rich sub-Neptunes could naturally evolve into water worlds through chemical processes occurring deep within their interiors. These findings completely reshape our understanding of how wet planets form throughout the galaxy.
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Table of Contents
Rewriting Planet Formation Theory
This research represents a fundamental paradigm shift in planetary science. For decades, the dominant theory held that water-rich planets must form beyond the snow line where ice can condense and accrete onto forming planetary bodies. The discovery that planets can create their own water internally through endogenic processes means we need to completely reconsider how we interpret the composition of exoplanets. When we detect water signatures in exoplanet atmospheres using telescopes like JWST, we can no longer automatically assume these planets migrated inward from cold formation zones. Instead, they might have formed exactly where we see them and generated their own oceans through these hydrogen-magma reactions.
The Chemistry of Planetary Water Creation
The key insight lies in understanding how extreme pressure conditions enable hydrogen to dissolve into molten rock and react with silicate minerals. At the incredible pressures found in sub-Neptune interiors—where hydrogen becomes a dense fluid rather than a gas—complete miscibility occurs between hydrogen, water, and silicates. This allows hydrogen to penetrate deep into the magma ocean and react efficiently with silicate components. The research shows silicon reduction is the dominant pathway, with silicon hydride (SiH4) forming as a key intermediate. What’s particularly remarkable is how pressure enhances these reactions—the experimental water production rates were 2,000-3,000 times higher than previous theoretical predictions that assumed ideal gas behavior at lower pressures.
Implications for Exoplanet Diversity
The implications for planetary evolution are profound. We now have a natural pathway for creating “hycean worlds”—planets with hydrogen-rich atmospheres covering deep water oceans. These could be incredibly common throughout the galaxy. The research suggests that as sub-Neptunes lose their hydrogen envelopes through atmospheric escape, the water produced internally could remain, potentially creating water-rich super-Earths. This explains how we might find ocean worlds in close orbits around their stars where traditional formation theories would predict dry, rocky planets. The varying mass fractions of different elements in planetary cores also means we should expect tremendous diversity in water production efficiency across different planetary systems.
The Role of Planetary Dynamics
Planetary convection plays a crucial role in sustaining these water-producing reactions. Vigorous mixing in both the molten core and atmosphere helps transport reactants to the reaction zone and remove water products, preventing the system from reaching chemical equilibrium that would stop the reactions. The research shows that in hotter planets with temperatures above 4,500 K, convection efficiently mixes water throughout the envelope for hundreds of millions of years. As planets cool, this mixing becomes less efficient, potentially creating layered structures with water-rich deep envelopes beneath hydrogen-rich outer layers. This dynamic interplay between chemistry and planetary physics creates complex evolutionary pathways that we’re only beginning to understand.
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Future Research Directions and Challenges
While these findings are revolutionary, they also highlight significant gaps in our understanding. We need better constraints on how planetary cooling rates affect reaction sustainability, and how atmospheric loss processes interact with internal water production. The research also suggests we need to reconsider how we model planetary interiors—traditional separate fluid models may not capture the complex miscibility behavior at extreme pressures. Future missions and telescopes should prioritize looking for chemical signatures that could distinguish between migrated water worlds and endogenically produced water worlds. The detection of specific hydrogen-silicate reaction products in exoplanet atmospheres could provide direct evidence for these processes occurring in real planetary systems across the galaxy.
