The story of hydrogen and water: new insights into the interaction of planet atmospheres and interiors

Recent studies suggest that most exoplanets—or their progenitors—begin life enveloped in hydrogen.  This primordial atmosphere interacts with the planet’s interior over timescales of millions to billions of years, making atmosphere–interior coupling essential for understanding planetary formation and evolution.  Yet these processes remain poorly constrained because they occur under extreme pressures and temperatures.  To probe them, we performed computational experiments using DFT-based molecular dynamics across a vast P-T regime typical of super-Earths and sub-Neptunes. We mapped out the critical curve which demarcates regimes where a single, well-mixed hydrogen-water fluid is stable and where it splits into distinct hydrogen-rich and water-rich phases. Our critical curve agrees well with existing experimental data and shows the influence of a change in fluid structure from molecular to atomic near 30-100 GPa and 3000-4000 K.

 

These results not only have far-reaching consequences for water-rich planets with hydrogen atmospheres like Uranus, Neptune, K2-18 b, and TOI-270 d but also bring into question the deeply ingrained notion in our community of a sharp boundary between the interior and an overlying atmosphere. Hot and young planets should have envelopes where hydrogen and water are entirely mixed, i.e., exist as a single homogeneous phase. However, as the planet cools, its deep interior should experience phase separation of hydrogen and water: leading to a "rainfall" or rainout of water towards the deeper interior, a consequent increase in internal luminosity, and the emergence of inner and outer envelopes that are hydrogen- and water-rich, respectively, and whose compositions or metallicities should depend on the planets age and instellation. Our results thus help improve our constraints on planets that are likely to have water oceans, which future surveys could leverage. Furthermore, our findings have implications for atmosphere loss and magnetic field generation. Our work thus demonstrates the importance of better understanding atmosphere-interior interactions, especially as we enter the era of James Webb Space Telescope, PLATO, the proposed Uranian Orbiter and Probe, and other next-generation observatories.