AI and the World's Hard Problems · Astronomy & space · arXiv · 2026-10-21
Ammonia-Hydrogen Phase Equilibria in Planetary Envelopes from First-Principles-Based Machine-Learning Molecular Dynamics
Ammonia and hydrogen are major constituents of planets within and beyond the solar system, yet how they interact in planetary envelopes is poorly known. Using machine-learning interatomic potentials trained on density functional theory calculations, large-scale two-phase coexistence simulations from 500 to 4000 K and 0.1 to 100 GPa map the coexisting hydrogen-rich and ammonia-rich compositions and the critical curve, whose temperature rises from 500 K at 0.1 GPa to 2000 K near 30 GPa. The phase boundary lies close to and intersects published thermal profiles of Uranus and Neptune, implying ammonia may be raining out in these planets today, whereas in warm sub-Neptunes such as TOI-270 d ammonia and hydrogen are likely miscible in all proportions.