  Many planets in the solar system and across the galaxy have hydrogen-rich
atmospheres overlying more heavy element-rich interiors with which they
interact for billions of years. Atmosphere-interior interactions are thus
crucial to understanding the formation and evolution of these bodies. However,
this understanding is still lacking in part because the relevant
pressure-temperature conditions are extreme. We conduct molecular dynamics
simulations based on Density Functional Theory to investigate how hydrogen and
water interact over a wide range of pressure and temperature, encompassing the
interiors of Neptune-sized and smaller planets. We determine the critical curve
at which a single homogeneous phase exsolves into two separate, hydrogen-rich
and water-rich phases, finding good agreement with existing experimental data.
We find that the temperature along the critical curve increases with increasing
pressure and shows the influence of a change in fluid structure from molecular
to atomic near 30 GPa and 3000 K, which may impact magnetic field generation.
The internal temperatures of many exoplanets, including TOI-270 d and K2-18 b
may lie entirely above the critical curve: the envelope is expected to consist
of a single homogeneous hydrogen-water fluid, that is much less susceptible to
atmospheric loss as compared with a pure hydrogen envelope. As planets cool,
they cross the critical curve, leading to rainout of water-rich fluid and an
increase in internal luminosity. Compositions of the resulting outer,
hydrogen-rich, and inner, water-rich envelopes depend on age and instellation
and are governed by thermodynamics. Rainout of water may be occurring in Uranus
and Neptune at present.
