The interior as the dominant water reservoir in super-Earths and sub-Neptunes

Water is an important component of exoplanets, with its distribution, that is, whether at the surface or deep inside, fundamentally influencing the planetary properties. The distribution of water in most exoplanets is determined by yet-unknown partition coefficients at extreme conditions. Here we first conduct ab initio molecular dynamics simulations to investigate the metal–silicate partition coefficients of water up to 1,000 GPa and then model planet interiors by considering the effects of water content on density, melting temperature and water partitioning. Our calculations reveal that water strongly partitions into iron over silicate at high pressures and, thus, would preferentially stay in a planet’s core. The results of our planet interior model challenge the notion of water worlds as imagined before: the majority of the bulk water budget (even more than 95%) can be stored deep within the core and the mantle, and not at the surface. For planets more massive than ~6 M⨁ and Earth-size planets (of lower mass and small water budgets), the majority of water resides deep in the cores of planets. Whether water is assumed to be at the surface or at depth can affect the radius up to 15–25% for a given mass. The exoplanets previously believed to be water-poor on the basis of mass–radius data may actually be rich in water. If water exists in super-Earth and sub-Neptune exoplanets, it is expected to be hidden deep in their cores and mantles, rather than at their surfaces. Exoplanets considered to be relatively dry might actually have abundant water sequestered in their interiors.