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.

水是系外行星的重要组成部分，其分布，无论是在地表还是在内部深处，都从根本上影响着行星的特性。大多数系外行星中水的分布是由极端条件下未知的分配系数决定的。在这里，我们首先进行从头算分子动力学模拟，研究高达 1,000 GPa 的水的金属硅酸盐分配系数，然后通过考虑水含量对密度、熔化温度和水分配的影响来模拟行星内部。我们的计算表明，在高压下，水比硅酸盐更强烈地分解成铁，因此，会优先留在行星的核心。我们的行星内部模型的结果挑战了之前想象的水世界的概念：大部分水预算（甚至超过 95%）可以储存在地核和地幔深处，而不是地表。对于质量大于 ~6 M _⨁的行星和地球大小的行星（质量较低且水预算较少），大部分水位于行星核心深处。对于给定质量，假设水位于表面还是深层，对半径的影响最多可达 15-25%。之前根据质量半径数据被认为缺水的系外行星实际上可能富含水。