  Water is an important component of exoplanets, with its distribution, i.e.,
whether at the surface or deep inside, fundamentally influencing the planetary
properties. The distribution of water in most exoplanets is determined by
yet-unknown partitioning coefficients at extreme conditions. Our new
first-principles molecular dynamics simulations reveal that water strongly
partitions into iron over silicate at high pressures and thus would
preferentially stay in a planet's core. Furthermore, we model planet interiors
by considering the effect of water on density, melting temperature, and water
partitioning. The results shatter 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 Earth's mass 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 by up to 25% for a given mass. This has drastic consequences for the
inferred water distribution in exoplanets from mass-radius data.
