Although water is necessary for life as we know it, a large fraction of water in planetary interiors may lead to unique conditions, under which the interior structure differs from the simple layered structure of a water ocean on top of a rocky surface, that supports life. We combined thermal evolution model with experimental data of ice-rock interaction at high pressure, and find that ice and rock are miscible in each other in planetary interiors. We show that water-rich planets are expected to have an interior structure of a mixed ice and rock ball, surrounded by a water / steam layer of less than 1% of the planet’s mass. In this case, the mass of the water layer is determined by the ice-rock interaction properties. We conclude that when water-ice is abundant in planetary interiors the ice and rock tend to stay mixed for billions of years, and the interior structure differs from the simple layered structure that is usually assumed.