Planetary objects that form in the outer solar system begin as a comet-like mixture of roughly 50% water and 50% rock by weight. Simulations of solar system formation have shown that planets are likely to migrate inward or outward as they form, presenting the possibility that icy planets could wind up in orbits where their water becomes liquid, turning them into Ocean Planets. This possibility was first discussed in the professional astronomical literature by Marc Kuchner and Alain Léger in 2003.
The oceans on such planets could be hundreds of kilometers deep, much deeper than the oceans of Earth. The immense pressures in the lower regions of these oceans could lead to the formation of a mantle of exotic forms of ice. This ice would not necessarily be as cold as we understand ice. If the planet is close enough to its sun that the water's temperature reaches the boiling point, the water will become supercritical and lack a well-defined surface.