Ocean-planets

 
Our team proposed the existence, among the extrasolar planets, of objects which don't exist in the Solar System, but may exist somewhere in the Universe : Ocean-planets, as an analogous to hot jupiters, as 51 Peg, the first extrasolar planet discovered around a solar type star.

Ocean-Planets would form in the protoplanetary disks beyond the snowline (distance at which the temperature is cool enough to let water freeze). The planet would then migrate in hoter regions of the Soalr System and their surface ices would melt into a deep ocean (~ 100 km). It would contain, in mass, 50% of metal-rocks and 50% of water (liquid and solid).
Their internal strucure is shown above (left) and compared to the structure of a rocky planet of the same mass (centre). Note that at equivalent mass, ocean planets have a radius bigger than rocky planets because their mean density is smaller. In certain cases, the extrasolar planet detection techniques allow the determination of their mass and radius. In the future, it should be possible to know if these planets really exist.
Some jovian satellites (Ganymede and Callisto) have a composition similar to ocean planet one, but they are much smaller.