  For most planets in the range of radii from 1 to 4 R$_{\oplus}$, water is a
major component of the interior composition. At high pressure H${}_{2}$O can be
solid, but for larger planets, like Neptune, the temperature can be too high
for this. Mass and age play a role in determining the transition between solid
and fluid (and mixed) water-rich super-Earth. We use the latest high-pressure
and ultra-high-pressure phase diagrams of H${}_{2}$O, and by comparing them
with the interior adiabats of various planet models, the temperature evolution
of the planet interior is shown, especially for the state of H${}_{2}$O. It
turns out that the bulk of H${}_{2}$O in a planet's interior may exist in
various states such as plasma, superionic, ionic, Ice VII, Ice X, etc.,
depending on the size, age and cooling rate of the planet. Different regions of
the mass-radius phase space are also identified to correspond to different
planet structures. In general, super-Earth-size planets (isolated or without
significant parent star irradiation effects) older than about 3 Gyr would be
mostly solid.
