  The physical state and properties of silicates at conditions encountered in
the cores of gas giants, ice giants and of Earth like exoplanets now discovered
with masses up to several times the mass of the Earth remains mostly unknown.
Here, we report on theoretical predictions of the properties of silica,
SiO$_2$, up to 4 TPa and about 20,000K using first principle molecular dynamics
simulations based on density functional theory. For conditions found in the
Super-Earths and in ice giants, we show that silica remains a poor electrical
conductor up to 10 Mbar due to an increase in the Si-O coordination with
pressure. For Jupiter and Saturn cores, we find that MgSiO$_3$ silicate has not
only dissociated into MgO and SiO$_2$, as shown in previous studies, but that
these two phases have likely differentiated to lead to a core made of liquid
SiO$_2$ and solid (Mg,Fe)O.
