  Helium is the second most abundant element in the universe, and together with
silica, they are major components of giant planets. Exploring the reactivity
and state of helium and silica under high pressure is of fundamental importance
for developing and understanding of the evolution and internal structure of
giant planets. Here, using first-principles calculations and crystal structure
predictions, we identify four stable phases of a helium-silica compound with
seven/eight-coordinated silicon atoms at pressure range of 600-4000 GPa,
corresponding to the interior condition of the outer planets in the solar
system. The density of HeSiO2 agrees with current structure models of the
planets. This helium-silica compound exhibits a superionic-like helium
diffusive state at the high pressure and high temperature conditions along the
isentropes of Saturn, a metallic fluid state in Jupiter, and a solid state in
the deep interiors of Uranus and Neptune. The reaction of helium and silica may
lead to the erosion of the rocky core of giant planets and form a diluted core
region. These results highlight the reactivity of helium under high pressure to
form new compounds, and also provides evidence to help build more sophisticated
interior models of giant planets.
