Demixing of Hydrogen and Helium at Megabar Pressures

We present results of ab initio finite-temperature density functional theory molecular dynamics simulations for fluid hydrogen-helium mixtures at megabar pressures. The location of the miscibility gap is derived from the equation of state data. We find a close relation between hydrogen-helium phase separation and the continuous nonmetal-to-metal transition in hydrogen. Our calculations predict that demixing of hydrogen and helium occurs in Saturn and probably also in Jupiter. These results will have a strong impact on interior models of giant solar and extrasolar planets.