Metallic helium in massive planets

Abstract

The most abundant planetary constituents in the universe are hydrogen and helium. In our own solar system, it is estimated that >70% of the planetary mass is of this form, with most of it residing in Jupiter and most of it in a metallic state. The likely metallic state of hydrogen at high pressure has long been recognized (1), although the exact nature of this state and its properties continue to be debated (2). Helium, on the other hand, is generally thought to be a reluctant partner to hydrogen at high pressure because its expected metallization pressure at low temperature is very high, perhaps around 100 Megabars (Mbar) or more. In comparison, the highest pressure in the hydrogen–helium part of Jupiter is perhaps only 40 or so Mbar (3). This suggests that most of Jupiter's interior consists of a mixture of metallic hydrogen fluid (protons in a degenerate electron sea) and neutral helium atoms, with the latter making up about a quarter of the mass but only 7% of the nuclei by number (Fig. 1). In this issue of PNAS, Stixrude and Jeanloz (4) show that band closure in pure helium occurs at lower pressures than previously thought, provided the effect of high temperatures is taken into account. This suggests that helium behaves as a metal, at least at the highest pressures encountered in Jupiter and perhaps over a wider range of pressures in the many, often much hotter, planets of Jupiter's mass and larger that are now evidently common in the universe (5). The full thermodynamic and transport properties of the relevant mixtures cannot be deduced from the behavior of the end members (pure hydrogen and pure helium) and are therefore an area of ongoing research.