FORMATION OF HYDRIDE PEROVSKITE AND WATER ON HOT HYDROGEN-RICH EXOPLANETS

Perovskite-structured (Mg,Fe)(Al,Si)O₃ is the most abundant mineral phase in the Earth's interior. The pioneering research conducted by Prof. Nancy Ross on a spectrum of perovskite-structured materials has laid the foundation for a deeper understanding of the properties of the magnesium silicate perovskite phase under the pressure-temperature conditions of the Earth's lower mantle. Our recent findings extend the significance of this research to the study of sub-Neptune exoplanets, a common type in our galaxy with sizes smaller than Neptune (between 1.8 and 3 Earth radii) and densities smaller than those of rocky planets.

These sub-Neptunes likely possess mineralogies that differ markedly due to the volatile-saturated conditions. We have developed techniques to heat hydrogen beyond 3000 K at pressures pertinent to the atmosphere-interior boundaries of sub-Neptunes (0-30 GPa). Our experiments reveal that (Mg,Fe)O melt can react with hydrogen under such conditions to form Mg₂FeH₆ in a vacancy-ordered double perovskite structure. This reaction facilitates electron exchanges, producing both hydride (H⁻ in Mg₂FeH₆) and proton (H⁺). Subsequently, the protons react with oxygen atoms from (Mg,Fe)O melt to generate H₂O. At pressures exceeding 26 GPa, (Mg,Fe)O melt becomes miscible with dense hydrogen liquid, which can result in compositional gradient between silicate magma and atmospheric layers in sub-Neptunes. This atomic-scale mixing may explain the rarity of planets in size between Neptune and Saturn, as efficient mixing inhibits the further growth of sub-Neptunes by promoting the ingassing of hydrogen into the interior rather than its accumulation in the atmosphere.

This investigation underscores how Prof. Nancy Ross’s foundational contributions to our understanding of perovskite-structured materials continue to influence our knowledge of volatile-rich exoplanets and will continue shaping our exploration of planetary science in the forthcoming decades.