The discovery of an extraordinary number of extrasolar planets, characterized by an unexpected variety of sizes, masses and orbits, challenges our understanding of the formation and evolution of the planets in the Solar System and the perception of the Earth as the prototypical habitable world. Many exoplanets appear to be rocky and yet more massive than the Earth, with expected pressures and temperatures of hundreds of gigapascal and thousands of Kelvin in their deep interiors. At these conditions, the properties of bridgmanite and ferropericlase, expected to dominate their mantles, are largely unconstrained, limiting our knowledge of their interior structure. Here we used nano-second X-ray diffraction and dynamic compression to experimentally investigate the atomic structure and density of iron oxide (FeO), one of the end-members of the (Mg,Fe)O ferropericlase solid solution, up to 700 GPa, a pressure exceeding the core–mantle boundary of a 5 Earth masses planet. Our data document the stability of the high-pressure cesium-chloride B2 structure above 300 GPa, well below the pressure required for magnesium oxide (MgO) to adopt the same phase. These observations, complemented by the calculation of the binary MgO–FeO phase diagram, reveal complex stratification and rheology inside large terrestrial exoplanets.
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Igor scripts used to analyse the X-ray diffraction and VISAR data described in this study can be obtained from the corresponding author upon reasonable request.
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We thank A. Correa Barrios, C. Davis, T. Uphaus and R. Wallace for assistance in target preparation, the operation staff at the OMEGA Laser Facility for supporting the experiments and S. Ritterbex for careful reading of the manuscript and constructive comments. F.C. is grateful to L. Stixrude and R. Wentzcovitch for insightful discussions and to R. Fischer, H. Ozawa and A. Boujibar for sharing some of their previous data. This work was performed under the auspices of US Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344 and supported, in part, by the US DOE, Office of Science, Office of Fusion Energy Sciences. The research was supported by NNSA/DOE through the National Laser Users’ Facility Program under contract numbers DE-NA0002720 and DE-NA0003611.
The authors declare no competing interests.
Peer review information Nature Geoscience thanks Yingwei Fei and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Primary Handling Editor: Rebecca Neely.
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Coppari, F., Smith, R.F., Wang, J. et al. Implications of the iron oxide phase transition on the interiors of rocky exoplanets. Nat. Geosci. 14, 121–126 (2021). https://doi.org/10.1038/s41561-020-00684-y