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# Implications of the iron oxide phase transition on the interiors of rocky exoplanets

## Abstract

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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## Data availability

Raw data were generated at the Omega large-scale facility. Derived data supporting the findings of this study are available in the Supplementary Information, from the corresponding author upon request and can be downloaded at https://doi.org/10.5061/dryad.pg4f4qrn8.

## Code availability

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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## Acknowledgements

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.

## Author information

Authors

### Contributions

F.C. designed and performed the experiments, analysed, interpreted and modelled the data and wrote the manuscript. R.F.S. contributed to the experiments’ execution and data analysis. M.M. and T.S.D. contributed to data interpretation and writing of the manuscript. J.W. helped with the experiments’ execution and, together with D.K., prepared the samples. S.H. helped to model the solid solution behaviour. J.H.E. and J.R.R. developed data analysis codes and contributed to data interpretation. T.S.D. was the principal investigator on the National Laser Users’ Facility (NLUF) experimental proposal. All authors discussed the results and reviewed the manuscript.

### Corresponding author

Correspondence to F. Coppari.

## Ethics declarations

### Competing interests

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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## Supplementary information

### Supplementary Information

Supplementary Figs. 1–14, Tables 1–5 and discussion.

## Rights and permissions

Reprints and Permissions

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

• Accepted:

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• DOI: https://doi.org/10.1038/s41561-020-00684-y

• ### Identification of CO2 adsorption sites on MgO nanosheets by solid-state nuclear magnetic resonance spectroscopy

• Jia-Huan Du
• Lu Chen
• Luming Peng

Nature Communications (2022)

• ### Structural transition and re-emergence of iron's total electron spin in (Mg,Fe)O at ultrahigh pressure

• Han Hsu
• Koichiro Umemoto

Nature Communications (2022)

• ### Structure and density of silicon carbide to 1.5 TPa and implications for extrasolar planets

• D. Kim
• R. F. Smith
• T. S. Duffy

Nature Communications (2022)