The density of liquid iron has been determined up to 116 GPa and 4350 K via static compression experiments following an innovative analysis of diffuse scattering from liquid. The longitudinal sound velocity was also obtained to 45 GPa and 2700 K based on inelastic x-ray scattering measurements. Combining these results with previous shock-wave data, we determine a thermal equation of state for liquid iron. It indicates that Earth’s outer core exhibits 7.5%–7.6% density deficit, 3.7%–4.4% velocity excess, and an almost identical adiabatic bulk modulus, with respect to liquid iron.

As the main constituent of planetary cores, pure iron phase diagram under high pressure and temperature is of fundamental importance in geophysics and planetary science. However, previously reported iron‐melting curves show large discrepancies (up to 1000 K at the Earth's core–mantle boundary, 136 GPa), resulting in persisting high uncertainties on the solid‐liquid phase boundary. Here we unambiguously show that the observed differences commonly attributed to the nature of the used melting diagnostic are due to a carbon contamination of the sample as well as pressure overestimation at high temperature. The high melting temperature of pure iron under core‐mantle boundary (4250 ± 250 K), here determined by X‐ray absorption experiments at the Fe K‐edge, indicates that volatile light elements such as sulfur, carbon, or hydrogen are required to lower the crystallization temperature of the Earth's liquid outer core in order to prevent extended melting of the surrounding silicate mantle.

There is a direct relation between the composition of a host star and that of the planets orbiting around it. As such, the recent discovery of stars with unusual chemical composition, notably enriched in carbon instead of oxygen, supports the existence of exoplanets with a chemistry dominated by carbides instead of oxides. Accordingly, several studies have been recently conducted on the Si–C binary system at high pressure and temperature. Nonetheless, the properties of carbides at the pressure‐temperature conditions of exoplanets interiors are still inadequately constrained, effectively hampering reliable planetary modeling. Here we present an in situ X‐ray diffraction study of the Si–C binary system up to 200 GPa and 3,500 K, significantly enlarging the pressure range explored by previous experimental studies. The large amount of collected data allows us to properly investigate the phase diagram and to refine the Clapeyron slope of the transition line from the zinc blende to the rock salt structure. Furthermore, the pressure‐volume‐temperature equation of state is provided for the high‐pressure phase, characterized by low compressibility and thermal expansion. Our results are used to model idealized C‐rich exoplanets of end‐members composition. In particular, we derived mass‐radius relations and performed numerical simulations defining rheological parameters and initial conditions which lead to onset of convection in such SiC planets. We demonstrate that if restrained to silicate‐rich mantle compositions, the interpretation of mass‐radius relations may underestimate the interior diversity of exoplanets.