Equation of State of SiC at Extreme Conditions: New Insight Into the Interior of Carbon-Rich Exoplanets

Key Points 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 200GPa and 3,500K, 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.