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

Abstract

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.

Capturing the oxidation of silicon carbide in rocky exoplanetary interiors

“…Because of speculations of silicon carbide in exoplanetary interiors, physical properties of silicon carbide are being studied extensively at high pressures and temperatures (Wilson & Militzer 2014;Nisr et al 2017;Daviau & Lee 2017;Miozzi et al 2018). Significant amounts of silicon carbide in exoplanetary interiors would have a major impact on the thermal evolution and geodynamical processes on such exoplanets because its thermal conductivity is abnormally high (Nisr et al 2017).…”

SiO2‐SiC Mixtures at High Pressures and Temperatures: Implications for Planetary Bodies Containing SiC

“…Further work on this system, such as heating for longer time periods or quantifying the amount of C liberated as heating goes on, would aid in better understanding the breakup process of SiC and whether we are observing an incomplete exsolution of C from SiC or a complete decomposition of SiC. An alternative explanation for our observations is that the nonequimolar SiC y +(1 − y ) C phase is stable at the P‐T conditions explored here and that additional heating would not further liberate C. The hypothesis that SiC will continue to break up and fully decompose with continued heating, however, is supported by observations of SiC decomposition at ambient pressure (Bhaumik et al, ), as well as the lack of any observation of nonequimolar SiC in previous works (Miozzi et al, ).…”

“…Recent work has explored the SiC system at high P‐T through the LHDAC (Kidokoro et al, ; Miozzi et al, ; Nisr et al, ) and through shock experiments (Tracy et al, ). While we observe indication of SiC decomposition in our present experiments, as well as in previous measurements (Daviau & Lee, ), other studies probing similar conditions do not report the appearance of diamond diffraction or other indications of SiC decomposition.…”

“…Such planets may be difficult to distinguish from their average density alone due to the high‐pressure phase transition in SiC (Daviau & Lee, ). Recent literature has explored the Si‐C system under planetary conditions with the goal of better characterizing exotic C‐rich worlds (Daviau & Lee, ; Kidokoro et al, ; Miozzi et al, ; Nisr et al, ; Wilson & Militzer, ).…”

Hot Super-Earths with Hydrogen Atmospheres: A Model Explaining Their Paradoxical Existence

“…Winn et al 2011) that could explain the observed ∼ 100 ppm modulation in the optical (Winn et al 2011;Dragomir et al 2014) and IR (Tamburo et al 2018) phase curves. Other proposed models for 55 Cancri e include the possibility of it being an icy planet (Zeng & Sasselov 2013;Zeng et al 2016) or having a carbon-rich composition (Madhusudhan et al 2012;Miozzi et al 2018). We understand that the situation concerning 55 Cancri e is complex as there is limited observational data that may appear contradictory in some occasions.…”