According to condensation models (Bond et al. 2010; Kuchner & Seager 2005), a key variable determining the chemical makeup of planets is the carbon-to oxygen ratio of the disk.
In stellar nebulae with carbon– oxygen ratios in excess of 0.8 (Bond et al. 2010), condensation models propose that solid bodies consist primarily of silicon carbide and carbon, leading to the formation of solid planets consisting of silicon, carbon, and possibly iron with minimal oxygen (Bond et al. 2010).
In such planets, diamond layers could for instance have huge consequences in terms of mantle dynamic as diamond possesses very different thermal conductivity properties than silicates.
Recent work on carbon-based planets has been particularly motivated by the detection of planets in the 55 Cancri system, whose C/O ratio is particularly high.
In particular, the planet 55 Cancri e, whose mass–radius relationship suggests that it may be insufficiently dense to have a silicate composition (Demory et al. 2011) and has been modeled as consisting of layers of carbon, SiC, and iron (Madhusudhan et al. 2012).
© Haven Giguere, Yale Univ
Interior view of 55 Cancri e (artistic view)