The phase and melting relations of the C-saturated C-Mg-Fe-Si-O system were investigated at high pressure and temperature to understand the role of carbon in the structure of the Earth, terrestrial planets, and carbon-enriched extraterrestrial planets. The phase relations were studied using two types of experiments at 4 GPa: analyses of recovered samples and in situ X-ray diffractions. Our experiments revealed that the composition of metallic iron melts changes from a C-rich composition with up to about 5 wt.% C under oxidizing conditions (ΔIW = -1.7 to -1.2, where ΔIW is the deviation of the oxygen fugacity (fO2) from an iron-wüstite (IW) buffer) to a C-depleted composition with 21 wt.% Si under reducing conditions (ΔIW < -3.3) at 4 GPa and 1,873 K. SiC grains also coexisted with the Fe-Si melt under the most reducing conditions. The solubility of C in liquid Fe increased with increasing fO2, whereas the solubility of Si decreased with increasing fO2. The carbon-bearing phases were graphite, Fe3C, SiC, and Fe alloy melt (Fe-C or Fe-Si-C melts) under the redox conditions applied at 4 GPa, but carbonate was not observed under our experimental conditions. The phase relations observed in this study can be applicable to the Earth and other planets. In hypothetical reducing carbon planets (ΔIW < -6.2), graphite/diamond and/or SiC exist in the mantle, whereas the core would be an Fe-Si alloy containing very small amount of C even in the carbon-enriched planets. The mutually exclusive nature of C and Si may be important also for considering the light elements of the Earth's core. © 2013 Springer-Verlag Berlin Heidelberg.