The Compositions of Planetary Cores

Understanding the composition of metallic cores in planetary bodies is crucial for unraveling planetary formation, differentiation, and evolution. On Earth, early seismic and density data suggested iron-dominated interiors alloyed with lighter elements such as sulfur, silicon, oxygen, carbon, hydrogen, and nitrogen. These elements influence core density, thermal conductivity, magnetic field generation, and surface habitability, and their incorporation depends on each planet's unique pressure, temperature, and redox conditions during differentiation. Experimental investigations of metal-silicate partitioning under extreme conditions show that many light elements are strongly siderophile at high pressures, contributing to the diversity of core compositions across the Solar System and beyond. This review synthesizes current knowledge on core compositions beyond Earth—spanning asteroids to exoplanets—and explores how laboratory experiments, cosmochemical evidence, and astrophysical observations collectivelyinform our understanding of core formation. By decoding core compositions, studies can better constrain the thermal histories and potential habitability of planetary bodies. ▪ Planetary core compositions reveal how planets form, differentiate, and evolve, shaping the density, heat flow, magnetic fields, and habitability of a planet. ▪ Experiments, cosmochemical abundances, and theoretical calculations explain the light element compositions of planetary cores from asteroids to exoplanets.