Small volume fractions of metal-rich, iron-nickel-sulfur-oxygen melt do not form an interconnected network in polycrystalline olivine at pressures up to 11 gigapascals. These results suggest that iron-rich melts could not have migrated downward along olivine grain edges to form Earth’s core. Core separation at pressures below 11 gigapascals within planetary bodies must have been facilitated by large degrees of partial melting, resulting in either separation of immiscible silicate and metal-sulfide melts or extraction of the silicate melt to concentrate the metallic remainder.