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The tungsten isotope composition of meteorites indicates that core formation in planetesimals occurred within a few million years of Solar System formation^,. But core formation requires a mechanism for segregating metal, and the ‘wetting’ properties of molten iron alloy in an olivine-rich matrix is thought to preclude segregation by permeable flow unless the silicate itself is partially molten^,,. Excess liquid metal over a percolation threshold, however, can potentially create permeability in a solid matrix, thereby permitting segregation. Here we report the percolation threshold for molten iron–sulphur compounds of approximately 5 vol.% in solid olivine, based on electrical conductivity measurements made in situ at high pressure and temperature. We conclude that heating within planetesimals by decay of short-lived radionuclides can increase temperature sufficiently above the iron–sulphur melting point (∼1,000 °C) to …