A hypothesis is proposed that outer cores of the terrestrial planets consist of a eutectic melt between the inner core iron and the lower mantle oxide. MgO, a major consituent of the lower mantle, is estimated to be substantially soluble in iron melt at high pressures and high temperatures reached in the outer core. Mars does not have an outer core, and Venus has a much smaller outer core than the Earth. On an assumption that the overall chemistry (Fe/Oxide ratio) is the same among the terrestrial planets except Mercury, or an alternative assumption that the metallic-iron/total-mass ratio increases in the decreasing order of distance from the sun, from 0.20 for Mars to 0.25 for Earth and Venus, the eutectic composition is estimated to be 0.41 by the first assumption or 0.24 by the second one. The Earth, when it was hotter, did not have a solid inner core. When it cooled, iron and oxide coprecipitated from the eutectic melt, iron sank to form the inner core, and oxide floated to form the D'' region.