Although all terrestrial planets are presently already separatedinto an iron-rich core and a silicate mantle, the process of theformation of a planetary core is still not well understood. Coreformation is generally thought to have occurred concurrently with orsoon after planet formation and happened rather rapidly. This workshows that planetary core formation by the settling of iron-rich meltbodies in a solid matrix is fast enough, if the viscosity of theplanetary mantle has a strong temperature dependence.