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Mercury's high uncompressed mass density suggests that the planet is largelycomposed of iron, either bound within metal (mainly Fe-Ni), or iron sulfide.Recent results from the MESSENGER mission to Mercury imply a low temperaturehistory of the planet which questions the standard formation models of impactmantle stripping or evaporation to explain the high metal content. LikeMercury, the two smallest extrasolar rocky planets with mass and sizedetermination, CoRoT-7b and Kepler-10b, were found to be of high density. Asthey orbit close to their host stars this indicates that iron rich innerplanets might not be a nuisance of the solar system but be part of a generalscheme of planet formation. From undifferentiated chondrites it is also knownthat the metal to silicate ratio is highly variable which must be ascribed topre-planetary fractionation processes. Due to this fractionation mostchondritic parent bodies - most of them originated in the asteroid belt - aredepleted in iron relative to average solar system abundances. The astrophysicalprocesses leading to metal silicate fractionation in the solar nebula areessentially unknown. Here, we consider photophoretic forces. As these forcesparticularly act on irradiated solids, they might play a significant role forthe composition of planetesimals forming at the inner edge of protoplanetarydiscs. Photophoresis can separate high thermal conductivity materials (iron)from lower thermal conductivity solids (silicate). We suggest that thesilicates are preferentially pushed into the optical thick disk. Subsequentplanetesimal formation at the edge moving outwards leads to metal richplanetesimals close to the star and metal depleted planetesimals further out inthe nebula.

Photophoretic separation of metals and silicates: the formation of Mercury like planets and metal depletion in chondrites