  Mercury's high uncompressed mass density suggests that the planet is largely
composed of iron, either bound within metal (mainly Fe-Ni), or iron sulfide.
Recent results from the MESSENGER mission to Mercury imply a low temperature
history of the planet which questions the standard formation models of impact
mantle stripping or evaporation to explain the high metal content. Like
Mercury, the two smallest extrasolar rocky planets with mass and size
determination, CoRoT-7b and Kepler-10b, were found to be of high density. As
they orbit close to their host stars this indicates that iron rich inner
planets might not be a nuisance of the solar system but be part of a general
scheme of planet formation. From undifferentiated chondrites it is also known
that the metal to silicate ratio is highly variable which must be ascribed to
pre-planetary fractionation processes. Due to this fractionation most
chondritic parent bodies - most of them originated in the asteroid belt - are
depleted in iron relative to average solar system abundances. The astrophysical
processes leading to metal silicate fractionation in the solar nebula are
essentially unknown. Here, we consider photophoretic forces. As these forces
particularly act on irradiated solids, they might play a significant role for
the composition of planetesimals forming at the inner edge of protoplanetary
discs. Photophoresis can separate high thermal conductivity materials (iron)
from lower thermal conductivity solids (silicate). We suggest that the
silicates are preferentially pushed into the optical thick disk. Subsequent
planetesimal formation at the edge moving outwards leads to metal rich
planetesimals close to the star and metal depleted planetesimals further out in
the nebula.
