Photophoretic Separation of Metals and Silicates: The Formation of Mercury-Like Planets and Metal Depletion in Chondrites

Abstract

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.

K2-106, a system containing a metal-rich planet and a planet of lower density

“…How robust is this conclusion that K2-106 b is metal rich? When we use models published by Fortney et al (2007) or Wurm et al (2013), we obtain the same results. Thus, regardless of which set of stellar parameters we use, whether we include the jitter term, and regardless of which models we use, in all cases we reach the conclusion that the iron core contains more than half of the mass of the planet.…”

“…Therefore it is reasonable to consider similar formation scenarios for K2-106 b and Mercury. In this respect, it is interesting to note that Wurm et al (2013) argued that the high iron abundance of Mercury is due to the photophoresis in the protoplanetary disk and not the result of a giant impact, as was previously thought. Photophoresis is a process in which iron and silicates are separated in the disk.…”

Is There a Temperature Limit in Planet Formation at 1000 K?

“…However, the local material available will not significantly change. Also, it is currently often argued that inner planetary systems are formed from material drifting inward (Boley & Ford 2013;Wurm et al 2013;Hu et al 2014). Inward drift is a classical mechanism for redistribution of matter (Weidenschilling 1977).…”

Did 26Al and impact‐induced heating differentiate Mercury?

“…These exoplanets, with high densities and large sizes, are orbiting their stars at very close distances (Wurm et al. ; Nayakshin ). These super‐Mercury planets have an internal structure identical with Mercury.…”

“…In a recently proposed scenario (Wurm et al. ), the photophoretic sorting of metal and silicate phases has been invoked during the formation of Mercury. The photophoretic sorting occurs due to distinct thermal gradients within silicate and metallic dust grains in an optically thin protoplanetary disk (Beresnev et al.…”