  Young stars and planets both grow by accreting material from the
proto-stellar disks. Planetary structure and formation models assume a common
origin of the building blocks, yet, thus far, there is no direct conclusive
observational evidence correlating the composition of rocky planets to their
host stars. Here we present evidence of a chemical link between rocky planets
and their host stars. The iron-mass fraction of the most precisely
characterized rocky planets is compared to that of their building blocks, as
inferred from the atmospheric composition of their host stars. We find a clear
and statistically significant correlation between the two. We also find that
this correlation is not one-to-one, owing to the disk-chemistry and planet
formation processes. Therefore rocky planet composition depends on the chemical
composition of the proto-planetary disk and contains signatures about planet
formation processes.
