Planetary heavy element fraction Z_pl divided by the heavy element fraction of the host star Z_star. The relative enrichment is shown as a function of planetary mass M. Red dots with error bars are from Miller & Fortney (2011) for transiting exoplanets. The solid red line is a least-squares fit to the exoplanet data, while the dotted red lines give the 1σ errors in the fit. Giant planets of the solar system are shown with black symbols. Blue dots are synthetic planets with 0.11 ≤ a/AU ≤ 1 (5 AU for the synthesis without migration). The blue line gives the fit to the synthetic data. The four panels show populations calculated with an f_opa indicated in the plot. The bottom right panel assumes in situ formation and Ṁ_Z,run = 0. The pale green dots are the same synthetic planets, but assuming that the accreted gas has the same metallicity as the host star instead of being metal free.