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The opacity due to grains in the envelope of a protoplanet regulates theaccretion rate of gas during formation, thus the final bulk composition ofplanets with primordial H/He is a function of it. Observationally, forexoplanets with known mass and radius it is possible to estimate the bulkcomposition via internal structure models. We first determine the reductionfactor of the ISM grain opacity f_opa that leads to gas accretion ratesconsistent with grain evolution models. We then compare the bulk composition ofsynthetic low-mass and giant planets at different f_opa with observations. Forf_opa=1 (full ISM opacity) the synthetic low-mass planets have too small radii,i.e., too low envelope masses compared to observations. At f_opa=0.003, thevalue calibrated with the grain evolution models, synthetic and actual planetsoccupy similar mass-radius loci. The mean enrichment of giant planets relativeto the host star as a function of planet mass M can be approximated asZ_p/Z_star = beta*(M/M_Jup)^alpha. We find alpha=-0.7 independent of f_opa insynthetic populations in agreement with the observational result (-0.71+-0.10).The absolute enrichment level decreases from beta=8.5 at f_opa=1 to 3.5 atf_opa=0. At f_opa=0.003 one finds beta=7.2 which is similar to theobservational result (6.3+-1.0). We thus find observational hints that theopacity in protoplanetary atmospheres is much smaller than in the ISM even ifthe specific value of the grain opacity cannot be constrained here. The resultfor the enrichment of giant planets helps to distinguish core accretion andgravitational instability. In the simplest picture of core accretion wherefirst a critical core forms and afterwards only gas is added, alpha=-1. If acore accretes all planetesimals inside the feeding zone, alpha=-2/3. Theobservational result lies between these values, pointing to core accretion asthe formation mechanism.

Grain opacity and the bulk composition of extrasolar planets. I. Results from scaling the ISM opacity