Schematic diagram illustrating how planet formation modifies the composition of the stellar surface. A star first accretes hydrogen, helium and heavy elements (equivalently, metals) from a protoplanetary disk of mass M_disk(t). After a time t_p, planet formation removes refractory elements from the accreting gas flow, leading to a dilution of the stellar surface. At this point, the mass in the gas disk (to be eventually accreted by the star) is ΔM₀ ≡ M_disk(t_p). Later, after disk dispersal, the accretion of rocky objects (e.g., planetesimals) can potentially increase the stellar metallicity (pollution).