Minimum masses of accreted material within white dwarf atmospheres as a function of temperature. The left panel shows the minimum mass of planetary material in the mixing zones of H- and He-dominated white dwarfs, represented by blue hexagons and orange pentagons, respectively. For He-dominated white dwarfs, the mixing zone is defined as the convection zone, whereas for warmer H-dominated white dwarfs, it is defined as the region above optical depth τ_R ≲ 5. Examples of a variety of Solar System objects are included for comparison. The right panel expresses the evolution of the mixing zone mass with decreasing T_eff and increasing cooling age. The much lower mixing zone masses of the two He-dominated white dwarfs with T_eff > 15 000 K (WD1536+520 (Farihi et al. 2016) and WD J2047–1259 (Hoskin et al. 2020)) arise from the large amounts of trace H (log(H/He) ≃−1 to −2) in their atmospheres.