The accretion of rocky material is responsible for the presence of heavy elements in the atmospheres of a large fraction of white dwarf stars .
Those objects represent a unique opportunity to infer the bulk composition of exoplanetesimals .
This chemical characterization requires the use of detailed atmosphere models to determine the elemental abundances at the photospheres of white dwarfs .
In this work , we use a state-of-the-art model atmosphere code to reanalyse the first large survey of metal-polluted white dwarfs for which abundances are found for multiple elements .
We show that the improved constitutive physics of our models lead to systematically higher Mg abundances than previous analyses .
We find an average \log { Mg / Ca } number abundance ratio of 1.5 .
This value is significantly above the reference abundance for chondrites , which is expected as current diffusion models predict that for the cool helium-atmosphere white dwarfs of our sample Mg should remain in the atmosphere longer than Ca .
This helps resolve a recently identified Mg depletion problem , where the planetesimals accreted by white dwarfs were reported to be Mg-deficient compared to the expected composition of their planetary systems .