is derived from the bulk density of transiting planets [e.g. @valenciaetal10-1], and transit spectroscopy provides some information on the chemical composition of their atmospheres [e.g. @grillmairetal08-1]. More detailed investigations of the chemistry of exo-planetary systems around main-sequence host stars are beyond the reach of present observational instrumentation. However, @zuckermanetal07-1 demonstrated in a pioneering paper that the photospheric abundances of polluted white dwarfs can be used to infer the bulk abundances of the planetary debris material detected around the white dwarf GD362, and showed that the composition of this material is broadly comparable to that of the Earth-Moon system.

The strong surface gravity of white dwarfs implies that metals will sink out of the photosphere on time scales that are orders of magnitude shorter than their cooling ages, and therefore white dwarfs are expected to have either pure hydrogen or helium atmospheres [@fontaine+michaud79-1]. Exceptions to this rule are only hot (${\mbox{$T_{\mathrm{eff}}$}}\ga25\,000$K) white dwarfs where radiative levitation can support some heavy elements in the photosphere [e.g. @chayeretal95-1], and cool (${\mbox{$T_{\mathrm{eff}}$}}\la10\,000$K) white dwarfs where convection may dredge up core material [@koesteretal82-2; @fontaineetal84-1]. Yet white dwarfs with metal-contaminated atmospheres have been known for nearly a century [@vanmaanen17-1], and accretion from the interstellar medium [e.g. 