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The presence of elements heavier than helium in white dwarf atmospheres isoften a signpost for the existence of rocky objects that currently orpreviously orbited these stars. We have measured the abundances of variouselements in the hydrogen-atmosphere white dwarfs G149-28 and NLTT 43806. Incomparison with other white dwarfs with atmospheres polluted by heavy elements,NLTT 43806 is substantially enriched in aluminum but relatively poor in iron.We compare the relative abundances of Al and eight other heavy elements seen inNLTT 43806 with the elemental composition of bulk Earth, with simulatedextrasolar rocky planets, with solar system meteorites, with the atmosphericcompositions of other polluted white dwarfs, and with the outer layers of theMoon and Earth. Best agreement is found with a model that involves accretion ofa mixture of terrestrial crust and upper mantle material onto NLTT 43806. Theimplication is that NLTT 43806 is orbited by a differentiated rocky planet,perhaps quite similar to Earth, that has suffered a collision that strippedaway some of its outer layers.

An aluminum/calcium-rich, iron-poor, white dwarf star: evidence for an extrasolar planetary lithosphere?