Alkali metals in white dwarf atmospheres as tracers of ancient planetary crusts

White dwarfs that accrete the debris of tidally disrupted asteroids 1 provide the opportunity to measure the bulk composition of the building blocks, or fragments, of exoplanets 2 . This technique has established a diversity of compositions comparable to what is observed in the Solar System 3 , suggesting that the formation of rocky planets is a generic process 4 . The relative abundances of lithophile and siderophile elements within the planetary debris can be used to investigate whether exoplanets undergo differentiation 5 , yet the composition studies carried out so far lack unambiguous tracers of planetary crusts 6 . Here we report the detection of lithium in the atmospheres of four cool (