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 (<5,000 K) and old (cooling ages of 5-10 Gyr ago) metal-polluted white dwarfs, of which one also displays photospheric potassium. The relative abundances of these two elements with respect to sodium and calcium strongly suggest that all four white dwarfs have accreted fragments of planetary crusts. We detect an infrared excess in one of the systems, indicating that accretion from a circumstellar debris disk is ongoing. The main-sequence progenitor mass of this star was 4.8 +/- 0.2 M-circle dot, demonstrating that rocky, differentiated planets may form around short-lived B-type stars.

Automated summary

White dwarfs that accrete debris from tidally disrupted asteroids provide insights into the bulk composition of exoplanetary building blocks, suggesting that rocky planet formation is a generic process. Analysis of lithophile and siderophile element abundances in planetary debris can help investigate planetary differentiation, although current studies lack clear tracers of planetary crusts. Detection of lithium in the atmospheres of certain white dwarfs indicates accretion of planetary crust fragments, with one system showing ongoing accretion from a circumstellar debris disk around a short-lived B-type star.