White dwarfs that accrete the debris of tidally disrupted asteroids¹ provide the opportunity to measure the bulk composition of the building blocks, or fragments, of exoplanets². This technique has established a diversity of compositions comparable to what is observed in the Solar System³, suggesting that the formation of rocky planets is a generic process⁴. The relative abundances of lithophile and siderophile elements within the planetary debris can be used to investigate whether exoplanets undergo differentiation⁵, yet the composition studies carried out so far lack unambiguous tracers of planetary crusts⁶. 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_⊙, demonstrating that rocky, differentiated planets may form around short-lived B-type stars.

白矮星吸积了被潮汐破坏的小行星^{1的碎片，这为测量系外行星2}的构件或碎片的整体成分提供了机会。这种技术已经建立了与太阳系³中观察到的成分相当的多样性，这表明岩石行星的形成是一个通用过程⁴。行星碎片中亲石元素和亲铁元素的相对丰度可用于研究系外行星是否经历分化⁵，但迄今为止进行的成分研究缺乏明确的行星地壳示踪剂⁶. 在这里，我们报告了在四颗凉爽（<5,000 K）和古老（冷却年龄为 5-10 Gyr 前）金属污染的白矮星的大气中检测到锂，其中一颗也显示出光球钾。这两种元素在钠和钙方面的相对丰度强烈表明所有四颗白矮星都吸积了行星地壳的碎片。我们在其中一个系统中检测到红外线过量，这表明来自星际碎片盘的吸积正在进行中。这颗恒星的主序前身质量为 4.8 ± 0.2  _M⊙，表明在短命的 B 型恒星周围可能形成岩石分化的行星。