  Tidal disruption and subsequent accretion of planetesimals by white dwarfs
can reveal the elemental abundances of rocky bodies in exoplanetary systems.
Those abundances provide information on the composition of the nebula from
which the systems formed, analogous to how meteorite abundances inform our
understanding of the early Solar System. We report the detection of Li, Na, K
and Ca in the atmosphere of the white dwarf Gaia DR2 4353607450860305024, which
we ascribe to accretion of a planetesimal. Using model atmospheres, we
determine abundance ratios of these elements, and with the exception of Li,
they are consistent with meteoritic values in the Solar System. We compare the
measured Li abundance to measurements in old stars and to expectations from Big
Bang nucleosynthesis.
