  White dwarf planetary systems provide a unique way to measure the bulk
composition of exoplanetary material. Extrasolar asteroids/comets/moons which
have survived the evolution of their host star can end up in the atmosphere of
the white dwarf. Asteroids and boulders appear to be the most common
pollutants, where we use the term "asteroids" to refer to the parent body that
is polluting the atmosphere. The presence of the planetary material is detected
via absorption lines of heavy elements. White dwarfs with these absorption
features are called "polluted" white dwarfs. Polluted white dwarfs were
expected to be rare objects because white dwarfs have high surface gravities,
therefore, these heavy elements will settle out of the white dwarf's
atmospheres in a short amount of time (Paquette et al. 1986). However,
high-resolution spectroscopic surveys found that 25-50% of white dwarfs are
polluted (Zuckerman et al. 2003, 2010; Koester et al. 2014). The mechanism
responsible for making a polluted white dwarf must be common and efficient.
There is strong theoretical and observational evidence that white dwarfs are
accreting from planetary material. There are different mechanisms that can
deliver exoplanetary material into the Roche lobe of the white dwarf. Debris
disks, transits from disintegrating bodies, and intact planets have all been
detected around white dwarfs (e.g., Jura et al. 2007; Vanderburg et al. 2015,
2020). This chapter will describe how the chemical autopsies are conducted, and
what is learnt about exoplanetary material from polluted white dwarfs.
