  Practically all known planet hosts will evolve into white dwarfs, and large
parts of their planetary systems will survive this transition - the same is
true for the solar system beyond the orbit of Mars. Spectroscopy of white
dwarfs accreting planetary debris provides the most accurate insight into the
bulk composition of exo-planets. Ground-based spectroscopic surveys of ~260,
000 white dwarfs detected with Gaia will identify >1000 evolved planetary
systems, and high-throughput high-resolution space-based ultraviolet
spectroscopy is essential to measure in detail their abundances. So far,
evidence for two planetesimals orbiting closely around white dwarfs has been
obtained, and their study provides important constraints on the composition and
internal structure of these bodies. Major photometric and spectroscopic efforts
will be necessary to assemble a sample of such close-in planetesimals that is
sufficiently large to establish their properties as a population, and to deduce
the architectures of the outer planetary systems from where they originated.
Mid-infrared spectroscopy of the dusty disks will provide detailed
mineralogical information of the debris, which, in combination with the
elemental abundances measured from the white dwarf spectroscopy, will enable
detailed physical modelling of the chemical, thermodynamic, and physical
history of the accreted material. Flexible multi-epoch infrared observations
are essential to determine the physical nature, and origin of the variability
observed in many of the dusty disks. Finally, the direct detection of the outer
reservoirs feeding material to the white dwarfs will require sensitive mid- and
far-infrared capabilities.
