Astro 2020 Science White Paper: Evolved Planetary Systems around White Dwarfs.

Practically all known planet hosts will evolve into white dwarfs, and large parts of their planetary systemswill survive this transition - the same is true for the solar system beyond the orbitof Mars. Spectroscopy of white dwarfsaccreting planetary debris provides the most accurate insight into the bulk composition of exo-planets. Ground-based spectroscopic surveys of ~260, 000 white dwarfsdetected 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 planetesimalsorbiting closely around white dwarfshas 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 planetesimalsthat is sufficiently large to establish their properties as a population, and to deduce the architectures of the outer planetary systemsfrom where they originated. Mid-infrared spectroscopy of the dustydisks will provide detailed mineralogical information of the debris, which, in combination with the elemental abundances measured from the white dwarfspectroscopy, 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 dustydisks. Finally, the direct detection of the outer reservoirs feeding material to the white dwarfswill require sensitive mid- and far-infrared capabilities.