The chemistry of extra-solar materials from white dwarf planetary systems

TL;DR

White dwarf planetary systems serve as natural laboratories for analyzing exoplanetary material, revealing compositions through pollution signatures in white dwarf atmospheres, which are more common than previously expected.

Contribution

This paper reviews how polluted white dwarfs are used to determine the bulk composition of exoplanetary material, highlighting recent findings and mechanisms of pollution.

Findings

25-50% of white dwarfs are polluted with planetary material

Polluted white dwarfs reveal diverse exoplanetary compositions

Multiple mechanisms deliver planetary debris to white dwarf atmospheres

Abstract

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.