esimal population (Akiba et al, 2024). While (exo-)Oort Cloud dynamics are identical for bodies orbiting a MS star or a WD, the combination of reduced stellar mass and increased orbital radius means that a body experiences a stronger effect from the Galactic tide following AGB mass loss. The presence of a planet or star between the Oort Cloud and the WD can reduce delivery of Oort Cloud material to the WD, as they can suppress eccentricity fluctuations induced by the Galactic tide and eject bodies that do get too close, preventing them from crossing the planet’s orbit (O’Connor et al, 2023;

Pham and Rein, 2024). Overall, exo-Oort cloud bodies may be responsible for the trace H seen in many He-atmosphere WDs, but are unlikely to be the dominant source of metal pollution (Veras et al, 2014c). Initial reservoirs of the parent bodies of comets are slowly depleted through a star’s MS lifetime by dynamical and collisional processes (Sec 2, 3). The strong mass loss at the end of a star’s life can destabilise formerly stable systems or reservoirs, and radiation forces can repopulate depleted reservoirs by moving bodies in the system (Section 10 and Figure 7). As a star loses mass, planet:star mass ratios increase, making interactions between planets and minor bodies stronger than previously when compared to the dominant interactions between planets and the host star (Duncan and Lissauer, 1998; Debes and Sigurdsson, 2002). The stability of a multi-planet system is governed by the planet:star mass ratio (Mpl/M⋆)α, where α = 1/3 for a two-planet system (Gladman, 1993) and α = 1/4 for a three-planet system (Quillen, 2011; Petit et al, 2020). Entire systems which survived the MS in stable configurations can therefore undergo scattering after stellar mass loss, in the process scattering small bodies down to the WD (Mustill et al, 2018). In more quiescent cases, an unstable “Kirkwood gap” around a mean-motion resonance can increase in width (Debes et al, 2012), as can the chaotic region of overlapping resonances close to a planet (Bonsor et al, 2011; Caiazzo and Heyl, 2017). In systems where one or more planets undergo tidal decay and/or engulfment, unstable secular resonances will move around the system, destabilising fresh, previously stable, reservoirs (Smallwood et al, 2021). Radiation forces on small bodies can also move the bodies into unstable regions (See Sec 10). These processes are reviewed in detail by Mustill (2026). Unfortunately, the large range of parameter space for setting up multiplanet systems, coupled with the lack of data on planets on wide orbits, means that there are many possible mechanisms with little observational guidance to choose between them. We do know that WD pollution is common, and that the accretion rates are relatively weakly decaying, over Gyr timescales (Blouin and Xu, 2022). This rules out instabilities in systems of giant planets as the dominant driver, as these planets are uncommon and the instabilities occur too quickly to explain late pollution (Mustill et al, 2014, 2018). Forcing from stellar binary companions can in principle explain late pollution (Hamers and Portegies Zwart, 2016; Petrovich and Mu˜noz, 2017), but observationally WD pollution does not seem to correlate with binarity (Wilson et al, 2019; Noor et al, 2024). Currently, two promising delivery mechanisms involve systems of high planetary multiplicity: destabilisation of high-multiplicity systems by mass loss may be common (Maldonado et al, 2021), and if the major planets remain stable there is scope for secular resonances to be moved around or repopulated during the AGB (Smallwood et al, 2021), so they can act on fresh reservoirs of material.