The collisional cascade model provides significant insight into debris disc evolution, but there are still several unanswered questions. First, in order for collisions to be destructive, the bodies must have high relative velocities. This means they must have excited orbits; i.e., their eccentricities and/or inclinations must be significantly non-zero. However, when planetesimals form they are thought to have relatively unexcited orbits, because gas in protoplanetary discs should significantly damp the orbits of young planetesimals. If true, this means that debris must get excited after the protoplanetary disc phase, in a process called “stirring” (Wyatt, 2008). We do not know the mechanism(s) behind stirring, although many have been suggested. One possible stirring mechanism is the gravity of the larger planetesimals within the disc itself, which could excite the other planetesimals (Kenyon and Bromley, 2001, 2008, 2010; Kennedy and Wyatt, 2010; Krivov and Booth, 2018). However, in some cases the disc masses required to grow the large planetesimals needed within the age of the disc would be implausibly high (Krivov and Wyatt, 2021; Pearce et al, 2022a). Alternatively, planets outside the disc could stir disc material through several processes, including secular interactions (Mustill and Wyatt, 2009), sweeping meanmotion resonances (Friebe et al, 2022), or by scattering material into the disc which itself excites other bodies (Costa et al, 2024).

[Figure: Fig. 3 Simulation of the early size distribution of the main Asteroid Belt (black line, Bottke et al 2020) and the trans-Neptunian Scattered Disc (“destabilized population”; red line, Bottke et al 2023). The Scattered Disc population is scaled to match the number of bodies with diameter above 0.01 km. Figure from Bottke et al (2023), reproduced under the CC-BY-4.0 license https://creativecommons. org/licenses/by/4.0/.]

However, these processes would sometimes require planets so massive that that they are ruled out by observations (Pearce et al, 2022a), and massive debris discs can also resist planetary stirring (Sefilian, 2024). This means that planets cannot be the only stirring mechanism. Other possibilities include “flyby stirring”, where material is stirred by another star as it passes close to the disc, although it is unclear whether this process is common enough to stir debris discs on a population level (Ida et al, 2000; Kobayashi and Ida, 2001; Kenyon and Bromley, 2002). Combinations of the above mechanisms, known as “mixed stirring”, may also occur (Mu˜noz-Guti´errez et al, 2023; Marshall et al, 2026). Finally, bodies could somehow already be stirred by the end of the protoplanetary disc phase (e.g. Walmswell et al 2013; Booth and Clarke 2016; Wyatt 2020), or get stirred as the protoplanetary disc dissipates (e.g. Zheng et al 2017; Best et al 2024). However, despite considerable work into these various mechanisms, it is still unclear how debris discs are stirred.

If debris discs act as reservoirs for high-eccentricity comets, then it is possible that the same process that drives some objects onto high-eccentricity orbits also excites other objects onto collisional orbits. If true, then we might expect the source reservoirs of high-eccentricity comets to be detectable, because they should have a high level of excitement, which would lead to increased collisions and hence dust.