species, and range of heliocentric distances over which observations are taken, though even the limited sample available to date indicates a complex taxonomy (Russo et al, 2016; Eistrup et al, 2019). The Rosetta results on 67P/Churyumov–Gerasimenko show an even more complex picture with adsorption and release processes affecting observed abundances (Luspay-Kuti et al, 2022; Mandt et al, 2024). These Rosetta results have not yet been fully linked to ground based observations (Saki et al, 2024).

For extrasolar systems, linking an exocomet to its source is more challenging. β Pic is so far the only well-characterised system with frequent observations of star-grazing comets (Lu et al., this issue). The system hosts two giant planets on well-determined orbits, along with multiple small body populations at a range of distances from the star (Smith and Terrile, 1984; Lagrange et al, 2010; Matr`a et al, 2019; Nowak et al, 2020). Beust et al (2024) used dynamical arguments to show that a hypothetical reservoir interior to β Pic’s inner planet could be the source, and this source could be consistent with detections of warm dust in the system. However, this is not a unique possibility, and it is also plausible that the comets originate further out in the main disc and move inwards through other processes (e.g. scattering by a chain of planets; Jaworska and Hoeijmakers 2026). For other exocomet systems we know even less. HD 172555 has comets detected in both spectroscopy and photometry, and hosts a debris disc that could be a comet source (Smith et al, 2012; Grady et al, 2018). However, much less is known about this system than β Pic, so confidently linking the comets to the disc is difficult. Likewise, many systems with spectroscopic absorption features attributed to comets also host debris discs (Iglesias et al, 2025), but we do not have enough evidence to say that the comets came from those discs. We saw in Section 2 that many of the dynamical delivery mechanisms involve planets. However, a major challenge in understanding the dynamics of exocomet systems is that the planets most favourable for driving delivery would be distant from their stars and of low mass, which makes them extremely difficult to detect. To date, very few planets have been detected in the outer regions of debris disc systems, and linking exocomets to their sources remains challenging. Furthermore, the host stars of spectroscopically-identified exocomets are more massive than the Sun (Korth et al, 2026b), as are the typical progenitors of polluted WDs (Section 11); around these stars, our knowledge of the planetary population is less than it is for FGK stars. Binary stellar companions are much easier to detect, however, and have been implicated in driving the dynamics for the photometric comet-host candidate KIC 8462852 (Young and Wyatt, 2024). There are ongoing efforts to find the wide-orbit exoplanets that could be responsible for generating exocomets. Microlensing will soon prove fruitful on a population level, with the advent of the Nancy Grace Roman Telescope (Akeson et al, 2019), but is ill-suited for particular systems. Coronagraphic direct imaging will push down to lower masses with Roman and possibly the Habitable Worlds Observatory (National Academies of Sciences, Engineering, and Medicine, 2021; Harada et al, 2024), though not necessarily for planets on wide orbits. Indirect dynamical traces may also prove possible in some specific systems; features seen in debris discs such as warps, eccentric belts, gaps, clumps and steep edges have all been used to infer the properties of unseen planetary perturbers (e.g. Mouillet et al 1997; Pearce and Wyatt 2014; Sefilian et al 2021; Friebe et al 2022; Booth et al 2023; Pearce et al 2024). Similarly, distortions of the gas kinematics of protoplanetary discs could identify outer planets during formation (Pinte et al, 2019).