JuanCastillo29/space-mission-intelligence-data huggingface dataset:
Bannister et al (2025) detail the material reservoirs whence we think comets originate. These reservoirs are typically located in the outer regions of planetary systems, so some mechanisms are needed to drive the bodies inwards towards the star, where they can sublimate and be observed as comets or exocomets. This section describes mechanisms that can remove bodies from reservoirs and place them on high-eccentricity, low-periapsis cometary orbits. In this Section, we focus on massive analogues of our own Asteroid and Kuiper Belts, which are often visible as debris discs around other stars. Delivery from Oort Cloud analogues is discussed in Section 4. Exocomets are detected or inferred around stars of diverse ages and spectral types (Iglesias et al 2025 and Korth et al 2026b). Similarly, indirect evidence of exocomets, such as exozodical dust in the inner regions of planetary systems, is also found around a high fraction of stars with diverse ages and types (e.g. Mennesson et al 2014; Nu˜nez et al 2017; Sezestre et al 2019; Ertel et al 2020; Pearce et al 2022b). This implies there must be mechanisms capable of delivering exocomets at both early and late stages of stars’ lives, and effective for different types of star. There is unlikely to be one universal Cometary pathway                    Alternative cometary pathway                     Ejection pathway | a)

Scattering by a chain of planets Migration | b) Scattering after a dynamical instability |

| --- | --- |
| c) Mean-motion resonances with a planet MMR | d) Secular resonances with a planet Secular resonance |
| e) Kozai by a highly inclined companion N e w o rb it | f) Stellar flyby |

[Figure: Fig. 1 Possible exocomet delivery and ejection mechanisms. Cometary nuclei are thought to initially]

form and reside in belts or discs of planetesimals, and are subsequently either driven towards the star or ejected by various dynamical mechanisms. Each panel shows a different potential mechanism, as described in Section 2. The large yellow circles are stars, the small grey points are small bodies which could become comets, and the brown circles are planets. The lines show various dynamical pathways; small bodies can either be driven towards the star to become comets (grey lines) or ejected from the system (dotted red lines). Solid and dashed grey lines show alternative pathways for getting bodies close to stars.
or even dominant mechanism; a combination of mechanisms is likely responsible, with different efficiencies at different times and around different stars. Figure 1 shows some dynamical mechanisms that could create exocomets, each of which is detailed below. These are the mechanisms most discussed in the literature.
A population of small bodies orbiting close to a planet will be slowly dynamically eroded, as bodies chaotically diffuse onto planet-crossing orbits (Wisdom, 1980; Mustill and Wyatt, 2012). Small bodies passing near a planet will then get scattered onto significantly different orbits around the host star. A planet near a reservoir of small bodies can hence scatter them inwards towards the star, or outwards and eject them from the system. However, it is difficult for a single planet to scatter bodies all the way into the inner regions, because this would require the body to lose a significant amount of orbital angular momentum during a single scattering event.

Instead, inward scattering is more easily achieved by a chain of multiple planets (e.g. Bonsor et al 2014; Marino et al 2018; Anslow et al 2023). In this scenario (Figure 1a), the outer planet scatters material inwards, where it encounters the next planet. That planet scatters material inwards again, where it encounters the next planet. This process continues, and can efficiently pass bodies from an outer reservoir into the inner regions. The act of scattering material inwards also drives the outer planet outwards to conserve the system’s total angular momentum, where the planet can encounter fresh material. The efficiency of this process depends on the specific architecture of the planetary system. Rodet and Lai (2024) performed a numerical investigation of two-planet systems, and find that two planets are much more effective at producing comets than single-planet systems. They find that the efficiency by which bodies are placed onto comet-like orbits is predominantly set by the parameters of the innermost planet. Generally the inner planet is more effective at perturbing material, at least over short time-scales, because it has a shorter orbital period, which results in shorter dynamical timescales for interacting with the small bodies.
Section 2.1 considered a chain of planets on relatively stable orbits, where the outermost planet lies near a reservoir of small bodies. However, it is also possible for planets to undergo rapid, significant orbital changes, and this can drive bodies inwards onto cometary orbits (Figure 1b). For example, after a long period of quiescent evolution, two or more planets could undergo a dynamical instability that drastically changes the planetary orbits over a short timescale (e.g., Chambers et al, 1996; Petit et al, 2020). Such planets could gain significant eccentricities and may be driven into the reservoirs, similar to the orbital evolution of Neptune in planetary instability models of the Solar System (e.g. Gomes et al 2005). This process could scatter a significant amount of small bodies both inwards and outwards, some of which could be passed inwards between planets like in Section 2.1. A late dynamical instability has been hypothesised to explain extremely dusty old debris discs such as η Corvi (Lisse et al, 2012).
Mean-motion resonances (MMRs) occur when two bodies have orbital periods that are approximately simple fractions of each other. For example, Neptune and Pluto are in the 3 : 2 MMR, which means that Neptune completes 3 orbits of the Sun for every 2 orbits completed by Pluto. Bodies in MMRs with a planet can have their eccentricities excited to high values by the interaction, especially if the planet has undergone migration (e.g. Wyatt 2003). Some resonances may be inherently unstable (Beust and Morbidelli, 1996), or they may be destabilised by the system’s global architecture (Lecar et al, 2001), forcing eccentricities still higher. These eccentricities can either be high enough that the bodies directly attain small periapsis distances (solid grey line on Figure 1c), or they can bring the bodies close to a planet, which then scatters them inwards (dashed grey line). Since MMRs take time to excite eccentricities, this mechanism could potentially supply material to the inner regions well into a system’s lifetime (e.g.

Faramaz et al 2017; Beust et al 2024). The Yarkovsky effect (Beekman, 2006), discussed below, can also force bodies to drift into unstable resonances from stable regions of a belt at late times (this process is ongoing in the Solar System’s Asteroid Belt). MMRs have been much discussed as a delivery mechanism in the reasonably well-characterised β Pictoris system; see Lu et al (this volume).
Section 2.3 described MMRs, which occur when the orbital periods of bodies are approximately simple fractions of each other. A similar situation can occur when the orbits themselves precess (i.e., revolve). Planets’ orbits change on long (secular) timescales with a characteristic set of eigenfrequencies that depends on the system architecture. If the precession period of an orbit is approximately in a simple ratio with one of these eigenfrequencies, then they form a secular resonance (e.g. Murray and Dermott 1999, Chapter 7). Planetary orbits can precess for several reasons, including the influence of other planets, or of a massive debris disc (Pearce and Wyatt, 2015; Yelverton and Kennedy, 2018; Sefilian et al, 2021, 2023). When this happens, an object near the secular resonance with that planet can have its eccentricity pumped up to high values, whilst its semimajor axis remains constant. This means that if a planet’s orbit precesses, and its secular resonance lies in a debris disc, then bodies in the disc can be driven onto cometary orbits with small periapsides close to the star (Figure 1d). Alternatively, objects could be driven onto orbits which cross that of a planet, and that planet could then directly scatter them inwards.
If bodies have mutual inclinations above ≈40◦, then the von Zeipel–Kozai–Lidov mechanism can occur (Kozai interactions for short; von Zeipel 1910; Lidov 1962; Kozai 1962). For systems with a debris disc and a planet or companion star on a highly inclined orbit, this mechanism can cause bodies in the disc to oscillate between loweccentricity orbits in the disc plane and high-eccentricity orbits in an inclined plane (Figure 1e). The semimajor axes remain constant in this interaction, so small bodies undergoing Kozai interactions can be driven onto cometary orbits with periapsides very close to the star. Note that there is some debate about the efficiency of this mechanism, because the effects of general relatively near the star can overcome the Kozai mechanism in some cases (Naoz, 2016).
An external star passing close to a planetary system can induce significant perturbations in both systems (Adams, 2010). This can directly drive small bodies from the outer regions of a system onto highly eccentric cometary orbits (Kenyon and Bromley, 2004; Morbidelli and Levison, 2004; Pfalzner et al, 2024b). Flybys can also trigger (possibly delayed) dynamical instabilities that destabilise planets (Malmberg et al, 2011), which then scatter small bodies onto potentially cometary orbits (Flammini Dotti et al, 2023). It is also possible for bodies to be transferred between stars during flyby encounters (Kenyon and Bromley, 2004;

Morbidelli and Levison, 2004; Levison et al, 2010; J´ılkov´a et al, 2016); the resulting orbits may have very high eccentricities, so a body can be ripped from one star and become a grazing comet around the other star (grey dashed line on Figure 1f). Stellar flybys could occur at any time, but they are much more likely in young systems, where stars are closer together in their birth environment (Lada and Lada, 2003). Within the Galactic field after a star has left its birth cluster, flybys that can directly affect a debris disc reservoir are extremely unlikely, but wider flybys that affect (exo-)Oort Cloud bodies are common, and they combine with the Galactic tide to help deliver such bodies to small periapsides (Rickman et al, 2008).
Our own Solar System is the best-studied system we have, and it offers a warning against over-reliance on single dynamical mechanisms when attempting to explain exocomet delivery, even within a single system, as most of the above mechanisms are known or suspected to have affected the dynamics of small bodies in the Solar System. Oort Cloud comets were originally implanted in the Oort Cloud via planetary scattering, some possibly during a global instability of the giant planets (see next Section). They then are affected both by stellar flybys when close to aphelion, and by planetary scattering when close to perihelion. The dynamics of bodies in the Asteroid and Kuiper Belts is affected by mean motion and secular resonances with the giant planets, and those Kuiper Belt bodies destabilised by such resonances can then be scattered inwards by the chain of giant planets to become Jupiter-Family Comets. Meanwhile, a relatively close stellar flyby may have sculpted the trans-Neptunian region, raising the perihelia of bodies such as Sedna which are no longer in direct dynamical contact with Neptune (Brasser et al, 2012), and early flybys may have contributed to populating the Oort Cloud (Levison et al, 2010). Looking at extrasolar systems, it is likely that many mechanisms are at play in each system.
Our best example of a cometary reservoir that has been dynamically depleted is the Kuiper Belt, located beyond the orbit of Neptune in the Solar System. The Kuiper Belt’s present-day mass is of order 0.01 M⊕(Fraser et al, 2014), but its initial mass is thought to have been around 20 M⊕(Nesvorn´y et al, 2018). This implies a depletion in mass by three orders of magnitude. The Kuiper Belt’s depletion is thought to have been largely the result of a planetary instability early in the Solar System’s history. The Kuiper Belt contains scattered and resonant populations, which indicate that Neptune once migrated outwards into the disc (Malhotra, 1995;

Gomes et al, 2005). The Solar System’s giant planets may originally have had a much more compact configuration than today (e.g. Tsiganis et al 2005; Morbidelli et al 2007), mainly due to planet migration in the protoplanetary disc (e.g. Masset and Snellgrove 2001; Morbidelli and Crida 2007; Kley and Nelson 2012). However, this compact planetary configuration would have been unstable after the gas disc disappeared, possibly following migration of Neptune outwards as it scattered TNOs inwards, ultimately leading to a dynamical instability (Gomes et al, 2005; Levison et al, 2011; Griveaud et al, 2024). The timing of the instability is not well constrained relative to the disappearance of the gas disc (e.g. Deienno et al 2017; Clement et al 2018; de Sousa et al 2020), but as a result Neptune was driven further outwards into a primordial Kuiper Belt that was at that time much more massive than today. This process would have scattered significant amounts of material, leading to the depletion of the Kuiper Belt. The current structure of the Kuiper Belt, and of the Jupiter Trojan population, can be well explained by this paradigm (Nesvorn´y et al, 2013, 2018). There may also be evidence that exoplanets scatter and deplete small body reservoirs in extrasolar systems. Some debris discs have sharp edges, which may be indicative of sculpting by unseen planets (Imaz Blanco et al, 2023; Pearce et al, 2024). In addition, Geiler et al (2019) found evidence that the disc of HR 8799 (where four giant planets have been detected) may contain a scattered population, and Matr`a et al (2019) find evidence of both kinematically hot and cold3 populations in the β Pictoris debris disc, which may also be due to scattering. The gaps in several debris discs, like HD 107146, may have been depleted via scattering from a hypothetical planet embedded in the disc (Friebe et al, 2022); alternatively, these may have been depleted by secular resonances with an internal planet (Pearce and Wyatt, 2015; Sefilian et al, 2021, 2023). It therefore appears that planets may deplete small body reservoirs in extrasolar systems too.
The extreme dynamics occurring during a stellar flyby could also significantly deplete small body reservoirs. This has been suggested in the Solar System, where a close stellar flyby potentially restructured the Kuiper Belt (Pfalzner et al, 2024a). This interaction may have directly ejected a quarter of the Belt, and injected a further 9% of the debris disc’s mass into the region inside Neptune, where it would have contributed to the Centaur and short-period-comet populations. That injected population would then undergo further depletion due to interactions with planets; Figure 2 shows the depletion of the injected small body population over time, following a flyby (Pfalzner et al, 2024a). On timescales of 10 Myr to 1 Gyr after the flyby, the injected population is significantly depleted by the planetary interactions, particularly with Jupiter. Hence much of the material that was injected onto cometary orbits by the flyby would be quickly depleted by planetary interactions.
3“Hot” and “cold” here refers to the degree of orbital excitation rather than thermodynamic temperature, a nomenclature also used for the hot and cold populations of the Kuiper Belt, and for stellar populations in the Galactic disc.

[Figure: Fig. 2 Depletion of the population of small bodies injected into the inner regions of the Solar System]

by a stellar flyby, from a simulation by Pfalzner et al (2024a). The plot shows the number of former TNOs whose perihelion lies within each bin. The material depletes over time (measured from the perihelion of the flyby), through interactions with the giant planets.
Even without planets or flybys, a small body reservoir would naturally deplete over time. This is due to the collisional erosion of the constituent bodies. Unlike collisions in protoplanetary discs, which can be agglomerative and cause objects to grow into larger bodies, collisions in debris discs are destructive and cause objects to break apart (Wyatt, 2008); this is because of the lack of damping and resulting high velocity dispersion of the population of small bodies. The debris from these collisions then undergoes further collisions, breaking into even smaller objects, and this process continues until objects are ground down into dust (Dohnanyi, 1969). Small dust is strongly affected by radiation and/or wind from the star, so when debris breaks into small dust grains, these forces overcome gravity and the grains are blown out of the system. This continuous collisional grinding and removal of material causes debris discs to lose mass over time (Wyatt et al, 2007; L¨ohne et al, 2008). The short lifetime of dust grains also means that observed debris discs must be currently collisionally active.
Evidence for this collisional process comes from multiple sources. First, there is a clear trend between debris disc brightness and system age (see Bannister et al, 2025). This means that the mass of dust in the system decreases with age, as would be expected from a collisional model. This has actually been observed in real time in the white dwarf debris disc WD 0145+234 (Swan et al, 2024), where timescales are extremely short owing to the small orbital radius of the disc. Second, observations of debris discs mainly detect dust grains with sizes similar to the observing wavelength, and the dust we see would have collisional and dynamical lifetimes much shorter than the age of the system (Hughes et al, 2018; Pearce, 2026). This means that this dust must be continually replenished, and collisions between larger debris bodies is the favoured means to do this. Third, IR spectroscopy of some debris discs reveals the presence of species such as SiO gas likely arising from high-velocity collisions (Lisse et al, 2009). Fourth, two transient companions to the debris disc host Fomalhaut have been detected, which are now thought to be dust clouds generated by planetesimal collisions (Kalas et al, 2008, 2026; Gaspar and Rieke, 2020). Dust release following such collisions has also been observed in the Asteroid Belt (Jewitt et al, 2010; Bodewits et al, 2011). Fifth, many Solar System asteroids can be grouped into families with similar proper orbital elements, which was early realised to indicate an origin from a common parent (Hirayama, 1918; Nesvorny, 2026). The size distributions of these families are indeed in good agreement with a collisional origin (e.g., Davis et al, 1985; Marschall et al, 2022).
The steady state collisional cascade model in Section 3.3.2 provides a good explanation for debris disc properties. However, in some cases, there is evidence for more extreme, stochastic collisions between larger objects. These are called “giant collisions” or “giant impacts”. Giant collisions in young planetary systems have been invoked in several scenarios to explain different observables. For instance, in our Solar System, a giant collision is the most accepted explanation for the formation of the Moon (Canup and Asphaug, 2001), and may explain the extreme morphology of Mercury (Benz et al, 1988), although Mercury’s extremely high iron fraction may instead be explained by iron-rich grains in the protoplanetary disc (Johansen and Dorn, 2022). Outside the Solar System, some debris discs classified as “extreme” have been suggested to undergo giant collisions in the past years (Meng et al, 2014; Su et al, 2019), based on large flux increases observed in their lightcurves. Previously, the connection between giant collisions and the presence of exocomets seemed unclear, because until recently there were no systems where both occurrences were taking place. However, the advent of JWST has revealed at least two likely recent collisions in the β Pic disc, the most prolific exocomet system. The morphological structure dubbed the “cat’s tail” (Rebollido et al, 2024) has been suggested to have originated from a possible collision point where dust is being produced sporadically. However, the distance from the star of a hypothetical collision point (≳100 au) makes it unlikely that there is a direct connection between such a collision and the presence of the exocomets seen in transit. Much closer to the star is the collision claimed by Chen et al (2024), inwards of 10 au, who observed that the population of small grains detected two decades earlier (Chen et al, 2007; Lu et al, 2022) has now disappeared, attributing this to the removal from the inner system by radiation pressure of a large quantity of debris from a large stochastic collision. Such a collision could be linked with destabilization of material in the inner regions, and therefore with a similar mechanism that is producing the exocomets. However, pinpointing the precise location of the collision, along with a dedicated dynamical model, would be necessary for an accurate determination of a link between such an event and small bodies thrown to the inner region of the system.
on the star’s distance from the Galactic centre. The strength of the eroding tide also varies with the star’s position in the potential as it orbits in the Galaxy. While the tidal force is greatest when a star crosses the midplane of the Galactic disc, it also varies if the star’s Galactocentric location changes with time. This occurs for any star on a non-circular orbit, but even stars on near-circular orbits can radially migrate several kpc in their lifetimes through interactions with the Galactic bar and spiral arms, in a process called “churning” (Sellwood and Binney, 2002;

Minchev and Famaey, 2010). Whether this has happened through past orbital evolution for the Sun, and what it means for our own Oort cloud, is uncertain (e.g. Kaib et al, 2011; Minchev et al, 2013; Mart´ınez-Barbosa et al, 2015). While scattering by giant planets naturally gives rise to a large population of bodies on extremely eccentric orbits, Oort cloud formation is not necessarily universal for all systems; for instance, Oort clouds may have lower survival probabilities in dense cluster environments (Nordlander et al, 2017) where bodies are unbound at comparatively small distances owing to frequent stellar flybys and the strong cluster tide. In general, Oort cloud prevalence around other stars in the Galaxy is unknown. Oort Clouds form when planets weakly scatter small bodies so that they do not attain escape velocity from the star and instead remain bound, undergoing a random walk in orbital energy (reciprocal semimajor axis), as described in Duncan et al (1987). As their orbits lengthen, interaction with the Galactic environment (passing stars and the Galactic tide) lifts their periapsides and temporarily traps them in the Oort Cloud at large semimajor axis. For many bodies this trapping process does not occur, and they are ejected from their host system to become interstellar objects, discussed in Section 8.