ones et al (2015). Over the past few decades we have also made great strides in characterising both the populations and the dynamical and collisional conditions within the Solar System’s source reservoirs, with the number of known Trans-Neptunian Objects (TNOs) growing from one (the binary Pluto–Charon system) in 1990 to many thousands at present. The detected TNOs, however, are much larger than typical cometary nuclei owing to detection biases: we still lack a census
1This could include tidal disruption, as is likely the case for some bodies orbiting white dwarfs.

of TNOs at sizes below ∼100 km, while the Oort Cloud remains largely hypothetical and undetected, save for those few members that have intruded into the inner Solar System2, owing to the immense distances (≳1000 au) at which bodies are thought to reside there. The situation is much less understood when it comes to small body and cometary phenomena outside the Solar System. We now have spectroscopic and photometric evidence for exocomets transiting a handful of other Main Sequence stars, notably β Pic (Ferlet et al 1987; Lagrange-Henri et al 1988, and reviewed extensively by Lu et al., this issue). A large fraction of Main Sequence stars exhibit exo-zodiacal light or “exozodi”, a near- or mid-IR excess located at ≲1 au whose origin is uncertain but may in part arise from dust released by comets near perihelion (Beichman et al, 2005; Absil et al, 2013; Ertel et al, 2020). Potential source populations are reasonably well-characterised, at least when it comes to more massive analogues of the Kuiper Belt which are easily visible as mid- or far-IR excess, or resolved thermal emission or scattered light, in the form of debris discs (Aumann et al, 1984; Smith and Terrile, 1984). On the other hand, the dynamics of delivery from debris disc body to comet is much less clear than it is in the Solar System. This is because the bodies responsible for delivery are nearly all undetectable with current technology, since most exoplanet detection methods favour closer orbits ≲few au, and all favour only more massive planets. While we have a good understanding of stellar multiplicity (Raghavan et al, 2010), and some census of super-Jupiter planets on wide orbits from direct imaging surveys (Bowler, 2016; Nielsen et al, 2019; Vigan et al, 2021), the demographics of smaller planets at tens of au are mostly unconstrained. We have learned from the explosion of exoplanet discoveries in the past three decades that planetary systems are common, diverse, and often do not at all resemble the Solar System (Winn and Fabrycky, 2015; Zhu and Dong, 2021). For exocomets this situation is both vexing, since we know little for certain about what might be responsible for delivery, and inspiring, since a great range of planetary architectures and dynamical mechanisms can be considered. Only in the case of β Pic do we have a good enough characterisation of the planets, the cometary activity, and the source reservoir; this unique system is discussed in detail in Lu et al., this issue. In this work, we discuss the long-term changes experienced by cometary reservoirs, and the small bodies that become comets themselves, inside and outside the Solar System. We discuss processes that take place after the epoch of formation in the protoplanetary disc, which is discussed in Bannister et al (2025). Our discussion of the physical evolution of cometary nuclei focuses on long-term processes; for a detailed discussion of the physics of mass loss from the nucleus and the formation of the comae and tails on (sub-)orbital timescales, see Vrignaud et al (2026, this issue). We begin the work by discussing the different dynamical mechanisms that have been proposed to move bodies out of their debris disc reservoirs to small periapsides (Section 2). We then discuss the long-term evolution of the source reservoirs, both by the aforementioned dynamical processes as well as by collisions between small bodies, which are unimportant in the present TNO and Oort Cloud populations in the Solar System
2Specifically, we mean interior to the orbit of Jupiter, although some comets do show activity at larger heliocentric distances.