The transition phase is defined by the re-entry of an object from the reservoirs into the inner regions of the Solar System where outgassing begins. State-of-the-art dynamical simulations of the outer Solar System indicate that the orbits of some subpopulations of TNOs can be perturbed by Neptune and diffuse inwards to smaller perihelia (see Nesvorn´y et al, 2017). Bodies undergoing this process, whose perihelion has not yet come inside Jupiter’s orbit, are known as Centaurs. When encountering a planet along their way, these objects get scattered inwards and outwards in a random walk, getting passed to the next planet interior or exterior to it (Duncan et al, 2004). Centaurs begin to show comet-like activity as far as 10 au from the Sun (Jewitt, 2009), thought to follow recent (within a few 100 years) jumps in orbital semimajor axis (Lilly et al, 2024). At these heliocentric distances their activity cannot be driven by the sublimation of water ice as in comets, but instead can be explained either by the sublimation of hypervolatiles or by the crystallisation of amorphous ice (Jewitt, 2009; Guilbert-Lepoutre, 2012). Centaurs remain among the least understood objects in the Solar System: they are challenging to observe from the ground due to their faintness (though a handful of detections of sublimating gas have been made, Harrington Pinto et al 2022, 2023) and have not yet been visited by spacecraft. As with TNOs, recent JWST observations have improved our knowledge of their surfaces, which seem to be consistent with those of thermally processed bodies from the parent TNO populations (Licandro et al, 2025).
The other population in the transition phase is that of the Dynamically New Comets (DNCs). DNCs are believed to be Oort cloud objects whose perihelia are re-entering the planetary region for the first time. They are typically identified as being bodies with a semimajor axis of ≳104 au (Levison, 1996), although owing to the random walk in (reciprocal) semimajor axis caused by planetary encounters it is not guaranteed that an individual object at a > 104 au has never before had a small perihelion. These objects may be the most pristine material surviving in the Solar System6 because they have been emplaced in the Oort Cloud at large distances from the Sun, and have remained in cold storage until the current epoch. The dominant process affecting cometary nuclei below their surface layer is thermal heating from solar radiation (Weissman et al, 2020), and while this is negligible once a body is “in storage” in the Oort Cloud, thermal processing can take place during the phase of scattering off the giant planets while the cometary nucleus is undergoing implantation into the Oort Cloud. This processing can remove volatiles such as CO and CO2, and trigger the crystallisation of amorphous water ice (Gkotsinas et al, 2024): despite their supposed “primitive” nature, Gkotsinas et al (2024) predict that over 1/3 of Oort Cloud objects lost all their CO early in their evolution, based on dynamical models of the early Solar System, though the average effect is still less than for KBOs (Nesvorn´y et al, 2017; Vokrouhlick´y et al, 2019; Raymond et al, 2020). Counterintuitively, DNCs are predicted in these models to be more volatile-depleted than dynamically old comets, owing to their different early dynamical evolution. Harrington Pinto et al (2022) indeed found such a difference, though it may also be attributed to cosmic ray processing of
6Note that Weissman et al (2020) favour “primitive” over “pristine”, as the former implicitly allows for some evolution since formation.