TNO2024

Recent observational surveys of the outer solar system have nearly completed the inventory of dynamically hot and cold Kuiper belt objects with an absolute magnitude ($H_r$) below 5.3. The completeness level allows for challenging the former assumption that the size-frequency distribution (SFD) of both populations could be described by a broken power law with a distinct slope for large bodies. Nonetheless, new evidence shows remarkable similarity in the SFD between $H_r\simeq 5.3$ and 8.3 (~400 and ~100 km diameter) for such populations, suggesting a potential common formation mechanism in that range. We use a collisional and dynamical evolution model (ALICANDEP-22) to explore such conjecture, recently put forward also by planetesimal formation models (Lyra et al., 2023). Here, we propose that the primordial disk, between 22 – 30 au, and the cold population formed in situ around ~47 au, followed a common planetesimal formation process up to ~400 km in size and evolved under the giant planet instability framework. Our findings indicate that the observed SFD of the cold and hot KBOs can be accurately modeled by ALICANDEP-22, assuming a simple functional form. We find that an exponential taper function informed from the current cold population, suitably scaled, and combined with a power law for larger bodies in the inner trans-planetary disc – to account for the dwarf-planet-sized bodies observed today – can evolve to match the debiased current populations. Moreover, if primordial bodies are characterized by a very low strength, the evolved SFD shows a change in the slope at ~2 km, in agreement with estimations from the cratering record on Charon. The proposed primordial SFD carries significant cosmogonic implications, reconciling observational constraints with planetesimal formation mechanisms in both the primordial trans-planetary disc before the onset of dynamical instability and the cold Kuiper belt population formed in-situ.