Understanding the trans-Neptunian Solar System

Abstract

The most pristine remnants of the Solar System planet formation epoch orbit the Sun beyond Neptune. These are the small bodies of the trans-Neptunian object populations. The bulk of the mass is distributed in ~100 km objects, but objects at smaller sizes have undergone minimum collisional processing. The New Horizons mission recently revealed that the body (486958) Arrokoth, with its effective diameter of ~20 km, appears to be a primordial body and not a collisional fragment. This indicates that bodies at these (and perhaps smaller) sizes retain a record of how they were formed, and they are the most numerous record of this epoch. However, it is impractical to find these bodies with optical surveys because their brightnesses are very low. Their presence can be inferred from the observed cratering record of Pluto and Charon, and it can be directly measured by serendipitous stellar occultations. These two methods produce contradicting results: occultations measure roughly ten times the number of about kilometer-size bodies as are inferred from the cratering record. We used numerical models to explore how these observations can be reconciled with evolutionary models of the outer Solar system. We find that models in which the initial size of the bodies decreases with increasing semimajor axis of formation and models in which the surface density of the bodies increases beyond the 2:1 mean-motion resonance with Neptune can produce both sets of observations, but a comparison to various observational tests favours the former mechanism. We discuss how the astrophysical plausibility of these solutions can be evaluated, and we conclude that extended serendipitous occultation surveys with a broad sky coverage are the most practical approach.