Abstract

The minor planet populations of the outer solar system—including trans-Neptunian objects (TNOs), Centaurs, comets, and others—are believed to preserve key evidence from the epoch of planet formation. These populations also offer invaluable insights into the subsequent dynamical evolution of the outer solar system, which has shaped their current orbital distributions, physical characteristics, and surface properties.

The most widely accepted models that successfully reproduce most observed features of the outer solar system belong to the family of planet instability models derived from the original Nice model. These models have endured rigorous scrutiny over time and have been adapted to account for the expanding body of observational evidence. Some features of the outer solar system populations, however, remain difficult to explain within the planet-migration framework. Such challenges have motivated investigations into additional mechanisms for populating the trans-Neptunian region, such as the hypothesis of an undiscovered massive planet in the outer solar system and the stellar flyby hypothesis.

This ISSI Forum aims to consolidate the community’s understanding of how recent observational evidence aligns with the different dynamical models. It will bring together 25 scientists with transdisciplinary expertise, spanning planet instability models, stellar flyby hypotheses, and observations of the orbital, physical, chemical, and surface properties of TNOs, Centaurs, Trojans, irregular satellites, and comets.

The primary goal of the Forum is to evaluate the current agreement between dynamical models and observational evidence while identifying key observational tests that should be prioritized in the near future. The forum will also emphasize strategies for maximizing the potential of recent and forthcoming major observational facilities, such as JWST and the ELTs, with particular focus on preparations for Rubin Observatory’s LSST survey, scheduled to launch in 2025.