Observational Tests of the Dynamical Models for Outer Solar System Formation

The objects beyond Neptune are thought to be the most pristine material remaining from the formation process of our solar system. Therefore, one of the most important tasks in planetary science is to understand the architecture of the outer solar system and explain the remarkable diversity in the physical properties and compositions of trans-Neptunian objects (TNOs), Oort cloud comets, and irregular satellites. The most widely accepted models, which successfully reproduce many observed features of the outer solar system, belong to the family of planetary instability models (see Nesvorny, 2018, for a review) derived from the original Nice model by Tsiganis et al. (2005). These models have withstood the test of time and have been successfully adapted to account for the growing body of observational evidence.

Some features of the outer solar system populations, however, pose challenges to the planet-migration models. For example, the existence of Sedna-like TNOs on highly eccentric orbits and high-inclination TNOs are difficult to explain using planet instability model simulations on their own. These and other anomalies have opened avenues for exploring additional mechanisms to populate the trans-Neptunian region, notably the hypothesis for the existence of an undiscovered massive planet in the outer solar system (e.g. Batygin & Brown, 2016). Another scenario, which has recently shown promising results, is the stellar flyby hypothesis (see Pfalzner et al., 2024). In that framework, the outer solar system's architecture could be replicated by the flyby of a star several billion years ago. This event could have occurred either as an alternative or in addition to planet migration.

The dynamical models proposed to explain the solar system's architecture serve as a backbone of planetary science research. They shape our understanding of early solar system evolution and are incorporated into the assumptions of almost every major research project focused on minor planets. It is therefore essential that these models are rigorously tested and continuously refined based on state-of-the-art observations. 

We are now at a pivotal moment for evaluating theoretical hypotheses against new observations. The last few years have brought an abundance of new observational evidence, some of which is challenging the existing models. Following the first two cycles of JWST, we now have an unprecedented window into the direct compositional evidence of TNOs and irregular satellites. Additionally, recent large TNO survey programs (e.g., DES, OSSOS) have significantly advanced our understanding of the orbital distribution and the range of surface properties and physical characteristics of TNOs. Last but not least, the in-situ experiments of space missions (Rosetta and New Horizons) have provided unprecedented details about the properties of comets and TNOs.

In order to consolidate the community’s understanding of how recent observational evidence aligns with the different dynamical models, we are organizing a 3-day Forum on 3–5 September 2025, hosted by the International Space Science Institute (ISSI) in Bern, Switzerland. The forum will bring together around 25 key members of the community with transdisciplinary expertise, encompassing observations of the orbital, physical, chemical, and surface properties of TNOs, irregular satellites, and comets, as well as planetary instability and stellar flyby models. The primary focus will be to work toward consensus on the key observational tests of these dynamical models that should be prioritized in the coming years.  

We will aim to solidify agreement on the main priorities for making optimal use of recent and upcoming major observing facilities (including JWST and the ELTs), particularly in preparation for the Rubin Observatory’s LSST survey, launching in 2025. For example, LSST is expected to increase the number of observed TNOs from ~4,000 to more than 35,000 over the next five years. Given the volume of data expected from LSST and the limited resources for follow-up observations, it will be essential to identify the most pressing questions that need to be addressed in order to test the existing dynamical models and improve our understanding of the processes that shaped the early solar system. The forum’s main outcome will be a peer-reviewed publication summarizing the current level of agreement between models and observations, and outlining the diagnostic observational tests that should be prioritized in the near future. At EPSC/DPS, we will share the key outcomes of the forum, highlight the main insights, and open the discussion to the wider community. The EPSC/DPS presentation will offer an excellent opportunity to engage the wider community and to gather further input on how we can best test and refine current models for the formation and evolution of the outer solar system.

References 

• Batygin, K., & Brown, M. E. (2016), The Astronomical Journal, 151, 22
• Nesvorný, D. (2018), Annual Review of Astronomy and Astrophysics, 56, 137
• Tsiganis, K., Gomes, R., Morbidelli, A., & Levison, H. F. (2005), Nature, 435, 459
• Pfalzner, S., Govind, A., & Portegies Zwart, S. (2024), Nature Astronomy, 8, 1380