Planet formation through the lens of dynamics

Huang, S. (2026)

Doctoral Thesis

Over the past two decades, transit surveys have revealed a large population of sub-Neptunes – planets with thick atmospheres that must have completed their growth within protoplanetary disks. During this stage, gravitational interactions with the disk drive planetary migration, often locking neighboring planets into mean-motion resonances. Yet most planets today, including those in the Solar System, are not in resonance, implying that many primordial resonances were later disrupted. The preserved and broken resonances are imprinted by the early formation process and post-disk evolution history. 
  
In this dissertation, orbital dynamics is used as a diagnostic tool to constrain the origin and evolutionary pathways of planets. Mean-motion resonances, in particular, act as fossil records of disk properties, planetary growth histories, and later dynamical instabilities. First, the analytical theory of a new resonance trapping regime is developed based on classical theories. By applying to the observed near-resonant exoplanets, their natal disk mass is constrained to be comparable to the Minimum Mass Solar Nebula(Chapter 2). Second, the spacing of resonant chains is found to encode planetary growth timescales. Rapid sequential formation produces tightly packed configurations, whereas slower growth yields wider separations (Chapter 3). When applied to the TRAPPIST-1 system, this diagnostic framework supports formation near the water iceline. Third, broken resonances are equally informative. Chapter 4 demonstrated that the perturbations from outer giant planets could destabilize the primordial resonant chains among terrestrial planets, naturally reproducing the present-day architecture of the Solar System. Finally, the role of environmental dynamics is explored across larger scales. Stellar cluster environments can truncate disk lifetimes through external irradiation, thereby suppressing giant planet growth and migration while promoting the formation of Neptune-mass planets at wide separations (Chapter 5). At the scale of individual disks, observed ALMA dust morphologies provide dynamical tracers of embedded, closely spaced protoplanets (Chapter 6).
  
This thesis highlights the fundamental role of dynamics in planet formation and evolution. Resonant architectures encode information about formation and migration timescales, while non-resonant configurations often reflect subsequent dynamical instabilities. From disk–planet interactions to stellar cluster environments, dynamics governs both the emergence and the observable structure of planetary systems. Together, these results contribute to a coherent framework for understanding the general processes of planet formation and ultimately the origin of the Solar System. 

• Celestial mechanics
• Protoplanetary Disks
• Planet Formation
• Solar system

• All authors: Huang, S.
• Supervisor: Portegies Zwart, S.
• Co-supervisor: Ormel, C.
• Committee: Snellen, I.A.G.; Kenworthy, M.A.; Kokubo, E.; Miguel, Y.; Dominik, C.; Petit, A
• Qualification: Doctor (dr.)
• Awarding Institution: Leiden Observatory, Faculty of Science, Leiden University
• Date: 2026-06-19
• ISBN (print): 9789465344492
• DOI: doi:10.60602/1887-4306860