Armaan Goyal, 2025
Title: Statistical Investigations of Small-Planet Architectures
Abstract: In the current golden age of planetary discovery, the astronomical community has identified several hundreds of stars each hosting multiple “small” planets between the size of Earth and Neptune, where planets orbiting the same star themselves display “peas-in-a-pod” uniformity in their size, mass, and orbital spacing. These small-planet architectures thus comprise the most common type of system in the Milky Way, indicating that their population-scale characterization is imperative for constraining the most general processes that govern planet formation. Accordingly, I present in this dissertation the results from four novel, population-level statistical analyses that provide critical insight towards the origins and evolution of these preeminent small-planet systems. I first illustrate that peas-in-a-pod mass uniformity, which had previously been demonstrated only for near-resonant systems with significant transit timing variations (TTVs), is also exhibited by systems with non-TTV mass measurements, suggesting that such architectures likely emerge from a formation process that is not inherently linked to resonance. I subsequently show that while peas-in-a-pod architectures are thus available to both near-resonant and non-resonant systems, the actual strength of intra-system uniformity differs between the two populations, as the former systems exhibit enhanced size uniformity that may be indicative of a more quiescent dynamical history. Considering the existence of the radius valley, a population-wide dearth of planets near 1.6R⊕ which separates rocky worlds from those with more substantial volatile components, I then present evidence towards inherent links between system architectural uniformity and individual planetary composition, where rocky systems are less uniform in planetary mass but more uniform in size and spacing compared to their volatile-rich counterparts. Finally, I demonstrate that the conventional 1-day classification boundary for ultra-short period planets (USPs), which has long been considered a purely arbitrary cutoff, corresponds to an astrophysical transition below which USPs are systematically smaller than non-USP worlds, while an additional 2-day cutoff naturally emerges from a similar transition below which planets are architecturally detached from their companion planets. These statistical trends provide direct insight towards the origins of prototypical planetary systems throughout the Milky Way, as well as our contextualization of the solar system in this broader galactic landscape.