Simulating Planetesimal Formation in the Kuiper Belt and Beyond

Abstract

A critical step in planet formation is to build super-kilometer-sized planetesimals out of dust particles in gaseous protoplanetary disks. The origin of planetesimals is crucial to understanding the Solar System, exoplanetary systems, and circumstellar disks. In this thesis, I present my work on exploring and better understanding promising planetesimal formation pathways with extensive numerical modeling and robust statistical analyses, with a main focus on the streaming instability (SI), a mechanism to aerodynamically concentrate solids in disks and trigger gravitational collapse to form planetesimals. The first study focuses on the numerical robustness of the SI, where I demonstrate that the nonlinear particle clumping by the SI is robust to various numerical setups. In the next study, I carry out the SI simulations including particle self-gravity with the highest resolution to date, which produces a broad and top-heavy initial mass distribution of planetesimals. Necessitated by analyzing my simulations, I have built and published an efficient clump-finding code, PLAN, capable of robustly identifying and characterizing self-bound clumps. I then present the highlights from analyses of the demographics of planetesimals. I first apply a maximum likelihood estimator to fit a suite of parameterized models with different levels of complexity to the simulated mass distribution. I show that our simulations produce different mass distributions with different aerodynamic properties of the disk and participating solids. I will report the first evidence for a turnover in the low mass end of the planetesimal mass distribution. With PLAN, I also find that the clumps in our simulations possess excess angular momenta that might explain why all planetesimals formed as binaries/multiples and the high binary fraction among Cold Classical Kuiper Belt Objects. Furthermore, the predicted binary orbits show a broad inclination distribution with 80% of prograde orbits, excellently matching the observations of trans-Neptunian binaries. Finally, I conclude with the key results in this thesis and discuss the future directions for planetesimal formation studies, with some pioneering results from my on-going work.