UC Santa Cruz

With the rapid increase in our ability to observe exoplanets and exoplanetary systems over the past two decades, the amount of data available to planet formation theorists has grown considerably. This in turn has spurred development in our understanding of the physics of planet formation. In this dissertation, I discuss the work I have done to understand the physical processes that shape the architectures of the planetary systems we observe. This work can broadly be broken down into studies that cover three distinct epochs of planet formation, which I roughly term the "early," "middle," and "late" stages.

Chapters 2-5 discuss a new paradigm for the early stages of planetary growth generally referred to as "pebble accretion." In Chapter 2, I discuss in detail an analytic model I developed to calculate how planets grow through pebble accretion, with a focus on how this process varies as a function of planetary mass, particle size, and the level of turbulence in the protoplanetary disk. I demonstrate that over a wide range of parameter space turbulence can greatly reduce the efficiency of pebble accretion. In Chapter 3 I apply these considerations to the growth of gas giant planets at wide orbital separations. I derive an inverse relationship between the level of turbulence in a protoplanetary disk and the semi-major axis at which the core of giant planet can form. In Chapter 4 I discuss how our modeling of pebble accretion naturally predicts an upper mass limit that planets can reach, which I term the "flow isolation mass." I discuss the characteristics of this mass scale, and discuss predictions for the architectures of planetary systems that reach the flow isolation mass in the context of new observations. Finally, in Chapter 5 I contrast growth limited by flow isolation to another limiting mass scale known as the "pebble isolation mass." I demonstrate that because of the top-down manner in which pebble isolation inhibits particle accretion, analytic estimates of the pebble isolation mass can be off by factors as large as ~5, and analytic estimation of the planet's final mass is considerably more difficult. I also show that if pebble accretion is simultaneously inhibited by pebble and flow isolation, growth generally stops at observed super-Earth mass scales over a wide range of disk parameters, and final planet masses can once again be estimated analytically in a straightforward manner.

Chapter 6 discusses the intermediate stage of planet formation, where the cores of giant planets have reached sufficient size to undergo runaway gas accretion. I discuss the two-way feedback process between the growing planet and the protoplanetary disk it feeds from. I derive analytic expressions for the planet's perturbation to the surface density, which I broadly classify into the "consumption" and "repulsion" regimes. These analytic expressions are vetted against 1D numerical simulations, both for viscous disks that accrete due to local kinematic viscosity, and inviscid disks which accrete via magnetized winds.

Chapter 7 is concerned with the late stages of planet formation, where giant planets interact under their mutual gravity. I develop a method to fit radial velocity signals of planets where the mutual gravitational interaction is strong and the system must be tested for long-term stability, and apply this method to the planetary system around the star HD 200964. I demonstrate that the increased time baseline from additional observations moves the best-fit period ratio from 4:3 closer to 7:5, with the 3:2 also providing plausible fits that exhibit long-term stability. I discuss these different period ratios in the context of different formation pathways.