Date of Award

Fall 1-1-2025

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Astronomy

First Advisor

van den Bosch, Frank

Abstract

Planet formation is an inherently multi-scale process, spanning nine order of magnitude in spatial scale from micron-sized dust grains to planetary system. This thesis adopts a multi-scale perspective to investigate the physical mechanisms that drive this transformation, focusing on the coupled dynamics of dust and gas in protoplanetary disks, the formation of planetesimals, and the imprints of disk physics on orbital architectures in planet-hosting binary systems. At the smallest scales, I present a new hydrodynamical instability that operates in dust-enhanced regions of protoplanetary disks if mass diffusion (or dust viscosity) declines sufficiently steeply with increasing dust density – conditions that are likely met when turbulence is locally generated by the streaming instability. I argue that this newly discovered instability may be responsible for the filamentary features near-ubiquitously seen in simulations of the nonlinear saturation of streaming instability. The model also admits growing oscillatory modes that operate analogously to the viscous over stability in planetary rings. Although it remains unclear whether realistic protoplanetary disk conditions satisfy the criteria for these modes, the model broadens our theoretical understanding of dust-gas dynamics at the planetesimal-forming scale. Next, I derive an analytic prediction for the initial mass function (IMF) of planetesimals forming via gravitational collapse of a turbulent dust layer. The theory is grounded in the classical Toomre instability framework accounting for dust-gas coupling, turbulent diffusion, and self-gravity. The resulting IMF is governed by the dust stability parameter Qp, which depends on the particle Stokes number, local dust-to-gas ratio, gas Toomre Q, and the diffusivity of the dust layer. The predicted IMF matches our numerical simulations of the streaming instability and gravitational collapse, and is consistent with both the “planetesimals are born big” paradigm and power-law distributions commonly found in simulations. At intermediate scales, I explore how thermodynamic feedback processes at ice lines can alter gas dynamics and potentially prolong the lifetime of dust-trapping vortices. Specifically, I examine whether the latent heat released during water ice sublimation can act as a localized energy source analogous to the mechanism driving Earth’s tropical cyclones. Through analytical estimates and exploratory simulations, I show that under-saturated flows near the water ice line can indeed stabilize anticyclonic vortices against viscous dissipation, thereby enhancing their effectiveness as sites for dust concentration and planetesimal formation. Finally, I consider system-scale orbital architectures focusing on the observed alignment trend between planetary and binary orbital planes in moderately wide (ab ? 102–103 AU) s-type stellar binaries. I utilize a secular model for the coupled evolution of the stellar spin and disk angular momentum vectors under the influence of an inclined companion. In this framework, dissipation due to internal disk warping during nodal precession can drive systems toward orbit-orbit alignment. I show that this process can reproduce both the spin-orbit and orbit-orbit alignment distributions observed in Gaia and TESS data, provided that the binary has sufficiently high mass ratio or the disk is relatively extended and viscous. Notably, the model predicts that alignment is less efficient in unequal-mass binaries, for which I identify tentative observational support. Together, these studies emphasize the importance of linking physical processes across scales in order to build a coherent, predictive theory of planet formation.

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