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Copy file name to clipboardExpand all lines: _research/01-protoplanetary-disks.md
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permalink: /research/protoplanetary-disks/
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## Introduction
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Protoplanetary disks are rotating disks of gas and dust that form around young stars as a natural byproduct of star formation. They are the birth environments of planets, and their physical and chemical structure sets the conditions for what kinds of planetary systems can emerge. PFITS+ researchers model disk dynamics, evolution, and observational signatures to understand the full life cycle of planet-forming environments.
Protoplanetary disks evolve over millions of years through a combination of gravitational, magnetic, thermal, and radiative processes. As gas accretes onto the central star and dust grains grow and settle toward the midplane, the disk gradually disperses, transitioning from a massive gas-rich disk to a debris disk. We study these evolutionary pathways to understand how disk properties change over time and how they influence the architecture of emerging planetary systems.
Gas and solid material in a protoplanetary disk continuously lose angular momentum and spiral inward toward the central star—a process called accretion. The rates and mechanisms of accretion, including turbulence driven by magnetohydrodynamic and hydrodynamic instabilities, play a central role in disk evolution and in setting the mass and composition budgets available for planet formation. Our research investigates how accretion varies with disk structure, ionization, and magnetic field geometry.
The temperature and pressure profile of a protoplanetary disk controls the locations of ice lines, the rates of chemical reactions, and the development of thermally driven instabilities. Radiative transfer through gas and dust shapes the thermal structure from the hot inner disk to the cold outer regions, while heating from stellar irradiation and accretion further modulate the vertical and radial temperature gradients. We model disk thermodynamics to understand how the thermal environment influences planet formation conditions and observable disk signatures.
Large-scale anticyclonic vortices can develop in protoplanetary disks when radial pressure gradients become unstable, most notably through the Rossby wave instability at sharp disk features such as gaps and dead-zone edges. These long-lived structures act as efficient dust traps, concentrating solid particles and potentially triggering planetesimal formation. PFITS+ researchers study vortex formation, persistence, and their role as sites of enhanced particle accumulation.
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permalink: /research/planet-formation/
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## Introduction
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Planet formation begins with microscopic dust grains in a protoplanetary disk and proceeds through a cascade of growth processes—from grain collisions to pebbles, planetesimals, planetary embryos, and finally planets. PFITS+ researchers investigate the physical mechanisms and timescales of each stage using large-scale numerical simulations, testing theoretical models against astronomical observations and Solar System constraints.
The first step in planet formation is the growth of submicron interstellar grains into millimeter-to-centimeter-sized pebbles through repeated low-velocity collisions. The subsequent fate of these pebbles—whether they continue to grow, drift inward due to gas drag, or accumulate in pressure traps—is a central open question in planet formation theory. We study how grain properties, turbulence levels, and local disk conditions control the efficiency of dust growth and the production of a reservoir of pebbles for later stages of planet formation.
Planetesimals are kilometer-scale rocky bodies that serve as the building blocks of planets. Their formation from pebbles likely involves the aerodynamic concentration of solid material, particularly through the streaming instability, which can trigger gravitational collapse in dust-rich regions of the disk. PFITS+ researchers run high-resolution simulations to characterize how and where planetesimals form, what their initial size distribution is, and how formation efficiency depends on disk conditions.
Once planetesimals form, they can grow efficiently by sweeping up the pebble-sized particles that drift through the disk due to gas drag. This pebble accretion mechanism can grow planetary embryos to Earth-mass and even super-Earth-mass bodies on timescales far shorter than classical collisional growth, and may explain the rapid formation of giant planet cores. We study how pebble accretion efficiency depends on disk structure, particle properties, and the orbital dynamics of growing embryos.
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## Introduction
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Astrophysical fluid dynamics governs the motion of gas and dust in protoplanetary disks under the combined influence of gravity, pressure gradients, magnetic fields, and radiation. Understanding these dynamics is essential for modeling disk evolution, identifying the sites and mechanisms of planet formation, and interpreting disk observations. PFITS+ researchers develop and apply fluid-dynamical models—from analytic linear theory to large-scale numerical simulations—to probe the complex multiphysics environments of planet-forming disks.
In protoplanetary disks, solid particles are coupled to the surrounding gas through aerodynamic drag, creating a complex two-component fluid whose behavior depends sensitively on particle size and local disk properties. The relative drift between dust and gas drives collective instabilities, redistributes solid material radially and vertically, and ultimately sets the stage for planetesimal formation. We study dust–gas dynamics across a wide range of disk conditions to understand how particle concentrations develop, evolve, and trigger the onset of planet formation.
Ionized regions of protoplanetary disks are subject to magnetohydrodynamic (MHD) effects that drive turbulence and transport angular momentum outward, enabling gas to accrete onto the central star. The magnetorotational instability (MRI) is the primary mechanism for generating this turbulence in weakly magnetized, differentially rotating disks, though the level of ionization—and therefore MHD activity—varies significantly with disk radius and height. PFITS+ researchers model MHD turbulence and its implications for disk structure, accretion rates, and conditions for planet formation.
The thermal and luminosity structure of a protoplanetary disk is determined by the interplay between gas dynamics and radiation transport. Heating by stellar irradiation and viscous dissipation, combined with radiative cooling through dust emission, shapes the temperature profile and controls the location of key chemical boundaries such as the water ice line. We investigate how radiation hydrodynamics governs disk stability, drives thermally excited instabilities, and influences the conditions available for planet formation.
Protoplanetary disks are prone to a variety of hydrodynamic and magnetohydrodynamic instabilities that drive turbulence, shape disk structure, and promote the concentration of dust into planet-forming environments. PFITS+ researchers investigate several key instabilities—including the magnetorotational, Rossby wave, streaming, and vertical shear instabilities—studying their onset conditions, nonlinear saturation, and observational consequences.
The magnetorotational instability (MRI) operates in weakly magnetized, differentially rotating disks and is the leading candidate for driving turbulent angular momentum transport and accretion in the ionized inner disk and surface layers. Its saturation produces sustained turbulence whose properties depend on the magnetic field geometry and strength, with implications for disk structure and the stirring of dust particles. We study MRI onset, nonlinear saturation, and quenching in the context of realistic disk ionization models.
The Rossby wave instability (RWI) develops at sharp radial extrema in the disk pressure profile—such as the edges of gaps carved by planets or dead zones—and generates large, long-lived anticyclonic vortices. These vortices efficiently trap dust particles, creating local solid overdensities that may seed planetesimal formation and that produce observable asymmetric features in millimeter continuum images. Our work examines the conditions that trigger the RWI, the structure and persistence of the resulting vortices, and their role in concentrating solids.
The streaming instability is a resonant drag instability arising from the aerodynamic interaction between inward-drifting pebbles and the slightly sub-Keplerian disk gas. Even modest enhancements in the local dust-to-gas ratio can trigger exponential clumping, driving dense filaments to gravitational collapse and forming planetesimals directly. PFITS+ is home to several leading experts on the streaming instability, with research spanning linear theory, nonlinear particle-gas simulations, and the connection to Solar System and extrasolar planet populations.
The vertical shear instability (VSI) operates in the cold, weakly ionized outer regions of protoplanetary disks, where rapid radiative cooling allows vertical temperature gradients to drive oscillatory instabilities in the vertical velocity field. The resulting turbulence produces vertical mixing and moderate levels of angular momentum transport without requiring magnetic fields. We investigate how the VSI interacts with other instabilities, how it stirs and diffuses dust particles, and what role it may play in setting the conditions for planetesimal formation in the outer disk.
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