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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.
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Protoplanetary disks are structures of gas and dust that orbit newly formed stars, providing the raw material from which planets assemble.
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Spanning up to hundreds of astronomical units, these disks evolve over millions of years through accretion, dispersal, and the growth of solid bodies.
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Understanding their structure and evolution is central to explaining the origins of the Solar System and the thousands of exoplanetary systems discovered around other stars.
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- Form around protostars and persist for ~1–10 million years
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- Contain the gas, dust, and ice from which all planetary bodies ultimately originate
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- Observed across a wide range of stellar masses with instruments such as ALMA and the VLA
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- Link stellar formation to planetary system architectures
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**Learn more:**
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-[PFITS+ Research Overview](/research/)
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-[Planet Formation](/research/planet-formation/)
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-[Fluid Dynamics](/research/fluid-dynamics/)
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-[Disk observations with ALMA](https://almascience.nrao.edu/){:target="_blank"}
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.
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Over multi-million-year lifetimes, protoplanetary disks lose mass through accretion onto the central star, photoevaporation driven by stellar and external radiation, and the gradual incorporation of material into planetary bodies.
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The interplay of these processes sets the stage for each subsequent phase of planet formation and ultimately determines the mass and composition budget available to nascent planets.
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- Disk masses and lifetimes are constrained by infrared and millimeter surveys of star-forming regions
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- Turbulent spreading and wind-driven mass loss redistribute angular momentum over time
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- Stellar irradiation creates temperature gradients that drive thermally driven disk winds
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- Disk dissipation timescales vary with stellar mass, multiplicity, and environment
-[Magnetically Driven Turbulence in the Inner Regions of Protoplanetary Disks](https://ui.adsabs.harvard.edu/abs/2024ApJ...972..128R/abstract){:target="_blank"} — [Rea](/team/rea-david/) et al. (2024)
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-[High-resolution Simulation of Protoplanetary Disk Turbulence Driven by the Vertical Shear Instability](https://ui.adsabs.harvard.edu/abs/2024ApJ...977..272S/abstract){:target="_blank"} — Shariff & [Umurhan](/team/umurhan-orkan/) (2024)
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.
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Gas and solids spiral inward through the disk via angular momentum transport, feeding material onto the central star and setting disk lifetimes.
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Magnetically driven turbulence, laminar magnetic torques, and disk winds are key candidates for sustaining the accretion rates inferred from observations of young stellar objects.
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Our group investigates these mechanisms using high-fidelity [magnetohydrodynamic simulations](/research/fluid-dynamics/#magnetohydrodynamics).
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- Accretion rates in T Tauri disks are typically ~10⁻⁸ solar masses per year
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- The magnetorotational instability (MRI) is a leading driver of turbulent angular momentum transport
-[Magnetically Driven Turbulence in the Inner Regions of Protoplanetary Disks](https://ui.adsabs.harvard.edu/abs/2024ApJ...972..128R/abstract){:target="_blank"} — [Rea](/team/rea-david/) et al. (2024)
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-[Jacob B. Simon](/team/simon-jacob/) — expert in magnetically driven accretion processes
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.
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The temperature and density structure of a protoplanetary disk is set by the balance between stellar irradiation, turbulent heating, and radiative cooling.
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Radial and vertical gradients in these quantities influence gas and dust dynamics, the location of condensation fronts ("snowlines"), and the character of disk instabilities.
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Accurately modeling this structure is essential for making realistic comparisons with infrared and millimeter-wave observations.
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- Midplane temperatures range from ~1500 K near the star to ~10 K in the outer disk
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- Snowlines (e.g., the water ice line) mark transitions in dust composition and influence solid mass budgets
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- Disk flaring allows stellar irradiation to heat the disk surface at large radii
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- Turbulent diffusion couples the thermal and chemical evolution of disk material
-[Length and Velocity Scales in Protoplanetary Disk Turbulence](https://ui.adsabs.harvard.edu/abs/2024ApJ...966...90S/abstract){:target="_blank"} — [Sengupta](/team/sengupta-debanjan/) et al. (2024)
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-[Turbulence in Particle-laden Midplane Layers of Planet-forming Disks](https://ui.adsabs.harvard.edu/abs/2023ApJ...942...74S/abstract){:target="_blank"} — [Sengupta](/team/sengupta-debanjan/) & [Umurhan](/team/umurhan-orkan/) (2023)
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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Large-scale anticyclonic vortices can form in protoplanetary disks through the [Rossby wave instability](/research/fluid-dynamics/#rossby-wave) and other mechanisms, persisting for hundreds to thousands of orbital periods.
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These structures efficiently concentrate dust particles, potentially triggering rapid [planetesimal](/research/planet-formation/#planetesimal-formation) and protoplanet formation, and may leave observable imprints as the asymmetric dust rings detected by ALMA.
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- Vortices form at pressure bumps such as the edges of dead zones or gaps opened by planets
-[Rapid Protoplanet Formation in Vortices: Three-dimensional Local Simulations with Self-gravity](https://ui.adsabs.harvard.edu/abs/2024ApJ...970L..19L/abstract){:target="_blank"} — [Lyra](/team/lyra-wladimir/) et al. (2024)
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-[On the Origin of Dust Structures in Protoplanetary Disks: Constraints from the Rossby Wave Instability](https://ui.adsabs.harvard.edu/abs/2023ApJ...946L...1C/abstract){:target="_blank"} — [Chang](/team/chang-eonho) et al. (2023)
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-[Planets and planetesimals at cosmic dawn: vortices as planetary nurseries](https://ui.adsabs.harvard.edu/abs/2025MNRAS.542..641E/abstract){:target="_blank"} — Eriksson et al. (2025)
Copy file name to clipboardExpand all lines: _research/02-planet-formation.md
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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.
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Planet formation spans more than forty orders of magnitude in mass—from sub-micron interstellar dust grains to gas giants rivaling Jupiter.
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[Our group](/team/) investigates the physical processes that bridge these scales, focusing on the aerodynamic and gravitational mechanisms that convert diffuse [protoplanetary disk](/research/protoplanetary-disks/) material into the diverse planetary systems observed around other stars.
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A central theme of our work is the streaming instability: a resonant aerodynamic mechanism that concentrates drifting pebbles into dense clumps capable of gravitational collapse into [planetesimals](#planetesimal-formation).
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- Planet formation involves physics from aerodynamics to self-gravity and orbital mechanics
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- Key barriers—due to radial drift, bouncing, and fragmentation—motivate instability-based formation pathways
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- Our group leads multi-code comparison projects to rigorously test theoretical predictions
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- Complementary observations from ALMA and space missions continue to reshape our understanding
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.
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Planet formation begins when microscopic interstellar dust grains collide and stick together, building progressively larger aggregates.
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As particles grow from micrometers to millimeters and centimeters ("pebbles"), aerodynamic effects increasingly govern their motion, and collisions can lead to fragmentation or bouncing rather than growth—creating a challenging "growth barrier" that must be overcome on the path to planetesimals.
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- Sub-micron grains grow by van der Waals adhesion; centimeter aggregates require different sticking mechanisms
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- Radial drift causes pebbles to spiral inward, providing a limited time window for growth
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- Grain size and composition (silicate vs. ice) strongly affect sticking and fragmentation thresholds
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- Coagulation and fragmentation can work together with streaming instability to enhance planetesimal formation
-[Positive feedback: How a synergy between the streaming instability and dust coagulation forms planetesimals](https://ui.adsabs.harvard.edu/abs/2025A&A...696L..23C/abstract){:target="_blank"} — [Carrera](/team/carrera-daniel/) et al. (2025)
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-[Positive feedback: II. How dust coagulation inside vortices can form planetesimals at low metallicity](https://ui.adsabs.harvard.edu/abs/2025A&A...701L...1C/abstract){:target="_blank"} — [Carrera](/team/carrera-daniel/) et al. (2025)
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-[On the Mass Budget Problem of Protoplanetary Disks: Streaming Instability and Optically Thick Emission](https://ui.adsabs.harvard.edu/abs/2026ApJ...997..192G/abstract){:target="_blank"} — [Godines](/team/godines-daniel/) et al. (2026)
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.
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Planetesimals—solid bodies, roughly tens of km in size—represent the first generation of gravitationally bound objects in the planet-forming disk and the precursors of asteroids, comets, Kuiper Belt objects, and planetary cores.
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The [streaming instability](/research/fluid-dynamics/#streaming) is a leading mechanism for their formation: the mutual aerodynamic drag between inward-drifting pebbles and sub-Keplerian gas creates a positive feedback that concentrates solids into dense filaments and clumps that then collapse under self-gravity.
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- Streaming instability was first identified by [Youdin](/team/youdin-andrew/) & Goodman ([2005](https://ui.adsabs.harvard.edu/abs/2005ApJ...620..459Y/abstract){:target="_blank"})
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- Clumping requires a threshold dust-to-gas ratio that depends on particle size and disk pressure gradient
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- Gravitational collapse of clumps produces planetesimals with a characteristic size distribution
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- Simulations must resolve both the gas dynamics and the self-gravity of the collapsing dust layer
-[Streaming Instability and Turbulence: Conditions for Planetesimal Formation](https://ui.adsabs.harvard.edu/abs/2024ApJ...969..130L/abstract){:target="_blank"} — [Lim](/team/lim-jeonghoon/) et al. (2024)
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-[Probing Conditions for Strong Clumping by the Streaming Instability](https://ui.adsabs.harvard.edu/abs/2025ApJ...981..160L/abstract){:target="_blank"} — [Lim](/team/lim-jeonghoon/) et al. (2025)
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-[The Streaming Instability in 3D: Conditions for Strong Clumping](https://ui.adsabs.harvard.edu/abs/2026ApJ..1000..156L/abstract){:target="_blank"} — [Lim](/team/lim-jeonghoon/) et al. (2026)
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-[A Solution for the Density Dichotomy Problem of Kuiper Belt Objects](https://ui.adsabs.harvard.edu/abs/2024PSJ.....5...55C/abstract){:target="_blank"} — [Carrera](/team/carrera-daniel/) et al. (2024)
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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Once a planetesimal or protoplanetary embryo has formed, it can grow rapidly by gravitationally capturing inward-drifting centimeter-sized pebbles whose trajectories are deflected by gas drag.
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Pebble accretion rates can far exceed those of classical planetesimal–planetesimal collisions, offering a compelling pathway to assembling the cores of giant planets and super-Earths within disk lifetimes.
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- Pebble accretion efficiency depends on particle size, orbital location, and embryo mass
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- Polydisperse pebble size distributions alter growth rates compared to single-size models
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- The "pebble isolation mass" sets an upper limit beyond which an embryo cuts off its own pebble supply
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- Pebble accretion has implications for giant planet formation, Kuiper Belt objects, and exoplanet demographics
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**Learn more:**
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-[Dust Coagulation](#dust-coagulation)
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-[An Analytical Theory for the Growth from Planetesimals to Planets by Polydisperse Pebble Accretion](https://ui.adsabs.harvard.edu/abs/2023ApJ...946...60L/abstract){:target="_blank"} — [Lyra](/team/lyra-wladimir/) et al. (2023)
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-[Rapid Protoplanet Formation in Vortices: Three-dimensional Local Simulations with Self-gravity](https://ui.adsabs.harvard.edu/abs/2024ApJ...970L..19L/abstract){:target="_blank"} — [Lyra](/team/lyra-wladimir/) et al. (2024)
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-[Andrew N. Youdin](/team/youdin-andrew/) — pioneer of the streaming instability and pebble accretion theory
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