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Merge pull request #51 from pfitsplus/copilot/draft-research-pages-content
Draft public-facing content for all "In development" research pages
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‎_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 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"}
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## Evolution
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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
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**Learn more:**
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- [Accretion](#accretion)
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- [Thermodynamic Structure](#thermodynamic-structure)
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- [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)
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## Accretion
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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
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- Non-ideal MHD effects (Ohmic diffusion, ambipolar diffusion, Hall effect) regulate MRI activity
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- Magnetically driven disk winds may dominate angular momentum transport in weakly ionized regions
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**Learn more:**
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- [Magnetohydrodynamics](/research/fluid-dynamics/#magnetohydrodynamics)
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- [Magnetorotational Instability](/research/fluid-dynamics/#magnetorotational)
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- [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
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## Thermodynamic Structure
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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
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**Learn more:**
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- [Radiation Hydrodynamics](/research/fluid-dynamics/#radiation-hydrodynamics)
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- [Vertical Shear Instability](/research/fluid-dynamics/#vertical-shear)
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- [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)
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## Vortices
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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
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- Dust-to-gas ratios inside vortices can greatly exceed disk-average values, aiding gravitational collapse
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- 3D simulations with self-gravity show that vortices can produce protoplanets on surprisingly short timescales
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- Vortex formation has been proposed as a pathway to rapid planet formation
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**Learn more:**
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- [Rossby Wave Instability](/research/fluid-dynamics/#rossby-wave)
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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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- [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)

‎_research/02-planet-formation.md‎

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## Introduction
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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
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**Learn more:**
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- [Protoplanetary Disks](/research/protoplanetary-disks/)
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- [Fluid Dynamics](/research/fluid-dynamics/)
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- [Code Comparisons](/research/code-comparisons/)
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- [PFITS+ Publications](https://ui.adsabs.harvard.edu/public-libraries/_-AhcKuYSKyaIu_U5ebVsA){:target="_blank"}
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## Dust Coagulation
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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
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**Learn more:**
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- [Streaming Instability](#planetesimal-formation)
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- [Planetesimal Formation](#planetesimal-formation)
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- [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)
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## Planetesimal Formation
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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
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**Learn more:**
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- [Streaming Instability](/research/fluid-dynamics/#streaming)
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- [Code Comparisons: Streaming Instability](/research/code-comparisons/)
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- [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)
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## Pebble Accretion
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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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