Introduction

Planet formation spans more than forty orders of magnitude in mass—from sub-micron interstellar dust grains to gas giants rivaling Jupiter. Our group investigates the physical processes that bridge these scales, focusing on the aerodynamic and gravitational mechanisms that convert diffuse protoplanetary disk material into the diverse planetary systems observed around other stars. 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.

  • Planet formation involves physics from aerodynamics to self-gravity and orbital mechanics
  • Key barriers—due to radial drift, bouncing, and fragmentation—motivate instability-based formation pathways
  • Our group leads multi-code comparison projects to rigorously test theoretical predictions
  • Complementary observations from ALMA and space missions continue to reshape our understanding

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Dust Coagulation

Planet formation begins when microscopic interstellar dust grains collide and stick together, building progressively larger aggregates. 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.

  • Sub-micron grains grow by van der Waals adhesion; centimeter aggregates require different sticking mechanisms
  • Radial drift causes pebbles to spiral inward, providing a limited time window for growth
  • Grain size and composition (silicate vs. ice) strongly affect sticking and fragmentation thresholds
  • Coagulation and fragmentation can work together with streaming instability to enhance planetesimal formation

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Planetesimal Formation

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. The streaming instability 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.

  • Streaming instability was first identified by Youdin & Goodman (2005)
  • Clumping requires a threshold dust-to-gas ratio that depends on particle size and disk pressure gradient
  • Gravitational collapse of clumps produces planetesimals with a characteristic size distribution
  • Simulations must resolve both the gas dynamics and the self-gravity of the collapsing dust layer

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Pebble Accretion

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. 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.

  • Pebble accretion efficiency depends on particle size, orbital location, and embryo mass
  • Polydisperse pebble size distributions alter growth rates compared to single-size models
  • The “pebble isolation mass” sets an upper limit beyond which an embryo cuts off its own pebble supply
  • Pebble accretion has implications for giant planet formation, Kuiper Belt objects, and exoplanet demographics

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