The process by which small rocky or icy bodies collide to form larger protoplanets
Introduction: From Dust Grains to Planetesimals
When a new star forms within a molecular cloud, the surrounding protoplanetary disk—comprised of gas and dust—provides the raw materials for planet formation. Yet the path from submicron dust grains to Earth-sized or even Jupiter-sized planets is by no means straightforward. Planetesimal accretion bridges the early stages of dust evolution (grain growth, fragmentation, and sticking) with the eventual formation of kilometer- to hundreds-of-kilometer-scale bodies known as planetesimals. Once planetesimals appear, gravitational interactions and collisions allow these larger solids to become protoplanets, ultimately shaping the architecture of emerging planetary systems.
- Why It Matters: Planetesimals are the “building blocks” of all terrestrial and many giant-planet cores. They also survive in modern remnants like asteroids, comets, and Kuiper Belt objects.
- Challenges: Simple collisional sticking mechanisms stall at centimeter-to-meter scales due to destructive collisions or rapid radial drift. Proposed solutions—streaming instability or pebble accretion—provide ways to bypass this “meter-size barrier.”
In short, planetesimal accretion is the crucial phase that transforms a disk of small, sub-millimeter grains into the seeds of future planets. Understanding this process answers how worlds like Earth (and likely many exoplanets) took shape from cosmic dust.
2. The Early Roadblock: Growth from Dust to Meter-Sized Objects
2.1 Dust Coagulation and Sticking
Dust grains within the disk begin at micron scales, which can form aggregates by:
- Brownian Motion: Tiny grains collide gently at low relative velocities, sticking via van der Waals or electrostatic forces.
- Turbulent Motions: In the disk’s turbulent gas, slightly larger grains meet more often, enabling mm- to cm-sized aggregates to form.
- Icy Particles: Beyond the frost line, ice mantles can promote more effective sticking, potentially speeding up the grain growth process.
These collisions can build “fluffy” aggregates up to millimeter or centimeter sizes. However, as grains grow larger, collision speeds rise. Beyond certain thresholds (velocity or size), collisions can shatter aggregates rather than build them, leading to a partial stalemate (the “fragmentation barrier”) [1], [2].
2.2 The Meter-Size Barrier and Radial Drift
Even if grains manage to become cm- to meter-sized, they face a second major issue:
- Radial Drift: Gas in the disk orbits slightly slower than Keplerian speed due to pressure support, causing solids to lose angular momentum and spiral inward. Meter-sized bodies can drift into the star on short timescales (~100–1000 years), possibly never forming planetesimals.
- Fragmentation: Larger aggregates can experience destructive collisions at higher relative velocities.
- Bouncing: Sometimes collisions result in bouncing off each other, not effectively growing.
Hence, purely incremental growth from tiny grains to kilometer-sized planetesimals is difficult if collisions and drift dominate. Solving this conundrum is central to modern planet formation theories.
3. Overcoming Growth Barriers: Proposed Solutions
3.1 Streaming Instability
One proposed mechanism is the streaming instability (SI). In the SI scenario:
- Collective Dust-Gas Dynamics: Particles slightly decouple from gas, forming local overdensities.
- Positive Feedback: Concentrated particles locally accelerate the gas, reducing headwind, allowing even more particles to accumulate.
- Gravitational Collapse: Eventually, these dense clumps can collapse under self-gravity, bypassing the need for slow, incremental collisions.
This gravitational collapse rapidly yields 10–100 km scale planetesimals—pivotal for jump-starting protoplanet formation [3]. Numerical simulations strongly support streaming instability as a robust path for planetesimal formation, especially if dust-to-gas ratios are somewhat elevated or pressure bumps concentrate solids.
3.2 Pebble Accretion
Another approach is pebble accretion, focusing on protoplanetary seeds (maybe 100–1000 km objects) that then “hoover up” mm- to cm-sized pebbles swirling in the disk:
- Bondi/Hill Radius: If the protoplanet is big enough for its Hill sphere or Bondi radius to capture drifting pebbles, accretion rates can be extremely rapid.
- Growth Efficiency: Low relative velocities between pebbles and the seed core can result in high capture probabilities, thereby skipping incremental collisions among peers [4].
Pebble accretion might be more relevant at the protoplanet stage, but it also ties in with the formation and survival of initial planetesimals or “seeds.”
3.3 Disk Substructures (Pressure Bumps, Vortices)
Observations of ALMA ring-like structures suggest dust traps (e.g., pressure maxima, vortices) where solids accumulate. These local high-solids regions can either directly collapse via streaming instability or facilitate faster collisions. Such substructures help circumvent radial drift losses by “parking” dust in stable zones. Over timescales of thousands of orbits, planetesimals can form in these dust traps.
4. Growth Beyond Planetesimals: Protoplanet Formation
Once kilometer-scale bodies exist, gravitational focusing intensifies collision cross-sections:
- Runaway Growth: The biggest planetesimals grow fastest, fueling “oligarchic” growth. A small number of large protoplanets dominate local feeding zones.
- Damping: Mutual collisions and gas drag can damp random velocities, encouraging further accretion instead of fragmentation.
- Timescales: In the terrestrial region (close to the star), protoplanet formation might happen over a few million years, culminating in a few embryo-sized bodies that eventually collide into final terrestrial planets. In outer regions, gas giants’ cores must form even faster to capture disk gas.
5. Observational and Laboratory Evidence
5.1 Remnants in Our Solar System
Our Solar System retains asteroids, comets, and Kuiper Belt objects as leftover planetesimals or partially grown bodies. Their composition and distribution hint at the conditions of planetesimal formation in the early solar nebula:
- Asteroid Belt: Between Mars and Jupiter, we find a mix of rocky, metallic, and carbonaceous bodies, remnants of incomplete planetesimal growth or gravitational scattering by Jupiter.
- Comets: Icy planetesimals from beyond the snow line, preserving pristine volatiles and dust from the outer disk.
Their isotopic signatures (e.g., oxygen isotopes in meteorites) reveal details about local disk chemistry and radial mixing.
5.2 Exoplanet Debris Disks
Observations of debris disks (e.g., with ALMA or Spitzer) around older stars show belts of colliding planetesimals. Famous examples: the β Pictoris system with a huge dust disk, possible planet(esimal) lumps. Younger systems with protoplanetary disks are often more gas-rich, while older debris disks are gas-poor, dominated by collisions among leftover planetesimals.
5.3 Laboratory Experiments and Particle Physics
Laboratory drop-tower or microgravity experiments investigate dust grain collisions—how do grains stick or bounce at certain speeds? Larger-scale experiments test the mechanical properties of cm-size aggregates. Meanwhile, HPC simulations integrate these data to see how collisions scale up. Constraints on fragmentation velocities, sticking thresholds, and dust composition feed into planetesimal formation models [5], [6].
6. Timescales and Stochasticity
6.1 Rapid vs. Gradual
Depending on disk parameters, planetesimals might form rapidly (thousands of years) under streaming instabilities or more gradually if growth is limited by slower collisions. The outcome can vary widely:
- Outer Disk: Low densities can slow planetesimal formation, but ices can ease sticking.
- Inner Disk: Higher densities accelerate collisions, but higher impact speeds risk fragmentation.
6.2 “Random Walk” to Protoplanets
As planetesimals emerge, gravitational stirring among them leads to a chaotic interplay of collisions, merging, or sometimes ejections. Certain zones might form large embryonic bodies quickly (like Mars-sized embryos in the terrestrial region). Once enough mass accumulates, the system’s architecture can “lock in” or continue evolving via giant impacts, as happened in the Earth–Theia collision scenario for our Moon’s origin.
6.3 Variation Among Systems
Exoplanet discoveries show some planetary systems formed super-Earths or hot Jupiters close to the star, while others maintain wide orbits or resonant chains. Divergent planetesimal formation rates and migration episodes can produce surprisingly diverse architectures from seemingly modest differences in disk mass, angular momentum, or metallicity.
7. Key Roles of Planetesimals
7.1 Seed Cores for Gas Giants
In the outer disk, once planetesimals grow to ~10 Earth masses, they can gravitationally capture hydrogen-helium envelopes, forming Jupiter-like gas giants. Without a core of planetesimals, such gas capture might be too slow before the disk dissipates. So planetesimals are integral to building giant planet cores in the Core Accretion model.
7.2 Delivery of Volatiles
Planetesimals formed beyond the snow line contain ices and volatiles. Subsequent scattering or late-stage impacts can deliver water and organics to inner terrestrial planets, possibly crucial for habitability. Earth’s water could partly come from planetesimals in the asteroid belt region or scattered comets.
7.3 Source of Minor Bodies
Not all planetesimals merge into planets. Many remain as asteroids, comets, Kuiper Belt objects, or Trojan populations. These populations preserve pristine material from the early disk, providing archaeological clues about the conditions and timescales of formation.
8. Future Research in Planetesimal Science
8.1 Observational Gains from ALMA, JWST
Ongoing high-resolution imaging can potentially detect not just disk substructures but concentrations or filaments of solids consistent with streaming instability. Detailed chemistry (CO isotopologues, complex organics) in these filaments helps confirm conditions favorable for planetesimal collapse.
8.2 Space Missions to Small Bodies
Missions like OSIRIS-REx (Bennu sample return), Hayabusa2 (Ryugu), or upcoming Lucy (Trojan asteroids) and Comet Interceptor extend our knowledge of planetesimal composition and interior structure. Each sample return or close flyby refines disk condensation models, collisional histories, and organic content, clarifying how planetesimals formed and evolved.
8.3 Theoretical and Computational Advances
Refinements in particle-based or fluid-kinetic simulations enable better modeling of streaming instability, dust collision physics, and multi-scale approaches (from sub-mm grains to multi-kilometer planetesimals). Coupling these with advanced HPC resources helps unify microscopic grain interactions with the emergent behavior of entire planetesimal swarms.
9. Summary and Concluding Remarks
Planetesimal accretion lies at the heart of how “cosmic dust” transforms into tangible worlds. From micro-scale dust collisions to streaming instabilities culminating in kilometer-scale bodies, the formation of planetesimals is both complex and essential for building planetary embryos—and, ultimately, fully grown planets. Observations of protoplanetary and debris disks, alongside sample returns from small bodies in our solar system, confirm the messy interplay of collisions, drift, sticking, and gravitational collapse. Each stage—from dust grains to planetesimals to protoplanets—reveals a meticulously orchestrated (yet somewhat stochastic) dance of materials under gravity, orbital dynamics, and disk physics.
In bridging these processes, we link the minuscule scales of micrograin sticking in the disk to the majestic scale of orbital architectures in multi-planet systems. For Earth and countless exoplanets, it all began with these diminutive lumps of dust coming together—planetesimals—sowing the seeds of entire planetary families that, in time, might even support life.
References and Further Reading
- Weidenschilling, S. J. (1977). “Aerodynamics of solid bodies in the solar nebula.” Monthly Notices of the Royal Astronomical Society, 180, 57–70.
- Blum, J., & Wurm, G. (2008). “The Growth Mechanisms of Macroscopic Bodies in Protoplanetary Disks.” Annual Review of Astronomy and Astrophysics, 46, 21–56.
- Johansen, A., et al. (2007). “Rapid planetesimal formation in turbulent circumstellar disks.” Nature, 448, 1022–1025.
- Lambrechts, M., & Johansen, A. (2012). “Rapid growth of gas-giant cores by pebble accretion.” Astronomy & Astrophysics, 544, A32.
- Birnstiel, T., Fang, M., & Johansen, A. (2016). “Dust Evolution and the Formation of Planetesimals.” Space Science Reviews, 205, 41–75.
- Windmark, F., Birnstiel, T., Ormel, C. W., & Dullemond, C. P. (2012). “Breaking the growth barriers in planetesimal formation.” Astronomy & Astrophysics, 544, L16.
- Morbidelli, A., Lunine, J. I., O’Brien, D. P., Raymond, S. N., & Walsh, K. J. (2012). “Building Terrestrial Planets.” Annual Review of Earth and Planetary Sciences, 40, 251–275.