The process by which small rocky or icy bodies collide and form larger protoplanets
1. Introduction: from dust grains to planetesimals
When a new star forms in a molecular cloud, the surrounding protoplanetary disk – made of gas and dust – becomes the main raw material for planet formation. However, the path from dust grains measuring microns to planets the size of Earth or even Jupiter is far from simple. Planetesimal accretion links the early dust evolution (grain growth, fragmentation, and sticking) with the eventual formation of kilometer- or hundred-kilometer-scale bodies called planetesimals. Once planetesimals appear, gravitational interactions and collisions allow them to grow into protoplanets, which ultimately determine the arrangement of developing planetary systems.
- Why it matters: Planetesimals are the “building blocks” of all rocky and many gas giant planet cores. They also remain in present-day bodies such as asteroids, comets, and Kuiper Belt objects.
- Challenges: Simple collision and sticking schemes stall in the centimeter–meter range due to destructive collisions or rapid radial drift. Proposed solutions – streaming instability or pebble accretion – allow bypassing this “meter-size barrier.”
In short, planetesimal accretion is a crucial phase that creates the seeds of future planets from small, submillimeter grains in the disk. Understanding this process means answering how worlds like Earth (and probably many exoplanets) formed from cosmic dust.
2. The first barrier: growth from dust to meter-sized objects
2.1 Dust coagulation and sticking
Dust grains in the disk start at micrometer scales. They can combine into larger structures:
- Brownian motion: Small grain collisions occur slowly, so they can stick together via van der Waals or electrostatic forces.
- Turbulent motions: In a turbulent disk environment, slightly larger grains collide more frequently, allowing mm–cm sized aggregates to form.
- Ice particles: Beyond the frost line, ice mantles can promote more effective sticking, accelerating grain growth.
Such collisions can create “fluffy” aggregates growing to millimeter or centimeter sizes. However, as grains grow, collision speeds increase. Exceeding certain speed or size limits, collisions can break apart aggregates instead of growing them, creating a partial dead end (called the “fragmentation barrier”). [1], [2].
2.2 Meter-size barrier and radial drift
Even if grains manage to grow to cm–m sizes, they face another major challenge:
- Radial drift: Due to pressure-supported disk gas rotating slightly slower than Keplerian velocity, solid bodies lose angular momentum and spiral toward the star. Meter-sized particles can be lost to the star within ~100–1000 years without forming planetesimals.
- Fragmentation: Larger clumps can break apart due to higher collision speeds.
- Bouncing: In some situations, particles only bounce off each other without leading to effective growth.
Thus, gradual grain growth to kilometer-sized planetesimals is difficult if destructive collisions and drift dominate. Solving this dilemma is one of the key questions in modern planet formation theory.
3. How to overcome growth barriers: proposed solutions
3.1 Streaming instability
One possible mechanism is streaming instability (SI). In the case of SI:
- Collective interaction of particles and gas: Particles partially decouple from the gas, forming local overdensities.
- Positive feedback: Concentrated particles locally accelerate gas flow, reducing the headwind against them, thus further increasing particle concentration.
- Gravitational collapse: Eventually, dense clumps can collapse under their own gravity, avoiding slow, gradual collisions.
Such gravitational collapse quickly produces 10–100 km scale planetesimals, crucial for initial protoplanet formation [3]. Numerical models strongly suggest that streaming instability can be a reliable pathway for planetesimal formation, especially if the dust-to-gas ratio is enhanced or pressure bumps concentrate solids.
3.2 "Pebble" accretion
Another method is "pebble" accretion, where protoplanetary embryos (~100–1000 km) "collect" mm–cm sized particles drifting in the disk:
- Bondi/Hill radius: If a protoplanet is large enough that its Hill sphere or Bondi radius can "capture" pebbles, accretion rates can be very high.
- Growth efficiency: Low relative velocity between pebbles and the core allows a large fraction of "pebbles" to accrete, bypassing the need for gradual collisions between similarly sized particles [4].
"Pebble" accretion may be more important in the protoplanet stage but is also related to primary planetesimals or surviving "seeds."
3.3 Disk substructures (pressure "bumps", vortices)
ALMA-detected ring-shaped structures indicate possible dust "traps" (e.g., pressure maxima, vortices) where particles accumulate. Such locally dense regions can collapse via streaming instability or simply rapidly promote collisions. These structures help prevent radial drift by "creating places" for dust accumulations. Over thousands of orbits, planetesimals can form in these dust traps.
4. Further growth beyond planetesimals: formation of protoplanets
Once kilometer-scale bodies exist, due to gravitational "focusing", collisions become even more frequent:
- Runaway growth: The largest planetesimals grow fastest, initiating "oligarchic" growth. A small number of large protoplanets dominate local resources.
- Acceleration / "damping": Mutual collisions and gas drag reduce random velocities, favoring accretion rather than disruption.
- Timescale: In inner (terrestrial) regions, protoplanets can form over several million years, leaving a few embryos that later collide to form the final rocky planets. In outer regions, the cores of gas giants require even faster evolution to accrete the disk gas in time.
"5. Observational and laboratory evidence"
"5.1 Remaining objects in our Solar System"
Our system retains asteroids, comets, and Kuiper belt objects as unfinished accretion planetesimals or partially formed bodies. Their composition and distribution help understand planetesimal formation conditions in the early solar nebula:
- Asteroid belt: In the region between Mars and Jupiter, we find bodies of various chemical compositions (rocky, metallic, carbonaceous), remnants of incomplete planetesimal evolution or orbits disturbed by Jupiter's gravity.
- Comets: Icy planetesimals from beyond the snow line, preserving primordial volatile compounds and dust from the disk's outer part.
Their isotopic signatures (e.g., oxygen isotopes in meteorites) reveal local disk chemistry and radial mixing processes.
"5.2 Exoplanet debris disks"
Observations of debris (dust) disks (e.g., with ALMA or Spitzer) around older stars show belts where planetesimals collide. A famous example is the β Pictoris system with a huge dust disk and possible (planetesimal) body "clumps." Younger, protoplanetary systems have more gas, while older ones have less, dominated by collisions among remaining planetesimals.
"5.3 Laboratory experiments and particle physics"
Drop tower or microgravity experiments study dust grain collisions – how grains stick or bounce off each other at certain speeds? Larger-scale experiments examine the mechanical properties of cm-sized aggregates. Meanwhile, HPC simulations integrate this data to see how collision scales grow. Information on fragmentation rates, sticking thresholds, and dust composition complements planetesimal formation models [5], [6].
"6. Timescales and randomness"
"6.1 Fast versus slow"
Depending on disk conditions, planetesimals can form quickly (within thousands of years) due to streaming instability or more slowly if growth is limited by less frequent collisions. Results vary greatly:
- Outer disk part: Low density slows planetesimal formation, but ice facilitates sticking.
- Inner disk part: Higher density promotes collisions, but higher speed increases the risk of destructive impacts.
"6.2 The 'Random Path' Toward Protoplanets"
As planets began to form, their gravitational interactions caused chaotic collisions, mergers, or ejections. In some regions, large embryos could rapidly form (e.g., Mars-sized protoplanets in the inner system). Once enough mass accumulated, the system's architecture could "lock in" or continue to evolve due to massive collisions, as believed in the Earth-Theia impact scenario explaining the Moon's origin.
6.3 Diversity of Systems
Exoplanet observations show that in some systems super-Earths or hot Jupiters form close to the star, while others retain wide orbits or resonant chains. Different planetesimal formation rates and migration processes can produce unexpectedly diverse planetary configurations, even with slight differences in disk mass, angular momentum, or metallicity.
7. Key Roles of Planetesimals
7.1 Cores for Gas Giants
In the outer disk zone, when planetesimals reach ~10 Earth masses, they can attract hydrogen–helium envelopes, forming Jupiter-type gas giants. Without a planetesimal core, such gas accretion may be too slow before the disk disperses. Therefore, planetesimals are crucial in forming giant planets in the core accretion model.
7.2 Volatile Compounds
Planetesimals formed beyond the snow line contain much ice and volatiles. Later, due to scattering or late collisions, they can deliver water and organic compounds to inner rocky planets, possibly contributing significantly to habitability. Earth's water may have partly originated from planetesimals or comets in the asteroid belt region.
7.3 Smaller Remnants
Not all planetesimals merge into planets. Some remain as asteroids, comets, or Kuiper Belt objects and bodies considered Trojans. These populations preserve primordial disk material, providing “archaeological” evidence about formation conditions and timescales.
8. Future Research on Planetesimal Science
8.1 Observational Achievements (ALMA, JWST)
High-resolution observations can reveal not only disk substructures but also concentrations or filaments of solid particles corresponding to flow instabilities. Detailed chemical analysis (e.g., CO isotopologues, complex organic compounds) in these filaments would help confirm conditions favorable for planetesimal formation.
8.2 Space Missions to Small Bodies
Missions such as OSIRIS-REx (aiming to return Bennu samples), Hayabusa2 (Ryugu), the upcoming Lucy (Trojan asteroids), and Comet Interceptor expand understanding of planetesimal composition and internal structure. Each sample return or close flyby helps refine disk condensation models, collision histories, and the presence of organic compounds, explaining how planetesimals formed and evolved.
8.3 Theoretical and Computational Improvements
Better particle or fluid-dynamic-kinetic models will provide more opportunities to understand flow instability, dust collision physics, and processes at various scales (from submm grains to multi-kilometer planetesimals). Using high-performance HPC resources, we can combine microscopic grain interaction nuances with the collective behavior of planetesimal swarms.
9. Summary and concluding remark
Planetesimal accretion is a crucial stage where "cosmic dust" turns into tangible worlds. Starting from microscopic dust collision interactions and ending with flow instability promoting the formation of kilometer-sized bodies, the emergence of planetesimals is both complex and essential for growing planetary embryos and eventually fully developed planets. Observations in protoplanetary and debris disks, as well as sample returns from small Solar System bodies, reveal the chaotic interplay of collisions, drift, adhesion, and gravitational collapse. At every stage – from dust to planetesimals and protoplanets – a carefully choreographed (though somewhat random) dance of material unfolds, driven by gravity, orbital dynamics, and disk physics.
By combining these processes, we connect the sticking of the finest dust in the disk with the magnificent orbital architectures of multi-planet systems. Like Earth, many exoplanets begin from the gathering of these tiny dust clumps – planetesimals, sowing entire families of planets that over time can even become habitable.
Links 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.