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, around it protoplanetary disk – made up of gas and dust – becomes the main raw material for planet formation. However, the path from submicron-sized dust grains to Earth- or even Jupiter-sized planets is far from simple. Planetesimal accretion combines the early evolution of dust (grain growth, fragmentation and coalescence) with the final formation of kilometer- or hundred-kilometer-scale bodies called planetesimals, formation. Once planetesimals are formed, gravitational interactions and collisions allow them to grow into protoplanet, which ultimately determine the arrangement of developing planetary systems.
- Why is this important?: Planetary grinders are "building blocks" in the cores of all rocky and many gaseous planets. They also persist in present-day bodies such as asteroids, comets, and Kuiper belt objects.
- Challenges: Simple collision and accretion schemes stop in the centimeter-meter range due to destructive collisions or rapid radial drift. Proposed solutions – streaming instability whether "pebble" accretion – allows you to bypass this “meter-sized obstacle”.
In short, planetesimal accretion is a fundamental phase that creates the seeds of future planets from a disk of tiny, submillimeter grains. Understanding this process is key to answering how worlds like Earth (and likely many exoplanets) were born from cosmic dust.
2. The first barrier: growth from dust to metric objects
2.1 Dust coagulation and adhesion
Dust grains in the disk, they start at micrometer scales. They can connect into larger structures:
- Brownian motion: Minor collisions between grains occur slowly, so they can adhere through van der Waals or electrostatic forces.
- Turbulent movements: In the turbulent environment of the disk, slightly larger grains meet more frequently, allowing mm–cm-sized aggregates to form.
- Icy particles: Beyond the freezing limit, ice shells can promote more efficient adhesion by accelerating grain growth.
Such collisions can create "loose" aggregates that grow to millimetre or centimetre sizes. However, as the grains grow larger, so does the speed of the collisions. Beyond certain speed or size limits, collisions can break up the aggregates rather than growing them, creating a partial deadlock (called a "fragmentation barrier"). [1], [2].
2.2 Meter-sized barrier and radial drift
Even if grains manage to grow to cm–m in size, they face another major challenge:
- Radial drift: The pressure-supported disk gas rotates slightly slower than the Kelper speed, causing solid bodies to lose angular momentum and spiral toward the star. Metric particles may be lost to the star over ~100–1000 years without ever forming into planetesimals.
- Fragmentation: Larger clusters may disintegrate due to higher collision speeds.
- Bounce: In some situations, particles just bounce off without causing effective growth.
Thus, the mere gradual growth of grains to kilometer-sized planetesimals is difficult if destructive collisions and drift dominate. Resolving this dilemma is one of the fundamental questions in modern planetary formation theory.
3.How to overcome obstacles to growth: proposed solutions
3.1 Streaming instability
One possible mechanism is flow instability (streaming instability, SI). In the case of SI:
- Collective interaction of particles and gases: Parts are slightly detached from the gas, forming local overloads.
- Positive feedback: The concentrated particles locally accelerate the gas flow, reducing the headwind against them, so the concentration of particles increases even more.
- Gravitational collapse: Eventually, dense clumps may collapse under their own gravity, thus avoiding slow, gradual collisions.
Such gravitational collapse quickly yields 10–100 km scale planetesimals, fatal for the initial formation of protoplanets [3]Numerical models strongly suggest that flow instability can be a plausible pathway for planetesimal formation, especially if the dust-to-gas ratio is increased or pressure ridges concentrate solid particles.
3.2 Pebble accretion
Another way is "pebble" accretion, where protoplanetary germs (~100–1000 km) "gather" mm–cm sized particles orbiting in the disk:
- Bondi/Hill radius: If a protoplanet is large enough that its Hill sphere or Bondi radius can "catch" pebbles, accretion rates can be very high.
- Growth efficiency: The low relative velocity between the pebbles and the nucleus allows a large fraction of the "pebbles" to coalesce, bypassing the need for gradual collisions between particles of similar size [4].
"Pebble" accretion may be more important in the protoplanetary stage, but is also associated with the survival of the primary planetesimals or "seeds."
3.3 Disk substructures (pressure ridges, vortices)
The ring-shaped structures detected by ALMA indicate possible dust “traps” (e.g., pressure peaks, vortices) where particles accumulate. Such locally dense areas may collapse through streaming instability or simply rapidly promote collisions. Such structures help prevent radial drift by “making room” for dust accumulations. Over thousands of orbits, planetesimals may form in these dust traps.
4. Further growth beyond planetesimals: formation of protoplanets
As soon as we have kilometer-scale bodies, because gravitational "concentration" collisions become even more frequent:
- Runaway growth: The largest planetesimals grow the fastest, and "oligarchic" growth begins to prevail. A small number of large protoplanets control local resources.
- Acceleration/"slowdown": Collisions and gas friction reduce random velocities, favoring accretion rather than decay.
- Time scale: In the inner (terrestrial) regions, protoplanets can form within a few million years, leaving behind a few embryos that later collide to form the final rocky planets. In the outer regions, the cores of gas giants need even faster evolution to accretize the disk gas.
5. Observational and laboratory evidence
5.1 Remaining objects in our Solar System
Our system has remained asteroids, comets and Kuiper belt objects as incomplete accretions of planetesimals or partially formed bodies.Their composition and arrangement provide insight into the conditions for the formation of planetesimals in the early solar system:
- Asteroid belt: In the region between Mars and Jupiter, we find bodies of various chemical compositions (rocky, metallic, igneous), left over from the incomplete evolution of planetesimals or orbits detuned by Jupiter's gravity.
- Comets: Icy planetesimals from beyond the snowline, preserving primordial volatiles and dust from the outer part of the disk.
Their isotopic signatures (e.g. oxygen isotopes in meteorites) reveal local disk chemistry and radial mixing processes.
5.2 Exoplanetary debris disks
Observations of debris disks (e.g. with ALMA or Spitzer) around older stars show bands where planetesimals collide. A famous example is β Pictoris system with a large dust disk, possible "bumps" of (planetesimal) bodies. Younger, protoplanetary systems have more gas, and older ones have less, dominated by collisions between the remaining planetesimals.
5.3 Laboratory experiments and particle physics
Drop tower or microgravity tests investigate dust grain collisions – how do grains stick together or bounce off each other at a given speed? Larger-scale experiments study the mechanical properties of cm-sized aggregates. Meanwhile, HPC simulations integrate this data to see how the size of collisions grows. Information on fragmentation rates, adhesion thresholds, and dust composition complement models of planetesimal formation [5], [6].
6. Timescales and randomness
6.1 Fast vs. Slow
Depending on the conditions in the disk, planetesimals can form rapidly (over thousands of years) due to streaming instability, or more slowly if growth is limited by slower collisions. The results vary widely:
- Outer part of the disk: Low density slows the formation of planetesimals, but ice facilitates coalescence.
- Internal part of the disk: Higher density encourages collisions, but higher speeds increase the risk of damaging impacts.
6.2 The “random path” to protoplanets
As planets begin to form, their gravitational interactions cause chaotic collisions, mergers, or ejections. In some regions, large embryos can form rapidly (e.g., Mars-sized protoplanets in the inner system). Once enough mass has accumulated, the architecture of the system can "lock in" or continue to change due to giant collisions, as suggested in the Earth-Theia collision scenario that explains the origin of the Moon.
6.3 System diversity
Observations of exoplanets suggest that in some systems super-Earths or hot Jupiters form close to the star, while in others wide orbits or resonant chains are preserved. Different rates of planetesimal formation and migration processes can give rise to unexpectedly different planetary configurations, even with small differences in disk mass, angular momentum, or metallicity.
7. Key roles of planetesimals
7.1 Nuclei for gas giants
In the outer zone of the disk, when planetesimals When they reach ~10 Earth masses, they can attract hydrogen-helium atmospheres, forming Jupiter-like gas giants. Without a planetesimal core, such gas accretion may be too slow for the disk to disperse. Planetesimals are therefore crucial in the formation of giant planets. nuclear accretions in the model.
7.2 Volatile compounds
Planetesimals formed beyond the snowline contain a lot of ice and volatiles.Later, through ejecta or late collisions, they may have delivered water and organic compounds to the inner rocky planets, perhaps making a significant contribution to life. Earth's water may have come in part from planetesimals or comets in the asteroid belt.
7.3 Smaller residues
Not all planetesimals coalesce into planets. Some remain as asteroids, comets whether Kuiper belt objects and bodies considered Trojans. These populations preserve the original material of the disk, providing "archaeological" evidence of the conditions and rates of formation.
8. Future research on planetesimal science
8.1 Observational achievements (ALMA, JWST)
High-resolution observations can reveal not only the substructures of disks, but also concentrations or filaments solid particles consistent with flow instability. 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 like OSIRIS-REx (to bring back samples from Bennu), Hayabusa2 (Ryugu), upcoming Lucy (for Trojan asteroids) and Comet Interceptor expands our understanding of the composition and internal structure of planetesimals. Each sample return or close flyby helps refine models of disk condensation, collision histories, and the presence of organic compounds, explaining how planetesimals formed and evolved.
8.3 Theoretical and computational improvements
Better particulate whether fluid dynamic-kinetic models will provide greater opportunities to understand flow instabilities, the physics of dust collisions, and processes at various scales (from submm grains to multi-kilometer planetesimals). Using high-performance HPC resources, we can connect the microscopic nuances of grain interactions and the collective behavior of a planetesimal swarm.
9. Summary and concluding remarks
Planetesimal accretion is a crucial stage in the transformation of "space dust" into tangible worlds. From microscopic interactions between dust collisions to flow instabilities that drive the formation of kilometer-sized bodies, the emergence of planetesimals is both complicated, and necessary to grow planetary embryos and eventually fully developed planets. Observations in protoplanetary and debris disks, and sample returns from small Solar System bodies, reveal a chaotic interplay of collisions, drift, accretion, and gravitational collapse. At each stage—from dust to planetesimals to protoplanets—a carefully orchestrated (if somewhat random) dance of material is revealed, driven by gravity, orbital dynamics, and disk physics.
Combining these processes, we link the coalescence of the finest dust in the disk to the grand orbital architectures of multiplanetary systems. Like Earth, many exoplanets begin with the coalescence of these tiny dust clumps – planetesimals, seeding entire families of planets that may even become habitable over time.
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, CW, & Dullemond, CP (2012). "Breaking the growth barriers in planetesimal formation." Astronomy & Astrophysics, 544, L16.
- Morbidelli, A., Lunine, JI, O'Brien, DP, Raymond, SN, & Walsh, KJ (2012). "Building Terrestrial Planets." Annual Review of Earth and Planetary Sciences, 40, 251–275.