Lesson 2 of 6 · Sticking, fragmentation, drift & planetesimals
Cross the gap from dust to building blocks
8 minute lesson · 3 checks along the way
Your goal for this lessonIdentify the obstacles to growing solid bodies and evaluate whether an observation distinguishes a growth mechanism.
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Small grains do not behave like small planets
At the beginning of solid growth, microscopic grains collide within gas. Contact forces between their surfaces can let them stick into porous . Their mutual gravity is not the main glue at this tiny scale. The outcome depends on speed, material, size, and structure. A gentle collision that joins two fluffy clumps does not establish what happens when denser or faster clumps meet.
Growth also has competitors. Aggregates can bounce, break apart, or lose small fragments through erosion. Gas drag changes particle motion and can make solids drift inward. A successful formation model must produce larger, lasting bodies before too much material is lost to the star or dispersed. Simply repeating the word accretion does not explain how every difficult size range is crossed.
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Use gas as part of the mechanism
Regions of increased gas pressure can concentrate drifting solids. Under suitable conditions, interactions between the particles and gas further strengthen concentrations; one proposed mechanism is the . If a concentration becomes dense enough, its collective self-gravity can collapse it into a . The required particle properties, concentration, and disk conditions are active research questions, not an automatic guarantee for every ring.
Once a substantial seed exists, it may capture additional small drifting solids with help from gas drag. This process, called , differs from explaining how the first planetesimals formed. The word pebble describes particles relevant to their gas interaction, not a universal schoolyard size. Collisions among larger bodies provide another route to continued growth, and several processes can operate together.
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Work through a decision
Keep a mass budget through a growth claim
Use an invented simulation with 100 mass units of solids. At its endpoint, 30 units occupy bound clumps, 20 have left the modeled region, and 50 remain dispersed. The fraction incorporated into clumps is 30/100 = 30 percent of the starting mass. Counting only the 80 units still present would answer a different question: 30/80 = 37.5 percent of the retained mass.
Neither percentage tells you that 30 planets formed. Mass, number, and size are different quantities. A researcher should also inspect whether the clumps survive, whether numerical resolution changes the result, and whether simulated collisions use realistic material properties. A second run that forms no clumps can be informative if it changes one stated condition, such as concentration, rather than many hidden assumptions at once.
Follow the same 100 mass units
Invented simulation inventory. Every block is 10 mass units, not one body. The denominator determines whether the result describes initial or retained material.
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Look for traces of assembly and transport
Rosetta observed comet 67P/Churyumov–Gerasimenko’s two-lobed shape. Detailed mapping of layers and comparison with gravity directions supported an interpretation in which two separately assembled bodies joined. The single navigation-camera photograph here does not establish the collision speed or replay the encounter. Shape, internal structure, strength, and later surface modification must be considered together before selecting a formation history.
Another test reaches beyond our system. A 2023 Webb study compared water-vapor spectra in compact and extended young disks. The difference supported inward delivery of icy solids followed by ice turning to vapor. That is evidence for transport under the studied conditions, not a filmed journey of one pebble. Connecting laboratory collisions, disk measurements, simulations, and surviving bodies gives a stronger account than any one method alone.
Rosetta’s navigation camera observed comet 67P/Churyumov–Gerasimenko on March 25, 2015. Its two-lobed form is visible. A proposed merger history uses more evidence, including mapped layers and gravity directions; this frame does not show the encounter.
Look closely
Can the visible two-lobed outline alone determine how fast two precursor bodies met?
See what to notice
No. Shape suggests questions about assembly, but an impact speed must be inferred through additional structural evidence and a physical model. Later erosion and activity can also modify the surface.
Put it together
Your lesson progress
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