Explain Solar System formation by gravitational accretion
Trace a rotating interstellar cloud into a disc and growing orbiting bodies.
Section 6.1.2 · The Solar System
Core and Extended; Physics 0625, 2026–2028. Use supplied distances, periods and stated orbital models. Drawings are not to scale. Written practice is self-checked.
Before you begin
- Recognise gravitational attraction.
- Distinguish a process model from a single event.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What does accretion describe?
What you will learn
- Trace a rotating interstellar cloud into a disc and growing orbiting bodies.
- Explain the material, gravity and rotation stages of an accretion model.
Begin with material and gravity
The accretion model begins with an interstellar cloud of gas and dust containing many elements. Gravity draws material together. As a region contracts, material concentrates toward a forming central star while the rotating material develops a flattened accretion disc. The model connects gravity, existing material and rotation rather than creating planets from empty space.
Accretion means growth by material collecting together. Small solid particles gather into larger bodies, and larger growing objects attract more material through gravity. Not every fragment becomes a planet: remaining bodies include asteroids and other small objects.
Connect stages without claiming every collision sticks
A useful sequence is cloud contraction, rotating disc, growing small bodies and larger planets. Gravity is important throughout, but collisions can also fragment bodies. A simple accretion diagram represents the net growth model rather than proving that all impacts always join perfectly.
The many elements in the cloud supply the materials from which different objects form. The disc’s conditions vary with distance from the forming Sun. The next lesson uses that variation to explain the smaller rocky inner planets and larger outer giants; rotation alone does not sort all materials by itself.
Pause and explain
Which sequence fits the accretion model?
Worked example
Explain why a model with cloud → rotating disc → larger bodies needs gravity.
Show the worked solution
- Gravity concentrates the initial cloud.
- Material gathers in a rotating disc rather than appearing from nothing.
- Gravitational attraction supports growth of larger bodies by accretion.
Answer Gravity drives contraction and gathering of material during accretion.
Try a topic test
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Describe three roles of the initial cloud in Solar System formation.
Give me a hint
Name its material, attraction and motion separately.
Compare my reasoning
- Gas and dust supply many elements.
- Gravity draws material together.
- Rotation is associated with a flattened accretion disc.
Material supply, gravitational contraction and rotating-disc formation.
Look for these in your work
- Include gas and dust.
- Include both gravity and rotation.
Order these model stages: growing planets, interstellar cloud, rotating disc, gathering small bodies.
Give me a hint
Begin with the material before larger bodies exist.
Compare my reasoning
- Start with the cloud.
- Contraction produces a rotating disc with gathering small bodies.
- Further accretion produces growing planets.
Interstellar cloud → rotating disc → gathering small bodies → growing planets.
Look for these in your work
- Retain causal order.
- Explain growth from existing material.
A simulation shows one impact breaking two small bodies apart. Does that single event disprove net accretion?
Give me a hint
Distinguish a local event from the overall growth history.
Compare my reasoning
- Not every collision must stick.
- Other gathering events can produce net growth.
- The model needs the whole sequence of material evolution, not one selected collision.
No; fragmentation can occur within an overall accretion process.
Look for these in your work
- Reject the all-collisions-stick assumption.
- Discuss net growth over many events.
Common mistakes
- Accretion uses pre-existing gas and dust.
- Rotation and gravity play different roles.
- Not every collision sticks and not every fragment becomes a planet.
What can you explain now?
Which sequence fits the accretion model?
Compare with the explanation
Cloud contraction, rotating disc, growth of bodies
This sequence includes the initial material, gravitational contraction, disc formation and later accretion.
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Make it stick
Explain the material, gravity and rotation stages of an accretion model.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.