Explain why inner and outer planets differ
Relate disc conditions to the smaller rocky inner planets and larger outer giants.
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
- Recall the eight-planet order.
- Describe gravitational accretion.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which four planets form the rocky inner group?
What you will learn
- Relate disc conditions to the smaller rocky inner planets and larger outer giants.
- Explain the broad inner–outer contrast through accretion conditions and gravity.
Compare the two groups
Mercury, Venus, Earth and Mars are the four smaller rocky inner planets. Jupiter, Saturn, Uranus and Neptune are the four larger outer planets with extensive gaseous envelopes. Jupiter and Saturn are gas giants; Uranus and Neptune are often called ice giants because their compositions differ. “Ice giant” does not mean a solid exposed ice ball.
The syllabus groups the outer four as large and gaseous in comparison with the inner four. That is a broad structural comparison, not a claim that every part of a giant is gas or that all outer planets have identical composition.
Use the accretion disc’s temperature difference
The hotter inner disc allowed mainly heat-resistant solid material to gather. Cooler outer regions allowed more icy material to be retained in growing bodies. Larger outer seeds could gather substantial gas under gravity, producing the broad inner-rocky and outer-giant contrast.
The explanation needs the many elements in the original gas/dust cloud, gravity and a rotating accretion disc. Distance labels alone describe the groups but do not explain their formation. Present-day density or temperature also cannot be inferred from radius alone without considering composition and atmosphere.
Pause and explain
Which explanation connects the two planet groups to accretion?
Worked example
Why does “the outer planets are far away” describe but not fully explain their large envelopes?
Show the worked solution
- Distance identifies their group.
- Cooler disc regions retained more material during formation.
- Growing bodies attracted gas through gravity, building large envelopes.
Answer Formation conditions and gravitational accretion supply the mechanism.
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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.
Name the inner and outer groups and compare their broad structures.
Give me a hint
Use all four members of each group.
Compare my reasoning
- List Mercury, Venus, Earth, Mars as rocky and smaller.
- List Jupiter, Saturn, Uranus, Neptune as larger outer giants.
- State that the outer group has extensive envelopes, with gas/ice giant distinctions.
Four rocky inner planets; four larger outer giants with extensive envelopes.
Look for these in your work
- Include every member.
- Avoid claiming identical composition.
Explain how cooler outer-disc conditions could support larger growing seeds.
Give me a hint
Link retained material to later gravitational gathering.
Compare my reasoning
- Cooler regions could retain additional icy material.
- More gathered solid material supported larger seeds.
- Their gravity could gather substantial gas envelopes.
Cooler retained material plus accretion supported large outer bodies.
Look for these in your work
- Include a material difference.
- Connect growth to gravity.
A pupil calls Uranus a solid ice ball because it is an ice giant. Correct that interpretation.
Give me a hint
Treat the category as a composition distinction, not a literal surface description.
Compare my reasoning
- Uranus is a large outer planet with an extensive envelope.
- Ice giant distinguishes its composition from Jupiter/Saturn.
- It does not imply a solid exposed ice surface.
Ice giant is a structural/composition category, not a solid-ice-ball claim.
Look for these in your work
- Retain the outer-giant category.
- Explain the naming limit.
Common mistakes
- Outer giants are not identical pure-gas spheres.
- An ice giant is not a solid ice ball.
- A distance label is not an accretion explanation.
What can you explain now?
Which explanation connects the two planet groups to accretion?
Compare with the explanation
Different disc conditions affected retained material and gas gathering
Temperature differences in a rotating gas/dust disc and gravity help explain the broad contrast in formed bodies.
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Make it stick
Explain the broad inner–outer contrast through accretion conditions and gravity.
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.