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Space Physics: Question 6

Syllabus 6.1.2

Structured Core 6 marks

An astronomy club is building a scale model of the Solar System. Before choosing materials for each planet, the members research how the eight planets differ from one another.

(a) State how the four planets closest to the Sun differ from the four planets furthest from the Sun, in terms of their typical size and composition. [2]

(b) The club also reads about the accretion model, which explains how the Solar System formed from a collapsing interstellar cloud of gas and dust. Describe this model, including the role played by gravity, the range of elements present in the cloud, and the formation of a rotating disc of material, and use these ideas to explain why the model leads to small rocky planets forming close to the Sun and large gas/ice giants forming much further out. [4]

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Worked solution

Part (a): Rocky planets vs gas/ice giants

The eight planets of the Solar System are not all alike:

  • The four planets closest to the Sun (Mercury, Venus, Earth and Mars) are relatively small and rocky (solid).
  • The four planets furthest from the Sun (Jupiter, Saturn, Uranus and Neptune) are much larger and made mostly of gas (and ice).

Part (b): The accretion model

The accretion model describes how the Solar System is thought to have formed:

  • Gravity causes a huge interstellar cloud of gas and dust to begin collapsing inward under its own gravitational attraction.
  • This cloud already contains many elements, including heavier ones (not just hydrogen), produced by earlier generations of stars.
  • As the cloud collapses, it starts to rotate faster and flattens out into a spinning accretion disc, with most of the material pulled inward to form the Sun at the centre.

This same model explains why the planets differ with distance from the Sun:

  • Close to the hot young Sun, temperatures in the disc were high, so only rocky, metallic materials could condense into solid grains. These grains gradually stuck together (accreted) into the four small rocky planets.
  • Further out, the disc was much cooler, so lighter materials such as gases and ices could also condense and accrete alongside rocky material, allowing much larger gas/ice giant planets to grow.

Final answers

  • (a) The four inner planets are small and rocky; the four outer planets are large and gaseous
  • (b) Gravity collapses an element-rich interstellar cloud into a rotating accretion disc; the hot inner disc forms small rocky planets, while the cooler outer disc forms large gas/ice giants