Know that, in comparison to each other, the four planets nearest the Sun are rocky and small and the four planets furthest from the Sun are gaseous and large, and explain this difference by referring to an accretion model for Solar System formation,

6.1.2 The Solar System

Learning objective

Know that, in comparison to each other, the four planets nearest the Sun are rocky and small and the four planets farthest from the Sun are gaseous and large, and explain this difference by referring to an accretion model for Solar‑System formation. The explanation must mention:

1. the role of gravity,
2. the mixture of gases and dust in the original interstellar cloud, and
3. the rotation of the cloud and the resulting accretion disc.

1 The Solar System – main components

• Sun – a G‑type main‑sequence star at the centre of the system.
• Eight planets (listed in order from the Sun):

  1. Mercury
  2. Venus
  3. Earth
  4. Mars
  5. Jupiter
  6. Saturn
  7. Uranus
  8. Neptune

• Minor bodies – objects that are not planets: dwarf planets (e.g. Pluto), asteroids (main belt between Mars and Jupiter), comets and other small Solar‑System bodies.
• Moons – natural satellites orbiting the planets (e.g. Earth’s Moon, Jupiter’s four Galilean moons).

2 Daily motion, seasons and lunar phases (core AO1)

• Earth rotates once every ≈ 24 h, producing the daily cycle of daylight and darkness.
• Earth’s axis is tilted by ≈ 23.5° to its orbital plane; as Earth orbits the Sun in 365 days, this tilt causes the Sun’s apparent height in the sky to change, giving the four seasons.
• The Moon orbits Earth in ≈ 28 days. The changing illuminated portion of the Moon as seen from Earth produces the familiar lunar phases.

3 Comparison of the inner (rocky) and outer (gaseous) planets

Caption: Inside the frost line only refractory material can condense, producing the small, dense rocky planets. Beyond the frost line ices and abundant H/He can condense, allowing the formation of massive gaseous planets.

4 The accretion model for Solar‑System formation

4.1 Gravity – the driving force

• The original interstellar cloud (nebula) contained gas and dust.
• Mutual gravitational attraction caused the cloud to contract.
• As the cloud collapsed, gravitational potential energy was converted into heat, eventually forming a hot central protostar – the future Sun.

4.2 Material in the nebula

• Hydrogen and helium – ~98 % of the mass; remain gaseous unless the temperature is extremely low.
• Heavier elements (astronomical “metals”) such as C, O, Si, Fe exist as solid dust grains.
• Dust grains stick together on gentle collisions, growing into larger particles and eventually into planetesimals.

4.3 Rotation and formation of an accretion disc

• The nebula possessed a small initial rotation.
• During collapse, angular momentum is conserved; as the radius decreases, the rotation speed increases.
• The increasing spin flattens the material into a thin, rotating accretion disc surrounding the protostar.
• Within the disc, particles follow nearly circular orbits, allowing low‑velocity collisions that build up planetesimals and, later, planets.

4.4 How the model explains the inner–outer planet differences

1. Temperature gradient (the “frost line”) – The disc is hot close to the protostar.

   • Inside ~3–4 AU (the frost line) temperatures are too high for volatile gases (H₂, He) and ices (H₂O, CH₄, NH₃) to condense.
   • Only refractory materials (silicates, metals) can solidify, leading to the formation of small, dense, rocky planets.

2. Increased solid mass beyond the frost line – Cooler temperatures allow ices to freeze, dramatically increasing the mass of solid particles.

   • Larger planetesimals form quickly and, because of their greater gravity, can attract surrounding hydrogen and helium.
   • This runaway accretion produces the massive, low‑density gaseous giants.

3. Timescales – The inner disc loses its gas to the Sun relatively early, limiting the time available for gas capture.
4. In the outer disc the gas remains for a longer period, giving the giant planets enough time to accumulate thick envelopes.

4.5 Average orbital speed (core formula)

The orbital speed of a planet is given by

v = 2πr ⁄ T

where r is the orbital radius and T the orbital period. Example for Earth:

• r ≈ 1 AU = 1.5 × 10⁸ km, T ≈ 365 days = 3.16 × 10⁷ s
• v ≈ 2π(1.5 × 10⁸ km) ⁄ 3.16 × 10⁷ s ≈ 30 km s⁻¹

5 Diagram – collapsing nebula, accretion disc and frost line

6 Glossary (key terminology)

Protostar

A hot, contracting mass of gas that will become a star once nuclear fusion begins.

Planetesimal

A solid body, typically kilometre‑size, formed from the co‑addition of dust grains in the accretion disc.

Refractory material

Substances with high melting points (e.g., silicates, metals) that can remain solid at the high temperatures close to the Sun.

Volatile material

Substances with low condensation temperatures (e.g., water, methane, ammonia, hydrogen, helium) that remain gaseous unless the environment is cold.

Frost line (snow line)

The distance from the protostar beyond which temperatures are low enough for volatile ices to condense.

Accretion disc

A rotating, flattened disc of gas and dust surrounding a young star, where planets form.

7 Summary points

• The inner four planets are small and rocky because they formed inside the frost line where only refractory material could solidify.
• The outer four planets are large and gaseous because they formed beyond the frost line, where ices and abundant H/He could be incorporated and later captured by gravity.
• Gravity caused the original nebula to collapse; conservation of angular momentum produced a rotating accretion disc.
• The nebula’s mixture of gases (H, He) and dust (heavier elements) supplied the raw material for both rocky and gaseous planets.
• Minor bodies (asteroids, dwarf planets, comets) are leftovers from the same disc that never accreted into the eight major planets.
• Understanding how the Solar System formed explains why Earth’s position – just inside the frost line – gives it a solid surface and a stable climate, a point often used in discussions of planetary habitability and environmental stewardship.