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 The Solar System

6.1.1 The Earth

• Rotation: Earth rotates once about its axis every ≈ 24 h, producing the regular alternation of day and night.
• Day‑night explanation: Because the Sun illuminates only the half of the planet that is facing it, any point on the surface experiences a period of illumination (day) followed by a period of darkness (night) as the planet turns.
• Axis tilt: The rotation axis is inclined ≈ 23.5° to the orbital (ecliptic) plane.
• Seasonal explanation: The tilt means that during different parts of the year the Sun’s apparent height above the horizon is higher in one hemisphere and lower in the other, giving the four seasons.
• Lunar month: The Moon orbits Earth in ≈ 28 days; the changing Sun–Earth–Moon geometry produces the familiar lunar phases.
• Key consequence: The Earth’s rotation period (≈ 24 h) is much shorter than its orbital period (≈ 365 days), so a single location experiences a full day‑night cycle each rotation while the whole planet completes one revolution around the Sun each year.

6.1.2 Order of the planets, dwarf planets and other small bodies

Planets are listed in order of increasing distance from the Sun:

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

In addition to the eight planets the Solar System contains:

• Dwarf planets: e.g. Pluto, Eris, Haumea, Makemake, Ceres.
• Minor bodies: asteroids (main‑belt and near‑Earth), comets (short‑period and long‑period), meteoroids, and Kuiper‑belt objects.

Average orbital speed

6.1.3 Observed differences between inner and outer planets

6.1.4 The accretion model of Solar System formation

4.1 Role of gravity

Gravity is the driving force that causes a diffuse inter‑stellar cloud to collapse under its own weight, eventually forming a protostar (the future Sun) surrounded by a rotating disc of material.

4.2 Composition of the parent molecular cloud

• Dust grains (≈ 30 % of the mass): silicates, iron, carbonaceous material – the building blocks of rocky planetesimals.
• Gas (≈ 70 % of the mass): ~70 % hydrogen, ~28 % helium, plus trace heavy elements (C, O, N, Si, Fe, …) – the raw material for the giant‑planet envelopes.
• Heavy elements are initially locked in solid grains, allowing them to stick together more readily in the dense inner disc.

4.3 Formation of an accretion disc

The original cloud possessed a small net angular momentum. Conservation of angular momentum during collapse forces the material to spin faster, flattening into a rotating disc around the nascent Sun.

Key consequences of the disc:

• Temperature gradient: Very hot close to the Sun, progressively cooler toward the outer edge.
• Snow line (≈ 3 AU): Inside this distance water ice cannot survive; only refractory (high‑melting‑point) materials are solid.
• Beyond the snow line: Ices and volatile gases can condense, providing extra solid mass for planetesimals.

4.4 Growth of planetesimals

1. Inner disc (inside the snow line): Dust grains collide and stick, forming small, dense rocky planetesimals. The limited amount of solid material means these bodies remain relatively low‑mass.
2. Outer disc (beyond the snow line): Icy planetesimals grow larger because ices add mass. When a core reaches ≈ 10 M⊕ (Earth masses) its gravity becomes strong enough to attract surrounding hydrogen and helium.
3. Runaway accretion: Once the critical core mass is attained, the rate of gas capture accelerates dramatically, allowing the rapid build‑up of massive envelopes – the giant planets.

6.1.5 Linking the model to the observed inner/outer‑planet differences

1. Temperature gradient: Inside the snow line only metals and silicates condense → small, dense, rocky planets.
2. Availability of volatiles: Beyond the snow line ices and abundant H₂/He are present → icy cores can bind large gas envelopes.
3. Runaway gas accretion: Larger outer cores exert stronger gravity, pulling in massive amounts of hydrogen and helium.
4. Resulting architecture: Four small, rocky inner planets and four large, gaseous outer planets, exactly as observed.

6.1.6 Summary points for revision

• Gravity collapses a molecular cloud and drives material onto a central protostar.
• The cloud’s composition provides both solid dust (for rocky cores) and abundant light gases (for giant envelopes).
• Conservation of angular momentum creates a rotating accretion disc with a strong temperature gradient.
• Inside the snow line (≈ 3 AU) only refractory material condenses → small, dense, rocky planets.
• Beyond the snow line ices and gases condense → rapid core growth and runaway gas accretion → massive gaseous planets.
• These processes together explain why the inner Solar System contains four small, rocky worlds while the outer Solar System contains four large, gaseous giants.