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 furthest from the Sun are gaseous and large, and explain this difference by referring to an accretion model for Solar System formation, including:

1. The model’s dependence on gravity.
2. The presence of many elements in interstellar clouds of gas and dust.
3. The rotation of material in the cloud and the formation of an accretion disc.

1. Observed Differences Between Inner and Outer Planets

2. The Accretion Model of Solar System Formation

2.1 Role of Gravity

Gravity is the fundamental force that pulls together the material in a collapsing interstellar cloud. The attractive force between two masses \$m1\$ and \$m2\$ separated by distance \$r\$ is given by

\$F = G\frac{m1 m2}{r^{2}}\$

where \$G\$ is the universal gravitational constant. As the cloud contracts, gravity becomes stronger, increasing the density of the central region and leading to the formation of a protostar.

2.2 Composition of the Interstellar Cloud

The parent molecular cloud contains:

• Dust grains composed of silicates, iron, carbonaceous material – the building blocks of rocky planets.
• Gas, primarily hydrogen (≈ 70 %) and helium (≈ 28 %), plus trace amounts of heavier elements (oxygen, carbon, nitrogen, etc.) – the raw material for giant planets.

Because heavy elements are initially locked in solid grains, they can clump together more readily in the dense inner region of the disc, whereas the abundant light gases remain in the outer, cooler parts.

2.3 Rotation and Formation of an Accretion Disc

The original cloud possessed a small net angular momentum. As it collapsed, conservation of angular momentum caused the material to spin faster, flattening into a rotating accretion disc around the nascent Sun.

Key consequences of the disc:

• Temperature gradient – hotter near the Sun, cooler farther out.
• In the hot inner disc, volatile gases cannot condense; only refractory (high‑melting‑point) materials survive, leading to the formation of small, dense, rocky planetesimals.
• In the cool outer disc, ices and gases can condense, allowing planetesimals to grow rapidly by accreting large envelopes of hydrogen and helium, producing massive gaseous planets.

3. Linking the Model to the Observed Planetary Differences

1. Temperature gradient: Inside the “snow line” (the distance where water ice can survive), only metals and silicates are solid, so planetesimals remain small and dense.
2. Availability of gas: Beyond the snow line, abundant hydrogen and helium remain in the disc, and icy planetesimals can capture these gases, leading to rapid mass increase.
3. Gravitational runaway: Larger cores in the outer disc exert stronger gravity, pulling in more gas and growing into the giant planets we observe.
4. Resulting architecture: The inner Solar System contains four small, rocky planets; the outer Solar System contains four large, gaseous planets.

4. Summary Points for Revision

• Gravity drives the collapse of a molecular cloud and the subsequent accretion of material.
• The cloud’s composition provides both solid dust (for rocky cores) and abundant light gases (for giant envelopes).
• Rotation creates an accretion disc with a temperature gradient that determines which materials can condense where.
• These processes explain why the inner planets are small and rocky, while the outer planets are large and gaseous.