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 🌞

Inner Planets – Rocky & Small

The four planets closest to the Sun – Mars, Venus, Earth, and Mercury – are made mostly of silicate rocks and metals. They are relatively small, with diameters ranging from about 4 000 km to 12 000 km. Because they formed in the warm inner part of the protoplanetary disc, the high temperatures caused volatile gases (like water, methane, ammonia) to evaporate, leaving behind heavy, solid materials. Think of it like baking cookies: the heat on the top crust makes it hard and dry, while the inside stays softer. Here, the heat made the inner planets “dry” and rocky.

Outer Planets – Gaseous & Large

Farther from the Sun, the four giant planets – Jupiter, Saturn, Uranus, and Neptune – are composed mainly of hydrogen, helium, and other gases. They are huge, with diameters up to 140 000 km. The cooler temperatures in the outer disc allowed gases to condense and stick together, creating massive, gas‑rich planets. Imagine a snowball that can keep growing because it’s cold enough to keep the snow glued together; the outer planets grew big because the “snow” (gas) didn’t evaporate.

Accretion Model – How the Solar System Grew

The accretion model explains how the Sun and planets formed from a rotating cloud of gas and dust. Below are the key ideas that answer the objective.

1. Gravity’s Role – The cloud’s own gravity pulled material inward. As particles clumped together, their combined mass increased, making them pull in more material. This is like a snowball rolling down a hill: the bigger it gets, the faster it rolls and the more snow it gathers.
2. Element Diversity in Interstellar Clouds – The cloud contained many elements (hydrogen, helium, carbon, oxygen, silicon, iron, etc.). In the hot inner region, only heavy elements could stay solid; lighter gases escaped. In the cold outer region, gases could condense and stick, forming giant planets rich in hydrogen and helium.
3. Rotation & Accretion Disc Formation – The cloud was spinning. Conservation of angular momentum caused it to flatten into a rotating disc around the centre of mass. Material in the disc spiralled inward to form the Sun, while particles farther out collided and stuck together to build planets. Picture a spinning pizza dough: the centre stays dense (the Sun) and the outer dough spreads out, where toppings (planets) form.

Key Equations & Concepts

• Gravitational force: \$F = G \dfrac{m1 m2}{r^2}\$ – shows how mass and distance control attraction.
• Escape velocity: \$v_{\text{esc}} = \sqrt{2 G M / r}\$ – explains why lighter gases escape from inner planets.
• Mass of a planet: \$M_{\text{planet}} = \dfrac{4}{3}\pi r^3 \rho\$ – relates size to density.

Planet Comparison Table

Quick Recap & Analogy

- Inner planets are like the dry crust of a cookie – solid, small, formed where it was hot.

- Outer planets are like a snowball in a freezer – cold, gas‑rich, and able to grow huge.

- The accretion model shows that gravity pulls material together, elements in the cloud provide the building blocks, and rotation creates a disc that feeds the Sun and seeds the planets.

- Remember the key equation for escape velocity: \$v_{\text{esc}} = \sqrt{2 G M / r}\$ – it explains why inner planets cannot hold onto light gases.