The Solar System & The Accretion Model

Physics: Cambridge International GCSE Supplement, topic 6.1.3

The Solar System

Our solar system is a small part of the Milky Way Galaxy. It is made up of:

The four inner planets (Mercury, Venus, Earth, Mars) are small and rocky.
The four outer planets (Jupiter, Saturn, Uranus, Neptune) are large and gaseous.

Billions of years ago, the Solar System began as a huge cloud of gas and dust (called a nebula) in space.
The cloud contained many different elements, such as hydrogen, helium, carbon, oxygen, and iron.
Slowly, gravity began pulling the gas and dust particles closer.
As particles collided, they stuck together, forming bigger and bigger clumps.
As gravity pulled the cloud inward, it started to rotate.
This rotation caused the cloud to flatten into a thin, spinning accretion disc with most material near the centre.

Most of the material moved to the centre of the disc, where gravity made it collapse and heat up.
This eventually formed the Sun.
Smaller clumps of dust and rock in the disc collided and stuck together.
These growing bodies are called planetesimals. Over time, they grew larger, becoming the planets.

Close to the Sun:
It was very hot, so gases like hydrogen and helium escaped.
Only heavy, solid materials stayed, forming the small, rocky inner planets.
Far from the Sun:
It was cooler, so gases could stay.
Planets here gathered huge amounts of gas around a rocky core, becoming the large, gaseous outer planets.