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The Sun is the largest object in the solar system and was the first to form.

The Sun’s huge gravitational field keeps many other objects - planets, dwarf planets, asteroids and comets - in orbit around it.

The Sun alone contains 99.8% of the total mass in the solar system.

Planets

The Earth is one of eight planets in the solar system.

The planets orbit the Sun at different distances.

The following sentence may help with remembering the order of the names of the planets from the Sun outwards:

My Very Easy Method Just Speeds Up Naming

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

The different planets have different properties and conditions.

In general, as the distance from the Sun increases:

• the temperature decreases, for example, Mercury is 430 °C whereas Neptune is -200 °C;
• the time taken to orbit the Sun increases, for example, Mercury orbits once every 88 Earth days, but Neptune orbits once every 165 Earth years.

Astronomers have divided the planets into two groups of four:

• the four inner planets (Mercury, Venus, Earth and Mars), which are small, rocky, dense planets, close to the Sun;
• the four outer planets (Jupiter, Saturn, Uranus and Neptune), which are large balls of gas, far from the Sun. They are called the gas giants.

When the solar system formed, rocks (and other dense, heavy materials in the dust cloud such as iron and uranium) tended to gather closer to the Sun, and these materials combined together to form the inner planets.

Lighter gaseous substances gathered together further away from the Sun and formed the outer gas giants.

For a planet to form, its own gravity must be strong enough to make it round or spherical in shape.

Its gravitational field must also be strong enough to ‘clear the neighbourhood’, pulling smaller nearby objects into its orbit.