The universe contains over 100 billion galaxies. A galaxy is a group of billions of stars. Our own galaxy is called the Milky Way, and it contains about 300 billion stars (300,000,000,000) and one of these is our Sun.

Planets and other objects go round the Sun, and these make up the solar system, with the Sun at the centre. The solar system contains different types of objects including:

• a star - the Sun

• planets, which go around the Sun

• satellites, which go around planets

• smaller objects such as asteroids and comets

The planets in order from the Sun

Mercury to Neptune

There are eight planets in the solar system. Starting with Mercury, which is the closest to the Sun, the planets are:

• Mercury

• Venus

• Earth

• Mars

• Jupiter

• Saturn

• Uranus

• Neptune

If you can't remember the correct order, try this sentence, or make one up of your own:

My Very Easy Method Just Speeds Up Naming

Pluto and the dwarf planets

Scientists have discovered other objects orbiting the Sun. These include comets, asteroids and dwarf planets, like Pluto and Eris.

Pluto used to be considered the ninth planet in our solar system. But in 2006 scientists renamed it as a dwarf planet. So now we have 8 planets in the solar system.

Heliocentric model

People used to think that the Earth was at the centre of the universe, with everything going around it. We now know that this is not correct. Scientific observations and space exploration show that the Sun is at the centre of our solar system. The Earth and other planets go around it. We say that the planets are in orbit around the Sun.

This model is called the heliocentric model. (Helios is the ancient Greek word for Sun.)