Structure of the Solar System

The Milky Way is a galaxy containing billions of stars. The Sun is one of these stars.

The Sun

The Sun is the largest object in the Solar System. The Sun’s huge gravitational field keeps many other objects - planets, dwarf planets, asteroids and comets - in orbit around it.


The Earth is one of eight planets in the Solar System. The planets orbit the Sun at different distances.

Eight planets arranged in order of distance from the Sun.The Sun and its planets - Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and 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

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.


Moons are natural satellites that orbit a planet. Many planets have moons, and some planets have many moons - Saturn has more than 50. The Earth has just one moon - the Moon.

Dwarf planets

Pluto is a dwarf planet. The gravitational field of a dwarf planet is not strong enough to clear the neighbourhood, so there may be other objects in its orbit around the Sun. The Solar System contains hundreds of dwarf planets, including Ceres (the only dwarf planet in the asteroid belt).


The Solar System contains smaller objects called asteroids - these orbit the Sun in highly elliptical orbits, which are oval or egg-shaped and may take millions of years to complete. Asteroids are made of metals and rocky material. There are large numbers of asteroids orbiting the Sun in the asteroid belt between Mars and Jupiter. There are also many in a region beyond Neptune called the Kuiper Belt.


The Solar System also contains small objects called comets. Comets are similar to asteroids, but are made of rocky material, dust and ice. As a comet approaches the Sun, it begins to vaporise, which means that it turns into a gas. It then produces a distinctive tail.