Section 1 of 5Members of the Solar System
The Sun and everything held in orbit around it make up the Solar System.
The Sun
- A star at the centre of the Solar System, whose gravity holds all the other objects in orbit.
Planets
- There are eight planets, and each one orbits the Sun on a nearly circular path.
Moons
- Natural satellites that orbit planets, held in orbit by the planet's gravity.
Dwarf planets and asteroids
- Dwarf planets such as Pluto and rocky asteroids, many in a belt between Mars and Jupiter, also orbit the Sun.
Comets
- Balls of ice and dust that move on long elliptical orbits and speed up as they pass close to the Sun.
Section 2 of 5The Planets
The Planets
Rocky planets
- Mercury, Venus, Earth and Mars are small and solid and lie closest to the Sun.
Gas giants
- Jupiter and Saturn are very large planets made mostly of hydrogen and helium.
Ice giants
- Uranus and Neptune are far from the Sun and are made mainly of icy materials and gases.
Order from the Sun
- The order is Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.
| Object | Orbits | Example |
|---|---|---|
| Planet | The Sun | Earth |
| Moon | A planet | The Moon around Earth |
| Comet | The Sun, in a long ellipse | Halley's comet |
| Artificial satellite | Earth or another body | Weather satellite |
Section 3 of 5Formation
The Solar System formed from a cloud of gas and dust about 4.6 billion years ago.
1
A nebula of gas and dust, left from earlier stars, began to collapse under its own gravity.
2
Most of the material fell to the centre, heated up and formed a star, the Sun.
3
Leftover material formed a spinning disc around the new star.
4
Within the disc, clumps pulled together under gravity to make planets, moons and smaller bodies.
5
Heavier elements were present because they came from earlier stars that exploded as supernovae.
Section 4 of 5The Galaxy and the Universe
Galaxy
- The Solar System is in the Milky Way, a galaxy containing billions of stars held together by gravity.
Universe
- The Universe contains billions of galaxies, spread over a vast distance.
Size order
- From smallest to largest the order is: planet, Solar System, galaxy, Universe.
Distances
- Distances between stars are so great that astronomers measure them in light-years, the distance light travels in a year.
Section 5 of 5Exam Checklist & Tips
8.3.4 – Key Points for the Exam
- Solar System The Sun together with all the objects that orbit it.
- Galaxy A very large group of stars, gas and dust held together by gravity.
- Dwarf planet A round body that orbits the Sun but has not cleared its orbit of other objects.
- Light-year The distance that light travels in one year.
- Satellite An object that orbits a larger object, either natural or artificial.
Common Mistakes
- Thinking a galaxy and the Solar System are the same thing; the Solar System is a tiny part of a galaxy.
- Describing a light-year as a unit of time instead of distance.
- Saying that planets make their own light; they only reflect sunlight.
- Mixing up the order of the planets or including Pluto as a planet.