3.1 The Sun and the planets

The Solar System contains one star, the Sun, and the objects held in orbit by its gravitational field. The eight planets in order are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. The first four are relatively small, rocky planets; the outer four are much larger and contain a high proportion of gases or ices.

A mnemonic can help memorise the order, but examination answers must state the actual planet names. Pluto is not one of the eight planets; it is classified as a dwarf planet.

3.2 Moons and natural satellites

A natural satellite is a naturally occurring body that orbits a planet or another minor body. Earth’s Moon is one example. Many outer planets have numerous moons because their large masses produce strong gravitational fields and because the outer Solar System contains abundant material.

An artificial satellite is a human-made object placed in orbit. The syllabus phrase ‘smaller Solar System bodies’ includes natural objects, while questions may use satellite diagrams to test gravitational orbit concepts.

3.3 Dwarf planets and minor planets

Minor planets orbit the Sun but are smaller than the major planets. Dwarf planets such as Pluto are approximately spherical because of their own gravity but have not cleared neighbouring objects from their orbital region. At this level, the important classification is that they orbit the Sun but are not among the eight planets.

Asteroids are mainly rocky or metallic bodies. Many are concentrated in the asteroid belt between Mars and Jupiter, although some have other orbits. Their sizes range from small fragments to bodies hundreds of kilometres across.

3.4 Comets

Comets are small bodies containing ice, dust and rock. They often travel in highly elliptical orbits. When a comet approaches the Sun, heating releases gas and dust, forming a bright coma and a tail.

The tail points generally away from the Sun because of solar radiation and charged particles from the Sun. It does not simply trail behind the comet in the same way as dust behind a moving vehicle.

3.5 Classification by motion

The most reliable way to classify a body is to state what it orbits. Planets, dwarf planets, asteroids and comets orbit the Sun. Moons orbit planets or minor planets. All of these motions are governed primarily by gravitational attraction.

Diagrams are often not to scale. A correct answer should rely on labels and relationships rather than apparent sizes or distances in an illustration.

Worked examples

Practical focus

Examination guidance

Check your understanding

1. Name the planets in order from the Sun.
2. What is a natural satellite?
3. Where is the main asteroid belt?
4. Why does a comet become more visible near the Sun?