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This lesson covers the structure of our solar system, the properties of the planets, dwarf planets, moons, and other objects within it, as required by the AQA GCSE Physics specification (4.8.1). Space physics is a Physics-only topic and does not appear in Combined Science Trilogy. You need to understand the relative sizes, distances, and orbits of objects in our solar system, as well as how our understanding of the solar system has changed over time.
Our solar system consists of one star — the Sun — together with eight planets, their moons, dwarf planets, asteroids, and comets, all held in orbit by the Sun's gravitational field.
The Sun is a main sequence star that formed approximately 4.6 billion years ago from a collapsing cloud of gas and dust called a nebula. It contains about 99.86% of the total mass of the solar system. The Sun is composed mainly of hydrogen and helium, and it generates energy through nuclear fusion — hydrogen nuclei fuse to form helium nuclei, releasing enormous amounts of energy.
Key facts about the Sun:
Exam Tip: The Sun is not "burning" — it generates energy through nuclear fusion, not combustion. This is a common misconception. In fusion, hydrogen nuclei combine under extreme temperature and pressure to form helium, releasing energy according to E = mc squared.
There are eight planets in our solar system, divided into two groups: the inner rocky planets (terrestrial planets) and the outer gas giants (Jovian planets).
| Planet | Type | Approximate Distance from Sun (million km) | Approximate Diameter (km) | Number of Known Moons | Orbital Period |
|---|---|---|---|---|---|
| Mercury | Rocky | 58 | 4,879 | 0 | 88 days |
| Venus | Rocky | 108 | 12,104 | 0 | 225 days |
| Earth | Rocky | 150 | 12,742 | 1 | 365.25 days |
| Mars | Rocky | 228 | 6,779 | 2 | 687 days |
| Jupiter | Gas giant | 778 | 139,820 | 95 | 11.9 years |
| Saturn | Gas giant | 1,427 | 116,460 | 146 | 29.5 years |
| Uranus | Ice giant | 2,871 | 50,724 | 28 | 84 years |
| Neptune | Ice giant | 4,495 | 49,244 | 16 | 165 years |
| Feature | Rocky (Inner) Planets | Gas/Ice (Outer) Planets |
|---|---|---|
| Composition | Rock and metal | Hydrogen, helium, methane, ammonia |
| Size | Relatively small | Much larger |
| Density | High density | Low density (except cores) |
| Atmosphere | Thin or none (except Venus and Earth) | Thick, deep atmospheres |
| Moons | Few or none | Many moons |
| Rings | No rings | Ring systems |
| Distance from Sun | Close (inner solar system) | Far (outer solar system) |
Exam Tip: You do not need to memorise exact distances or diameters, but you must understand the relative order of the planets and be able to compare their properties. A useful mnemonic is: My Very Excellent Mother Just Served Us Nachos (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune).
The following diagram shows the organisation of objects in our solar system:
graph TD
A["Solar System"] --> B["The Sun (Star)"]
A --> C["Inner Rocky Planets"]
A --> D["Asteroid Belt"]
A --> E["Outer Gas/Ice Giants"]
A --> F["Kuiper Belt & Oort Cloud"]
C --> G["Mercury"]
C --> H["Venus"]
C --> I["Earth"]
C --> J["Mars"]
D --> K["Millions of rocky bodies between Mars and Jupiter"]
E --> L["Jupiter"]
E --> M["Saturn"]
E --> N["Uranus"]
E --> O["Neptune"]
F --> P["Dwarf planets, comets, icy bodies"]
style A fill:#2c3e50,color:#fff
style B fill:#f39c12,color:#fff
style C fill:#e74c3c,color:#fff
style D fill:#95a5a6,color:#fff
style E fill:#3498db,color:#fff
style F fill:#8e44ad,color:#fff
A dwarf planet orbits the Sun and has enough mass for gravity to pull it into a roughly spherical shape, but it has not cleared the neighbourhood around its orbit of other debris. The most well-known dwarf planet is Pluto, which was reclassified from a planet to a dwarf planet in 2006 by the International Astronomical Union (IAU).
Other dwarf planets include:
Exam Tip: If asked why Pluto was reclassified, the key reason is that it has not "cleared its orbital neighbourhood." Unlike the eight planets, Pluto shares its orbital region with many other Kuiper Belt objects.
A moon is a natural satellite that orbits a planet. Moons are held in orbit by the gravitational attraction of the planet they orbit. Earth has one moon (the Moon), while Jupiter and Saturn have dozens.
Asteroids are small, irregularly shaped rocky bodies that orbit the Sun. Most asteroids are found in the asteroid belt between Mars and Jupiter. They are remnants from the formation of the solar system — material that never formed into a planet due to the gravitational influence of Jupiter.
Comets are small bodies made of ice, dust, and rock — sometimes called "dirty snowballs." They travel in highly elliptical orbits around the Sun. When a comet approaches the Sun, the heat causes the ice to sublimate (turn directly from solid to gas), forming a glowing coma and a tail that always points away from the Sun due to the solar wind.
| Object | Composition | Orbit Shape | Location |
|---|---|---|---|
| Moon | Rock (varies) | Nearly circular around a planet | Orbiting planets |
| Asteroid | Rock and metal | Roughly circular around the Sun | Mostly asteroid belt |
| Comet | Ice, dust, and rock | Highly elliptical around the Sun | Kuiper Belt / Oort Cloud origin |
Our understanding of the solar system has changed dramatically over time.
The geocentric model (Earth-centred) was the dominant model for over 1,500 years. In this model, proposed by Ptolemy (around 150 AD), the Earth was at the centre of the universe and all other celestial bodies — including the Sun, Moon, planets, and stars — orbited around it. The model required complex additions called epicycles (small circles within larger circular orbits) to explain the observed retrograde motion of planets.
The heliocentric model (Sun-centred) was proposed by Nicolaus Copernicus in 1543. In this model, the Sun is at the centre and the planets (including Earth) orbit around it. This model was supported and refined by:
| Feature | Geocentric Model | Heliocentric Model |
|---|---|---|
| Centre | Earth | Sun |
| Proposed by | Ptolemy | Copernicus |
| Orbit shape | Circles with epicycles | Ellipses (Kepler) |
| Explains retrograde motion? | Poorly (needs epicycles) | Naturally (planets overtaking) |
| Supported by evidence? | No telescopic evidence | Galileo's observations |
Exam Tip: You may be asked to explain why the geocentric model was replaced. The key points are: Galileo observed the moons of Jupiter (showing not everything orbits Earth), the phases of Venus (only explained by the heliocentric model), and Kepler showed that orbits are ellipses, not circles. The heliocentric model was simpler and explained observations better.
The solar system is vast. To appreciate the scale:
The distances between objects in the solar system are enormously greater than the sizes of the objects themselves. If the Sun were the size of a football, the Earth would be a small grain of sand about 25 metres away, and Neptune would be a slightly smaller grain about 750 metres away.
Exam Tip: When answering questions about the solar system, always use correct scientific terminology. Say "gravitational attraction" not "gravity pulls," say "nuclear fusion" not "burning," and always specify whether you are talking about the orbit of a planet around the Sun or a moon around a planet.