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This lesson covers the structure and composition of our solar system — as required by the Edexcel GCSE Physics specification (1PH0), Topic 7: Astronomy. This is a Paper 2 topic. You need to understand the arrangement of objects in the solar system, the difference between rocky and gas giant planets, and appreciate the scale of the universe from planets to galaxies.
The Sun is the star at the centre of our solar system. It is a massive ball of hot gas (mainly hydrogen and helium) that produces energy by nuclear fusion in its core.
Key facts about the Sun:
Exam Tip: The Sun is a star, not a planet. It produces its own light and heat through nuclear fusion. Planets do not produce their own light — they reflect light from the Sun.
There are eight planets in our solar system, orbiting the Sun. In order of increasing distance from the Sun, they are:
| Order | Planet | Type | Approximate Distance from Sun (million km) | Key Feature |
|---|---|---|---|---|
| 1 | Mercury | Rocky (terrestrial) | 58 | Smallest planet, no atmosphere |
| 2 | Venus | Rocky (terrestrial) | 108 | Hottest planet (thick CO₂ atmosphere) |
| 3 | Earth | Rocky (terrestrial) | 150 | Only planet known to support life |
| 4 | Mars | Rocky (terrestrial) | 228 | Red planet, thin atmosphere |
| 5 | Jupiter | Gas giant | 778 | Largest planet, Great Red Spot |
| 6 | Saturn | Gas giant | 1,427 | Famous ring system |
| 7 | Uranus | Ice giant | 2,871 | Rotates on its side |
| 8 | Neptune | Ice giant | 4,498 | Strongest winds in the solar system |
A useful mnemonic: My Very Excited Mother Just Served Us Nachos (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune).
Exam Tip: You must know the order of the planets from the Sun. Pluto is no longer classified as a planet — it was reclassified as a dwarf planet in 2006 by the International Astronomical Union (IAU).
The planets are divided into two main categories based on their composition:
| Feature | Rocky (Terrestrial) Planets | Gas Giant Planets |
|---|---|---|
| Which planets | Mercury, Venus, Earth, Mars | Jupiter, Saturn, Uranus, Neptune |
| Position | Inner solar system (closer to Sun) | Outer solar system (further from Sun) |
| Surface | Solid rocky surface | No solid surface — layers of gas and liquid |
| Size | Relatively small | Much larger |
| Density | Higher density | Lower density |
| Atmosphere | Thin or no atmosphere (except Venus) | Very thick atmospheres of hydrogen and helium |
| Moons | Few or none | Many moons |
A dwarf planet orbits the Sun and has enough mass for its gravity to pull it into a roughly spherical shape, but it has not cleared its orbit of other debris. The most well-known dwarf planet is Pluto. Others include Eris, Ceres, Haumea and Makemake.
A moon (natural satellite) is a body that orbits a planet. Moons are held in orbit by the gravitational attraction of the planet.
Asteroids are small, irregularly shaped rocky bodies that orbit the Sun. Most are found in the asteroid belt between Mars and Jupiter. They range in size from a few metres to hundreds of kilometres across.
Comets are small icy bodies that orbit the Sun in highly elliptical (elongated) orbits. When a comet passes close to the Sun, the ice heats up and produces a visible tail of gas and dust that always points away from the Sun (due to the solar wind).
graph TD
A["The Solar System"] --> B["The Sun<br/>(star at the centre)"]
A --> C["Inner Rocky Planets"]
A --> D["Asteroid Belt"]
A --> E["Outer Gas/Ice Giants"]
A --> F["Other Objects"]
C --> G["Mercury"]
C --> H["Venus"]
C --> I["Earth"]
C --> J["Mars"]
E --> K["Jupiter"]
E --> L["Saturn"]
E --> M["Uranus"]
E --> N["Neptune"]
F --> O["Dwarf Planets<br/>(e.g. Pluto)"]
F --> P["Comets"]
F --> Q["Moons<br/>(natural satellites)"]
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:#2980b9,color:#fff
style F fill:#8e44ad,color:#fff
style G fill:#1a1a2e,color:#fff
style H fill:#1a1a2e,color:#fff
style I fill:#1a1a2e,color:#fff
style J fill:#1a1a2e,color:#fff
style K fill:#1a1a2e,color:#fff
style L fill:#1a1a2e,color:#fff
style M fill:#1a1a2e,color:#fff
style N fill:#1a1a2e,color:#fff
style O fill:#1a1a2e,color:#fff
style P fill:#1a1a2e,color:#fff
style Q fill:#1a1a2e,color:#fff
The orbital period of a planet is the time it takes to complete one full orbit around the Sun. Orbital periods increase with distance from the Sun — planets further out take longer to orbit.
| Planet | Average Distance from Sun (AU) | Orbital Period |
|---|---|---|
| Mercury | 0.39 | 88 days |
| Venus | 0.72 | 225 days |
| Earth | 1.00 | 365.25 days (1 year) |
| Mars | 1.52 | 687 days (~1.9 years) |
| Jupiter | 5.20 | 11.9 years |
| Saturn | 9.54 | 29.5 years |
| Uranus | 19.2 | 84 years |
| Neptune | 30.1 | 165 years |
Exam Tip: Note that the further a planet is from the Sun, the longer its orbital period. This is because it has a greater distance to travel AND it moves more slowly (orbital speed decreases with distance).
The solar system is just a tiny part of a much larger structure:
| Scale | Description |
|---|---|
| Planet | A body orbiting a star (e.g. Earth orbiting the Sun) |
| Solar system | A star and all objects orbiting it |
| Galaxy | A collection of billions of stars held together by gravity |
| Universe | Everything that exists — all galaxies, all space, all matter and energy |
Exam Tip: Make sure you understand the hierarchy: planets orbit stars → stars exist within galaxies → galaxies exist within the universe. A very common exam mistake is confusing "solar system" with "galaxy" or "universe."