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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."
Understanding the true scale of the solar system is one of the most demanding requirements of this topic. Distances are so large that numbers become hard to visualise, so examiners often ask candidates to compare relative sizes using a simple model.
Question: If the Sun is represented by a football of diameter 22 cm, how far away (on the same scale) would Earth be? How far would Neptune be? (Use: Sun diameter = 1.4 × 10⁶ km; Earth–Sun distance = 1.5 × 10⁸ km; Neptune–Sun distance = 4.5 × 10⁹ km.)
Step 1 — Scale factor. A 22 cm ball representing 1.4 × 10⁶ km gives a scale of 22 cm : 1.4 × 10⁶ km, or 1 cm : ~6.36 × 10⁴ km.
Step 2 — Earth's position. 1.5 × 10⁸ km ÷ 6.36 × 10⁴ = ~2,360 cm ≈ 23.6 m from the football-Sun.
Step 3 — Neptune's position. 4.5 × 10⁹ km ÷ 6.36 × 10⁴ = ~70,800 cm ≈ 708 m from the football-Sun — the length of seven football pitches.
This shows that the solar system is mostly empty space. Even with the Sun shrunk to a football, Neptune is more than half a kilometre away.
Common mistake: Candidates often confuse the diameter of the Sun with its distance from Earth, or quote the Earth–Moon distance when asked about the Earth–Sun distance. Always check the label on the number before substituting.
| Body type | Orbits around? | Light source? | Typical size |
|---|---|---|---|
| Star | Centre of a galaxy (or galactic core) | Produces own light via nuclear fusion | 10⁶ km (diameter) |
| Planet | A star | Reflects starlight | 10³–10⁵ km |
| Dwarf planet | A star (orbit not cleared) | Reflects starlight | 10²–10³ km |
| Moon (natural satellite) | A planet | Reflects starlight | 10¹–10³ km |
| Asteroid | A star (rocky) | Reflects starlight | 10⁰–10² km |
| Comet | A star (icy, elliptical orbit) | Reflects starlight; tail glows | 10⁰–10¹ km |
Exam Tip: In a 4-mark "describe" question on the structure of the solar system, examiners look for precise taxonomy — correctly naming the Sun as a star, naming at least four planets in order, identifying asteroids as being in the asteroid belt between Mars and Jupiter, and describing comets as having highly elliptical orbits. Vague words like "space rocks" or "balls of gas" lose marks.
flowchart LR
A["Moon (satellite)"] --> B["Planet"]
B --> C["Solar system<br/>(star + orbiting bodies)"]
C --> D["Galaxy<br/>(~100 billion stars)"]
D --> E["Observable Universe<br/>(~200 billion galaxies)"]
style A fill:#34495e,color:#fff
style B fill:#2980b9,color:#fff
style C fill:#f39c12,color:#fff
style D fill:#8e44ad,color:#fff
style E fill:#2c3e50,color:#fff
Exam-style question (6 marks): Describe the structure of the solar system and explain how it fits into the wider universe.
Grade 4–5 answer: The solar system has the Sun in the middle. Eight planets go around the Sun. The inner planets (Mercury, Venus, Earth, Mars) are rocky and the outer planets (Jupiter, Saturn, Uranus, Neptune) are big gas planets. There are also moons, asteroids and comets. The solar system is in the Milky Way galaxy. The Milky Way is one of lots of galaxies in the universe.
Grade 8–9 answer: The solar system consists of the Sun — a main sequence G-type star powered by the nuclear fusion of hydrogen into helium — together with eight planets held in near-circular orbital motion by the Sun's gravitational field. The four terrestrial inner planets (Mercury–Mars) are separated from the four outer gas/ice giants (Jupiter–Neptune) by the asteroid belt. Dwarf planets such as Pluto lie in the Kuiper Belt beyond Neptune. Natural satellites (moons) orbit most planets, while comets follow highly elliptical orbits and develop tails of ionised gas and dust that always point away from the Sun due to solar wind. The solar system is located within the Orion Arm of the Milky Way spiral galaxy, roughly 26,000 light years from the galactic centre. The Milky Way is one of approximately 200 billion galaxies in the observable universe, which has a radius of ~46.5 billion light years.
Edexcel alignment: This content is aligned with Edexcel GCSE Physics (1PH0) specification Topic 8 Astronomy (separate-science only) — specifically 8.1 Our solar system (structure and the Sun as a star), 8.2 Orbital motion (planets, moons, dwarf planets, asteroids and comets), and the introductory content for 8.5 Galaxies and the scale of the Universe. Assessed on Paper 2.