# The Solar System

> Physics · CIE IGCSE 0625 (2026-2028)
> Source: https://www.owlsprep.com/study/cie-0625-u6-the-solar-system/

This guide covers all Core and Extended content for the Solar System subtopic in CIE IGCSE Physics 0625 Unit 6 Space Physics, including system components, the accretion model of Solar System formation, orbital motion, and Extended orbital speed calculations.

**Prerequisites:** [Basic understanding of forces and gravitational attraction](https://www.owlsprep.com/study/cie-0625-u1-forces-and-motion/)

## Learning objectives

- Identify all components of the Solar System per CIE 0625 syllabus requirements
- Explain the accretion model of Solar System formation and why inner planets are small/rocky and outer planets large/gaseous (Core)
- Describe the orbital motion of planets, asteroids and comets for Core tier questions
- Describe qualitatively how a planet's surface gravitational field strength depends on its mass and decreases with distance from the planet (Core)
- Calculate the time light takes to travel between Solar System objects using $t=d/c$ with $c=3.0\times10^8$ m/s (Core)
- Calculate orbital speed using $v=2\pi r/T$ for Extended tier assessments
- Distinguish between solar system object types to answer 2-3 mark exam questions

## Core: Components and Formation of the Solar System

**Solar System** — A system consisting of a star and all objects that orbit it, held together by gravitational attraction. Our Solar System's central star is the Sun.

*Example:* Our Solar System includes 8 major planets, dwarf planets, asteroids, comets, and natural satellites (moons).

Our Solar System formed approximately 4.6 billion years ago from a collapsing cloud of gas and dust. The Sun accounts for 99.8% of the total mass of the Solar System, making it the dominant gravitational force holding all orbiting objects in place.

1. Inner rocky planets (closest to the Sun): Mercury, Venus, Earth, Mars
2. Asteroid belt: Region between Mars and Jupiter containing millions of small rocky asteroids
3. Outer gas/ice giants: Jupiter, Saturn, Uranus, Neptune
4. Outer Solar System: Dwarf planets (e.g. Pluto, Ceres), icy Kuiper belt objects, and distant Oort cloud comets

> **mnemonic**
>
> My Very Easy Method Just Speeds Up Naming Planets = Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune

**Worked example:** A student lists the following objects in order of increasing distance from the Sun: Mercury, Venus, Mars, Earth, Jupiter. Identify the error in their list and write the correct order.

1. Recall the order of inner rocky planets from the Sun: Mercury first, then Venus, Earth, then Mars.
2. The student swapped the positions of Earth and Mars in their list.
3. Correct order: Mercury, Venus, Earth, Mars, Jupiter

**Why are the four planets nearest the Sun small and rocky, while the four furthest planets are large and gaseous?** Cambridge 0625 explains this difference using an **accretion model** of Solar System formation. The Solar System began as a rotating **interstellar cloud** of gas and dust.

- **The model depends on gravity.** Gravitational attraction pulls the gas and dust of the cloud together, so material clumps and grows (accretes) into larger bodies.
- **The interstellar cloud contains many elements.** The cloud of gas and dust holds many different elements, providing the raw material for both rocky and gaseous planets.
- **The rotating cloud forms an accretion disc.** As the cloud collapses under gravity it rotates, flattening into a spinning **accretion disc** around the young Sun, and the planets grow within this disc.

Close to the Sun it was too hot for gases and ices to condense, so only rocky material with a high melting point remained to accrete — forming the four small, rocky inner planets. Further from the Sun it was cold enough for large amounts of gas and ice to be captured as well, so the four outer planets grew into large gas giants.

**Worked example:** Explain, using the accretion model, why the four planets nearest the Sun are small and rocky while the four furthest planets are large and gaseous.

1. State the starting point: the Solar System formed from a rotating interstellar cloud of gas and dust containing many elements, pulled together by gravity into a spinning accretion disc.
2. Near the Sun it was very hot, so only materials with high melting points (rock/metal) could stay solid and accrete, forming the small, rocky inner planets.
3. Far from the Sun it was much colder, so gases and ices could also be captured and accrete, allowing the outer planets to grow large and gaseous.

> **Exam tip:** For the accretion model, learn the three marking points explicitly: (a) gravity pulls material together, (b) the interstellar cloud contains many elements, and (c) the rotating cloud forms an accretion disc.

## Core: Orbital Motion of Solar System Objects

**Orbit** — The regular, repeating curved path of an object around a more massive object, maintained by gravitational attraction between the two objects.

Different objects in the Solar System follow different shaped orbits at different speeds, as outlined below:

- Planets and asteroids follow almost circular orbits around the Sun, all lying in roughly the same flat plane and moving in the same direction
- Comets are small icy bodies that follow highly elliptical (oval-shaped) orbits, taking them very close to the Sun at one end and far out into the outer Solar System at the other
- A comet's gas and dust tail always points away from the Sun, pushed by the solar wind, whichever way the comet is moving

**Worked example:** State two differences between the orbit of a comet and the orbit of a planet.

1. Difference 1: Planet orbits are almost circular, while comet orbits are highly elliptical.
2. Difference 2: Comet orbits are often tilted far outside the flat plane of planet orbits, while all major planet orbits lie in roughly the same plane.

> **Exam tip:** When describing comet orbits at Core, focus on their highly elliptical shape and that the tail always points away from the Sun. How a comet's speed changes with its distance from the Sun is Extended-only content (see the Extended section).

## Core: Gravitational Field Strength and Light-Travel Time

Beyond naming the objects in the Solar System, CIE 0625 Core also expects you to describe how gravity varies from planet to planet, and to work out how long light takes to travel the huge distances between Solar System objects.

**Gravitational field strength** — The gravitational force acting on each kilogram of mass at a point. Near a planet's surface it fixes the weight of an object. At Core the syllabus treats it only qualitatively.

- The gravitational field strength at the **surface** of a planet depends on the **mass** of the planet: a more massive planet produces a stronger surface field, so an object of the same mass weighs more there.
- The gravitational field strength **around** a planet **decreases as the distance from the planet increases**: the further away you go, the weaker the field becomes.
- This is why the giant outer planets (large mass) have much stronger surface gravity than the small inner planets, and why a spacecraft feels a weaker pull the further it travels from a planet.

> **info**
>
> At Core you only need the qualitative relationships (stronger field for greater planet mass; weaker field at greater distance). You are **not** expected to use an equation such as $g = GM/r^2$ in 0625.

**Worked example:** Two planets, X and Y, have the same radius, but planet X has a greater mass than planet Y. State and explain which planet has the greater gravitational field strength at its surface.

1. The gravitational field strength at the surface of a planet depends on the mass of the planet.
2. Planet X has the greater mass, so it produces the stronger surface gravitational field.
3. Therefore an object of the same mass would weigh more on planet X than on planet Y.

**Speed of light** — Light travels through space (a vacuum) at a constant speed of $c = 3.0 \times 10^8$ m/s. Because Solar System distances are so large, light still takes a measurable time to cross them.

*Notation:* c

The time taken for light to travel a distance is found from the standard speed relationship, using the speed of light $c$:

$$t = \frac{d}{c}$$

Where:
- $t$ = time taken for the light to travel, in seconds (s)
- $d$ = distance travelled, in metres (m)
- $c = 3.0 \times 10^8$ m/s = the speed of light in a vacuum

**Worked example:** The average distance from the Sun to the Earth is $1.5 \times 10^{11}$ m. Calculate the time it takes light from the Sun to reach the Earth, in seconds and in minutes. Use $c = 3.0 \times 10^8$ m/s.

1. Step 1: Write the relationship for light-travel time:
2. $$t = \frac{d}{c}$$
3. Step 2: Substitute the distance and the speed of light:
4. $$t = \frac{1.5 \times 10^{11}}{3.0 \times 10^8}$$
5. Step 3: Calculate the time in seconds:
6. $$t = 500 \text{ s}$$
7. Step 4: Convert to minutes: $500 \div 60 \approx 8.3$ minutes, so sunlight takes about 8 minutes 20 seconds to reach the Earth.

> **info**
>
> The same method works for the distance between any two Solar System objects, for example the light-travel time from the Sun to Jupiter or between two planets. Always keep the distance in metres and $c$ in m/s so the time comes out in seconds.

> **Exam tip:** For light-travel time, remember $c = 3.0 \times 10^8$ m/s and use $t = d/c$, keeping $d$ in metres. For gravity, only the qualitative rules are needed at Core: more planet mass gives a stronger surface field, and greater distance gives a weaker field.

## Extended Only: Orbital Speed Calculations

**Orbital speed** — The linear speed of an object as it travels along its orbital path, calculated by dividing the circumference of the orbit by the time taken to complete one full orbit.

*Notation:* v

$$v = \frac{2\pi r}{T}$$

Where:
- $v$ = orbital speed in metres per second (m/s)
- $r$ = average radius of the orbit in metres (m)
- $T$ = time period for one full orbit in seconds (s)

Always convert given values to SI units before substituting into the formula to avoid unit mismatch errors. If a question asks for speed in km/h, you may keep $r$ in kilometres and $T$ in hours to save calculation time.

**Worked example:** The Earth orbits the Sun at an average radius of $1.5 \times 10^{11}$ m. It takes 365 days to complete one full orbit. Calculate the orbital speed of the Earth in m/s, giving your answer to 2 significant figures.

1. Step 1: Convert the time period from days to seconds: $365 \text{ days} = 365 \times 24 \times 60 \times 60 = 3.1536 \times 10^7 \text{ s}$
2. Step 2: Substitute values into the orbital speed formula:
3. $$v = \frac{2 \times \pi \times 1.5 \times 10^{11}}{3.1536 \times 10^7}$$
4. $$v \approx 3.0 \times 10^4 \text{ m/s}$$
5. Final answer: $3.0 \times 10^4$ m/s (2 significant figures)

> **info**
>
> **Extended — elliptical orbits and changing speed:** An object in an elliptical orbit does not move at a constant speed. It travels **faster when it is closer to the Sun** and **slower when it is further away**. This is why a comet speeds up as it approaches the Sun and slows down as it moves out to the far end of its highly elliptical orbit. This speed-with-distance behaviour is Extended-only content and should not be used to earn Core marks.

> **Exam tip:** If a question gives you the diameter of an orbit instead of the radius, remember to divide the diameter by 2 to get $r$ before substituting into the formula, as the formula uses orbit radius to calculate circumference. Extended candidates should also remember that an object in an elliptical orbit moves faster when closer to the Sun.

*Calculator:* allowed

## Common pitfalls

- **Wrong:** Listing Pluto as the 9th major planet in order from the Sun
  - Why it fails: Pluto is classified as a dwarf planet in the CIE 0625 syllabus, and only the 8 major planets are required for exam questions.
  - Correct: Only list the 8 major planets (Mercury to Neptune) when asked for the order of planets, and mention dwarf planets as separate objects.
- **Wrong:** Stating that a comet's tail points behind its direction of travel
  - Why it fails: Comet tails are pushed by solar wind, so they always point directly away from the Sun, regardless of the comet's direction of movement.
  - Correct: Explicitly state that comet tails point away from the Sun in all exam answers.
- **Wrong:** Using orbit diameter instead of radius in the $v=2\pi r/T$ formula
  - Why it fails: The formula uses orbit radius to calculate the circumference of the orbit, so using diameter will give a result double the correct value.
  - Correct: Always use the average distance from the orbiting object to the central body (orbit radius) in orbital speed calculations.
- **Wrong:** Forgetting to convert time period to seconds when calculating speed in m/s
  - Why it fails: Unit mismatch leads to incorrect results by factors of 3600 or more, which will lose you calculation marks.
  - Correct: Convert $T$ from hours/days/years to seconds before substituting into the formula for m/s results.
- **Wrong:** Describing asteroid orbits as highly elliptical like comets
  - Why it fails: Most asteroids in the main asteroid belt have nearly circular orbits, unlike comets.
  - Correct: Distinguish asteroid orbits (roughly circular) from comet orbits (highly elliptical) in descriptive answers.

## Cheatsheet

| Object Type | Core Key Features | Extended Key Content |
| --- | --- | --- |
| Sun | Medium star at centre, 99.8% of system mass | Central mass for orbital speed calculations |
| Inner Planets | Rocky, small, close to Sun: Mercury, Venus, Earth, Mars | Orbital radii ~$5.8 \times 10^{10}$ m to $2.3 \times 10^{11}$ m |
| Outer Planets | Gas/ice giants, large, far from Sun: Jupiter, Saturn, Uranus, Neptune | Orbital radii ~$7.8 \times 10^{11}$ m to $4.5 \times 10^{12}$ m |
| Asteroids | Rocky bodies in asteroid belt, near-circular orbits | Orbital speed calculations use average orbit radius |
| Comets | Icy bodies, highly elliptical orbits, tail points away from Sun | Orbital speed highest when closest to the Sun |
| Orbital Speed | N/A for Core | Formula: $v=2\pi r/T$, SI units: v (m/s), r (m), T (s) |

## What's next

Now that you have mastered the Solar System content for CIE IGCSE Physics 0625, you can move on to learning about stars, galaxies, and the Big Bang theory, which are the remaining core topics in Unit 6 Space Physics. If you are taking the Extended tier, make sure you practice plenty of orbital speed calculation questions to avoid common unit and formula errors. You should also work through structured practice questions covering all Space Physics topics to reinforce your knowledge and prepare for 3-5 mark long answer questions in your exam.

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