The Solar System

This lesson explains The Solar System for Cambridge IGCSE Physics 0625. It covers the Core knowledge required by every candidate. Focus on the cause-and-effect explanation and the exact quantities being compared. The goal is to explain the idea accurately and apply it in an unfamiliar context.

What is in the Solar System?

The Solar System contains one star, the Sun, and eight planets in this order: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune. It also contains minor planets such as Pluto, dwarf planets, moons orbiting planets, asteroids (mostly between Mars and Jupiter), and comets on highly elliptical orbits.

The four inner planets are small and rocky. The four outer planets are large gas giants. The accretion model explains this split. Around 4.6 billion years ago, gravity pulled a slowly rotating cloud of gas and dust (a nebula) inwards. Most material collapsed to form the Sun. The rest flattened into a rotating accretion disc. Close to the hot young Sun, only rock and metal could remain solid, forming small rocky planets. Far out, it was cold enough for ice and gases to collect, forming the giants.

Distances are huge, so we describe them by light travel time. Light from the Sun takes about 500 seconds (8 minutes) to reach Earth.

How do you calculate orbital speed?

Average orbital speed equals the circumference of the orbit divided by the orbital period. In words: orbital speed = 2π × orbital radius ÷ orbital period. In symbols: $v = \dfrac{2\pi r}{T}$.

Quantity	Symbol	Unit
Orbital speed	$v$	m/s
Orbital radius	$r$	m
Orbital period	$T$	s

The Sun’s gravitational field weakens with distance. So planets further from the Sun travel more slowly and take longer to complete one orbit. Mercury orbits in 88 days; Neptune takes 165 years.

Worked example

Mars orbits the Sun at an average radius of $2.3 \times 10^{11}\ \text{m}$. Its orbital period is 687 days. Calculate the average orbital speed of Mars. Give your answer in m/s. [3]

Solution:

1. Equation: $v = \dfrac{2\pi r}{T}$
2. Convert the period to seconds: $T = 687 \times 24 \times 3600 = 5.94 \times 10^{7}\ \text{s}$
3. Substitute: $v = \dfrac{2\pi \times 2.3 \times 10^{11}}{5.94 \times 10^{7}}$
4. Answer: $v = 2.4 \times 10^{4}\ \text{m/s}$ (2 significant figures)

Mark scheme:

• M1: correct equation $v = \dfrac{2\pi r}{T}$ stated or used
• M1: period correctly converted to seconds ($5.9 \times 10^{7}\ \text{s}$)
• A1: answer $2.4 \times 10^{4}\ \text{m/s}$ (accept 24 000 m/s; unit required)

Common errors and how to correct them

• Leaving T in days. The single biggest mark-loser. Convert days to seconds first: multiply by 24, then 3600. Write the conversion as its own line.
• Using diameter instead of radius. Some questions give the diameter of the orbit. Halve it before substituting into $v = \dfrac{2\pi r}{T}$.
• Reversing the planet order. Examiners ask for planets “in order from the Sun”. A mnemonic helps: My Very Easy Method Just Speeds Up Naming.
• Vague accretion answers. “Dust came together” may not earn credit. Name the force (gravity), the starting object (a rotating cloud of gas and dust), and the disc.
• Calling comet orbits circular. Comets follow highly elliptical orbits with the Sun far from the centre. Planets follow orbits that are very nearly circular.

How to apply this in an exam

For any $v = \dfrac{2\pi r}{T}$ question, write three labelled lines before touching the calculator: $r$ in metres, $T$ in seconds, then the substitution. Method marks survive a calculator slip; a single unlabelled answer does not. If the answer is not between $10^{3}$ and $10^{5}\ \text{m/s}$ for a planet, recheck the time conversion, because that range is a quick sanity test worth memorising.

Where this skill matters

Solar System content appears in Paper 1 and Paper 2 as multiple-choice recall: planet order, rocky versus gaseous, light travel time. Papers 3 and 4 use structured questions mixing accretion-model description (2-3 marks) with an orbital speed calculation (3-4 marks). Because this is Core, the content is identical for Core and Extended candidates; Extended papers just embed the calculation in harder contexts. There is no practical work here, so Papers 5 and 6 rarely touch it. Malaysian candidates usually sit variant 2, and Space Physics has featured in every June and November series since the 2023 syllabus began.