The Solar System and Gravity: KS3 Physics Guide

The solar system is a family of eight planets, their moons, and countless smaller bodies all bound by the gravity of one star — the Sun. Understanding gravity, orbits, and the structure of the solar system is a key part of the KS3 physics curriculum, typically taught in Year 9.

What objects make up the solar system?

The solar system contains the Sun and everything that orbits it. The main categories are:

Planets — large bodies that orbit the Sun and have cleared their orbital neighbourhood. There are eight planets.

Dwarf planets — smaller bodies that orbit the Sun but have not cleared their neighbourhood (e.g. Pluto, Eris).

Moons (natural satellites) — bodies that orbit planets rather than the Sun directly (e.g. the Moon orbits Earth).

Asteroids — rocky, metallic bodies, most of which orbit in the asteroid belt between Mars and Jupiter.

Comets — small icy bodies with highly elliptical orbits; they develop a bright tail when close to the Sun as ice vaporises.

The eight planets in order from the Sun

Planet | Approx. distance from Sun (million km) | Number of moons
Mercury | 58 | 0
Venus | 108 | 0
Earth | 150 | 1
Mars | 228 | 2
Jupiter | 778 | 95
Saturn | 1,430 | 146
Uranus | 2,870 | 28
Neptune | 4,500 | 16

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The inner four planets (Mercury, Venus, Earth, Mars) are small and rocky. The outer four (Jupiter, Saturn, Uranus, Neptune) are gas and ice giants, much larger in size. Jupiter is the largest planet — more than 1,300 Earths would fit inside it.

What is gravity?

Gravity is an attractive force that acts between any two objects that have mass. Every object with mass pulls every other object with mass towards it — the more massive the object, the stronger the pull.

Two key rules:

Gravitational force increases as the masses of the objects increase.

Gravitational force decreases as the distance between the objects increases.

The Sun is by far the most massive object in the solar system (it contains about 99.8% of all the mass), so it exerts the dominant gravitational pull that holds all the planets in their orbits.

Why do planets orbit the Sun?

A planet in orbit is constantly falling towards the Sun due to gravity — but because it also has a sideways velocity, it keeps missing. The result is a curved path: an orbit.

Gravity provides the centripetal force (the inward force needed to keep an object moving in a circle). Without gravity, a planet would fly off in a straight line (Newton's first law). The planet's orbital speed must be precisely matched to the gravitational pull at that distance: too fast and it escapes; too slow and it spirals inward.

Planets closer to the Sun orbit faster because the gravitational pull is stronger. Mercury, the innermost planet, completes one orbit in just 88 Earth days. Neptune, the outermost, takes about 165 Earth years.

What is the difference between mass and weight?

This is one of the most commonly confused pairs in KS3 physics.

| Mass | Weight
Definition | Amount of matter in an object | Gravitational force acting on that mass
Unit | Kilogram (kg) | Newton (N)
Depends on location? | No — constant everywhere | Yes — varies with gravitational field strength
Measured with | Balance (e.g. kitchen scales) | Newtonmeter (spring scale)

Weight is calculated using:

W = m × g

where W = weight in newtons (N), m = mass in kilograms (kg), and g = gravitational field strength in N/kg.

On Earth's surface: g = 9.8 N/kg (often rounded to 10 N/kg at KS3)

On the Moon's surface: g = 1.6 N/kg (about one-sixth of Earth's)

Worked example: weight on Earth and on the Moon

A student has a mass of 60 kg. Calculate her weight on Earth and on the Moon.

On Earth: W = m × g = 60 × 9.8 = 588 N

On the Moon: W = m × g = 60 × 1.6 = 96 N

Her mass remains 60 kg in both places — mass never changes. Her weight falls to about one-sixth because the Moon's gravitational field is weaker.

What is a light-year?

A light-year is a unit of distance, not time. It is the distance that light travels in one year in a vacuum:

1 light-year ≈ 9.46 × 10¹⁵ metres (about 9.46 trillion kilometres).

The nearest star to the Sun, Proxima Centauri, is about 4.2 light-years away. Even at the speed of sound in air (340 m/s), it would take over 37 million years to reach it — illustrating the enormous scale of even our local galactic neighbourhood.

The Department for Education's Science Programmes of Study for Key Stage 3 requires pupils to describe the solar system, understand the role of gravity in maintaining orbits, and distinguish between weight and mass. BBC Bitesize KS3 Physics covers all of these topics with diagrams, videos, and practice questions (see bbc.co.uk/bitesize/subjects/zh2xsbk). NASA's Solar System Exploration site provides authoritative data on planetary distances, moon counts, and orbital periods at solarsystem.nasa.gov.

Frequently asked questions

What is the difference between weight and mass?

Mass is the amount of matter in an object, measured in kilograms (kg). It is the same wherever you go. Weight is the gravitational force acting on that mass, measured in newtons (N). Weight depends on the local gravitational field strength (g), which varies from planet to planet. The formula W = m × g connects them. On Earth g ≈ 9.8 N/kg, so a 10 kg object weighs 98 N. On the Moon (g = 1.6 N/kg) the same 10 kg object weighs only 16 N — lighter, but not less massive.

Why do planets orbit the Sun?

Planets orbit the Sun because gravity pulls them towards it, and their sideways (tangential) velocity stops them from falling straight in. The result is a curved, approximately circular path. Gravity provides the centripetal (inward) force that continuously changes the planet's direction of travel. If gravity suddenly disappeared, a planet would fly off in a straight line. Planets closer to the Sun need to travel faster to maintain their orbit, which is why Mercury (closest) moves much faster than Neptune (farthest).

How many planets are in the solar system?

There are eight planets in the solar system: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Pluto was reclassified as a dwarf planet in 2006 by the International Astronomical Union because it has not cleared its orbital neighbourhood of other debris. This definition change is sometimes controversial but is the current scientific consensus.

What is a light-year?

A light-year is a unit of distance — the distance that light travels through a vacuum in one year (approximately 9.46 × 10¹⁵ metres). It is used in astronomy because the distances between stars and galaxies are so vast that kilometres become impractical. The nearest star to our Sun (Proxima Centauri) is about 4.2 light-years away; the Andromeda Galaxy is about 2.5 million light-years away.

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