Our Solar System
The structure of the Solar System - the Sun, the eight planets and the smaller bodies - and how gravity sets an object's weight and keeps everything in orbit.
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Our place in space
The Solar System is our corner of the Universe. At its centre is the Sun, a star, and orbiting it are eight planets (including the Earth), along with their moons and a swarm of smaller bodies. In this module you will learn what is in the Solar System, how gravity sets your weight on different worlds, and how gravity keeps the planets in orbit.
What is in the Solar System
The Sun is orbited by several kinds of body:
Order the planets
Put the eight planets in order, starting nearest the Sun and moving outwards.
- Mercury
- Venus
- Earth
- Mars
- Jupiter
- Saturn
- Uranus
- Neptune
Match each body
- The Sun
- A natural satellite
- An asteroid
- A comet
- A star at the centre of the Solar System
- A moon that orbits a planet
- A small rocky body between Mars and Jupiter
- A small icy body on a long, stretched orbit
Nearest the Sun
Which planet orbits closest to the Sun?
- Mercury
- Earth
- Jupiter
- Neptune
Mass and weight
These two words are often mixed up, but in physics they mean different things.
Weight on the Moon
An astronaut has a mass of 80 kg. On the Moon the gravitational field strength is 1.6 N/kg. Use weight = mass x g to work out the astronaut's weight on the Moon. Give your answer in newtons.
Why you weigh less on the Moon
Your mass never changes, but your weight does, because it depends on the gravitational field strength (g) of the world you are on. On Earth g is about 10 N/kg, but on the Moon it is only about 1.6 N/kg, roughly a sixth as strong. So the same astronaut weighs about six times less on the Moon, even though they are made of exactly the same amount of matter.
Mass and weight
Select the TWO statements that are true.
- An object has the same mass on the Moon as on Earth
- Weight is a force measured in newtons
- Weight is the same everywhere in the Solar System
- Mass is a type of force
The Solar System in words
At the centre of the Solar System is the _____, a giant ball of hot gas. Eight _____ orbit it, held in place by the force of _____. The planet closest to the Sun is _____. Small, icy bodies that travel on long, stretched orbits are called _____.
The changing model
Long ago people believed the Earth sat at the centre of everything (the geocentric model). Today we use the heliocentric model. Tap the TWO statements that belong to the modern, heliocentric model.
- The Sun is at the centre of the Solar System
- The Earth is at the centre of everything
- The planets, including Earth, orbit the Sun
- The Sun and planets all orbit the Earth
- The Earth does not move at all
Gravity and orbits
Work through what keeps a planet in orbit and how our ideas changed.
- A planet moves around the Sun on a nearly circular orbit. What provides the force that keeps it in orbit?
- The planet travels at a steady speed. Why is its velocity still changing?
- Astronomers replaced the Earth-centred picture with a Sun-centred one. What is the Sun-centred model called?
Explain the difference
An astronaut weighs less on the Moon than on Earth, yet their mass is exactly the same. Explain why. Use the idea of gravitational field strength in your answer. Write in clear, full sentences.
- State what mass is and that it does not change
- State what weight depends on (gravitational field strength)
- Give the values of g on Earth and on the Moon
- Use weight = mass x g to explain why the weight is smaller on the Moon