KS3 · Physics

Our solar system

Picture a poster with the planets lined up neatly beside the Sun. It's handy, but it hides two big truths: where the planets really are, and how far apart.

Start here

Travel outward from the Sun

Start at the Sun and step outward one region at a time. Tap each band. Every band is drawn the same size, so this shows the ORDER going outward, not the real distances.

↑ The Sun, at the centre

↓ Far beyond Neptune

Zone 1 of 6

The Sun

This is where everything starts. The Sun is a star: an extremely large sphere of hot gas that gives out light in all directions. Everything in our solar system orbits the Sun, and the force that holds the whole solar system together is gravity.

  • A star
  • At the centre

Going outward: small rocky planets, then the asteroid belt, then large gas planets, then dwarf planets and the Kuiper belt. The other stars are far, far beyond all of it.

A star or a planet?

A star (like our Sun)vsA planet (like Earth)

Start with the top row. It's the difference people mix up most.

Focus

Light

A star (like our Sun)

Gives out its own light (and other radiation) in all directions

A planet (like Earth)

Gives out no light of its own. It only reflects a star's light

The insight

This is the big one. When a planet shines in the night sky, you are seeing sunlight bouncing off it.

What it is

A star (like our Sun)

An extremely large sphere of hot gas

A planet (like Earth)

A very large sphere of rock or gas

Who orbits whom

A star (like our Sun)

Planets orbit it

A planet (like Earth)

It orbits a star

Size

A star (like our Sun)

Usually far bigger

A planet (like Earth)

Usually far smaller

Temperature

A star (like our Sun)

Usually much hotter, because of nuclear reactions inside it. It is NOT burning

A planet (like Earth)

Usually much cooler than a star

In the night sky

A star (like our Sun)

The other stars keep their pattern

A planet (like Earth)

Can look like a bright star, but moves slowly against the pattern of stars

What makes it that kind of object?

Answer the questions, find the object

Pick an object in space and answer each question about it. Your answers lead to exactly one kind of object.

Own light? → What does it orbit? → Round? → Orbit cleared?

5 kinds of object this tree can reach.

On An object in space. 2 branches to choose from.

Only one route fits any object. Try the dwarf-planet route: it's the one that catches people out.

How far is far?

How many Suns would reach Earth?

Imagine lining up copies of the Sun side by side, starting at the Sun and heading towards Earth. How many Sun-widths would it take to reach us?

Your estimate

125 Sun-widths

0 Sun-widths250 Sun-widths

Check your picture

What does a row of planets really show?

A classroom poster shows the Sun on the left and the eight planets lined up in a neat row beside it, nicely spaced out.

Which is closest to what you think the row of planets shows?
How sure are you?

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

The Sun, everything that orbits it, and the huge gaps no poster can show.

What you need to know

  • Our Sun is a star. The solar system is the Sun and everything that orbits it: 8 planets (many with moons), dwarf planets and smaller objects such as asteroids.
  • Going outward from the Sun: Mercury, Venus, Earth and Mars (small, rocky), the asteroid belt, Jupiter, Saturn, Uranus and Neptune (large, made of gas), then dwarf planets such as Pluto and the Kuiper belt.
  • The planets orbit the Sun in roughly circular orbits, all in the same direction and on the same flat surface. They are not in a line.
  • The distances in the solar system are huge compared with the sizes of the objects in it.

The big picture

Our solar system is the Sun (a star) and everything that orbits it. Going outward, there are the small rocky inner planets, the asteroid belt, the large gas outer planets, and then dwarf planets and the Kuiper belt beyond Neptune. The planets move around the Sun in roughly circular orbits on one flat surface, and the distances between everything are enormous compared with the sizes of the objects.

Key points

1A star gives out its own light and is hot because of nuclear reactions inside it, not because it is burning. A planet only reflects a star's light.
2A planet orbits a star, has been pulled into a sphere by its own gravity, and has cleared its orbit. A dwarf planet is round but hasn't cleared its orbit. An asteroid is neither round nor has it cleared its orbit.
3Moons orbit planets. Earth has one moon; some planets have many and some have none.
4Gravity is the force that holds the solar system together.
5The other stars are not part of our solar system. They are over 10 000 times further away than Neptune.

Worked example

Problem

Object X gives out no light of its own. It orbits the Sun, somewhere between the orbits of Mars and Jupiter. Its own gravity has not pulled it into a sphere: it's lumpy. What kind of object is Object X?

⚠ Watch out

Saying the Sun is 'burning'. Burning needs oxygen, and there is no oxygen in space for the gas to burn with. Stars are hot because of nuclear reactions inside them.

🧠

Memory hook

Rock, belt, gas, beyond: that's the journey out from the Sun. For the planets in order, try: My Very Easy Method Just Speeds Up Naming (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune).

✓

Check yourself

Without looking back: name the inner planets in order, say where the asteroid belt and the Kuiper belt are, and explain why a planet can look like a bright star.

Flashcards

(13)
What kind of object is our Sun?
A star: an extremely large sphere of hot gas that gives out light (and other radiation) in all directions.
Why are stars hot?
Nuclear reactions inside them. They are not burning: burning needs oxygen, and there is none in space.
What is our solar system?
The Sun and all the objects that orbit it: 8 planets (many with moons), dwarf planets and smaller objects such as asteroids.
List the eight planets in order, starting nearest the Sun.
Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.
How are the inner planets different from the outer planets?
The four inner planets are smaller and made mostly of rock. The four outer planets are larger and made of gas.
What three things must be true for an object to be a planet?
It orbits a star, its own gravity has pulled it into a sphere, and its gravity has cleared its orbit of similar-sized objects.
What is a dwarf planet? Give two examples.
An object whose gravity has made it round but hasn't cleared its orbit. Ceres orbits between Mars and Jupiter; Pluto orbits beyond Neptune.
What is a moon?
A large natural object that orbits a planet. Earth has one; some planets have many and some have none.
Where are the two big collections of asteroids?
The asteroid belt, between the orbits of Mars and Jupiter, and the Kuiper belt, beyond Neptune.
Describe how the planets move.
They orbit the Sun in roughly circular orbits at different distances, all in the same direction and on the same flat surface.
Why can't a picture show the solar system's distances and sizes to scale?
The distances are huge compared with the objects. Earth is about 107 Sun-widths from the Sun, so at true spacing the planets would be too small to see.
Are the other stars in the night sky part of our solar system?
No. They are not between the planets; they are over 10 000 times further away than Neptune.
What force holds the solar system together?
Gravity. For example, there are gravity forces between Earth and the Sun, and between Earth and the Moon.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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How this lesson was checked. This KS3 Physicslesson was published through Lightbulb Learning's human-designed editorial process — the educational standards, accuracy rules and publication checks it must pass were authored and approved by Philip Halpin. It passed subject-specific assessment, automated educational checks and technical publication verification before going live (publication checks completed 30 September 2026). Published pages are monitored, human spot-checking is ongoing across the lesson library, and anything found wrong is corrected or withdrawn. How our lessons are made and checked. Spotted a mistake? Email hello@lightbulblearning.co and we'll review it.