The Solar System, Grade 8
Every diagram of the solar system is a lie about distance.
The planets get drawn a few centimetres apart because a page cannot hold the truth. Lay the model out on a school field and Neptune is 562 metres from the Sun. That walk is the only way a class ever feels the scale, and it is the one piece of hands-on work this topic offers.
This page covers the Sun and the objects that orbit it: the Sun, the eight planets, dwarf planets, moons, asteroids, comets, meteors, the Kuiper Belt, the shape of the whole system, and why Earth of all the planets supports life. For light years, the Milky Way and galaxies see beyond the solar system, and for telescopes and the Southern Cross see looking into space.
Where this fits in the curriculum
| Subject | Natural Sciences |
|---|---|
| Grade | 8 |
| Term | 4 |
| Strand | Planet Earth and beyond |
| Topic | The solar system |
| Status | Informal activities. This grade puts all three assessed pieces in Terms 1, 2 and 3, so there is nothing prescribed here |
| Marks | 45 on our worksheet. The topic revision section is 30 |
The Sun
A medium-sized star, and the only one close enough to matter to us.
| Distance from Earth | 150 million km |
|---|---|
| Light takes | 8 minutes 20 seconds |
| Diameter | About 1,4 million km, so about 109 Earths side by side |
| Mass | 333 000 times the mass of Earth |
| Share of the solar system's mass | 99,8 % |
| Core temperature | About 15 million degrees Celsius |
| Surface temperature | About 5 500 degrees Celsius |
| Age | 4,6 billion years, with about 5 billion still to go |
That surface figure is worth holding on to. Grade 7 material on the Sun and the seasons prints about 6 000 degrees, which is the kelvin value with the wrong unit on it. 5 500 degrees Celsius is the number.
Check the light time yourself: 150 000 000 km divided by 300 000 km per second is 500 seconds, and 500 seconds is 8 minutes 20 seconds. That is the same sum our visible light page does from the other direction.
The Sun does not burn
You will see the Sun described as a burning ball of gases, and as having been burning for 4,6 billion years. It is not burning and it never has been.
Burning means combustion, combustion needs oxygen, and there is no oxygen. What the Sun does is fuse hydrogen into helium, turning about 600 million tonnes of hydrogen into helium every second and releasing energy as it goes.
The wrong word does real damage. A learner who thinks the Sun is on fire has no way to answer why it has not run out of air, and no route into understanding what a star actually is.
The same mistake is printed a year earlier. Grade 7 material on sources of energy says of the Sun that "this source will burn out", inside a unit on renewable and non-renewable sources, which is where the idea takes hold.
Sunspots, solar flares and prominences
| Feature | What it is |
|---|---|
| Sunspot | A cooler, darker patch on the surface, caused by the Sun's magnetic field. Still about 3 500 degrees, so it only looks dark next to 5 500 |
| Solar flare | A sudden burst of energy and radiation thrown off the surface |
| Prominence | A loop of glowing gas arching out and back along the magnetic field |
Flares reach us. The particles they throw out disturb satellites, radio signals and power grids, and they are what makes the aurora.
Never look at the Sun
Not directly. Not through sunglasses. Not through binoculars, a telescope, a lens, a prism or a pinhole.
Damage to the retina is permanent, and it does not hurt while it is happening, so nothing warns you to stop.
To show sunspots, project the image onto card and stand with your back to the Sun. Never view through anything.
The three tests for a planet
Something is a planet only if all three are true.
- It orbits the Sun
- It is big enough that its own gravity has pulled it into a round shape
- It has cleared its neighbourhood, meaning it is gravitationally dominant in its own orbit
The third one is the one learners leave out, and it is the only one that does any work in the next question.
Why is Pluto not a planet?
Because it passes the first two tests and fails the third.
Pluto orbits the Sun, and it is round. But it sits in the Kuiper Belt among thousands of other icy objects and has not cleared them out of its path. It is one of a crowd rather than the boss of its own orbit.
It was reclassified in 2006 as a dwarf planet, along with Ceres in the asteroid belt and Eris, Haumea and Makemake further out.
Pluto was not demoted for being small. Size is not one of the three tests. What happened in 2006 is that the word "planet" was defined properly for the first time, and Pluto fell on the wrong side of the third test. Pluto did not change. The definition did.
Orbits are ellipses, not circles
You will see a planet defined as travelling in a regular circular path, with the key words repeating "clear circular path". Orbits are ellipses: squashed circles.
Grade 8 does not need the mathematics, but it does need the word, because the consequence is visible. Mars is sometimes about 3 light minutes from us and sometimes about 22. A circular orbit could not do that.
The inner and outer planets
| Inner | Outer | |
|---|---|---|
| Which | Mercury, Venus, Earth, Mars | Jupiter, Saturn, Uranus, Neptune |
| Made of | Rock and metal | Mostly hydrogen and helium |
| Size | Small | Large |
| Moons | 0, 0, 1, 2 | Many |
| Rings | None | All four have them. Saturn's are simply the ones you can see |
The asteroid belt separates them, and the split exists because of heat. Close to the young Sun it was too hot for ice and gas to hold together, so only rock and metal survived. Further out they could hold together, so the outer planets grew enormous.
A gas giant has no surface. You would sink through thickening gas until it became liquid. Any temperature quoted for Jupiter, Saturn, Uranus or Neptune is a cloud-top temperature, not a surface one, and it is worth saying so rather than letting a learner picture standing on Jupiter.
And Jupiter's temperature is not set by its distance from the Sun alone. Jupiter radiates more energy than it receives, left over from the slow contraction it is still going through.
How many moons does Jupiter have?
Over 90, as counted in 2026. Saturn has over 270.
| Planet | Moons, counted in 2026 |
|---|---|
| Mercury, Venus | 0 |
| Earth | 1 |
| Mars | 2 |
| Jupiter | over 90 |
| Saturn | over 270 |
| Uranus | at least 28 |
| Neptune | at least 16 |
| Pluto | 5 |
You will see Jupiter and Saturn given "just over 60 moons each", and Pluto given three. Pluto has five, and Saturn's count has more than quadrupled.
These numbers move every time a survey finishes, because the new moons are small, dark and a long way off. So write the number with the year next to it. That is what a scientist does, and it is what an examiner should accept.
The part of the answer that never goes out of date: Saturn has the most, Jupiter is second, and the inner planets have almost none.
The asteroid belt
A band of rocky bodies orbiting the Sun between Mars and Jupiter.
It is far emptier than the pictures suggest. Mostly empty space, with rocks a very long way apart. Spacecraft fly through it without difficulty.
Ceres, about 950 km across, is the largest object in it and it is classed as a dwarf planet rather than an asteroid.
One claim to qualify: you will read that asteroids have iron cores just like Earth's. The large ones that melted early did separate into a core and a crust. Most asteroids are rubble and have no core at all.
Meteoroid, meteor and meteorite
Three words for the same rock in three different places.
| Word | Where it is |
|---|---|
| Meteoroid | Out in space |
| Meteor | Burning up in our atmosphere. The streak of light |
| Meteorite | The piece that survives and reaches the ground |
Comets, and which way the tail points
A comet is ice, dust and rock. Close to the Sun the ice turns to gas and it grows a glowing head and a tail.
| Part | What it is |
|---|---|
| Nucleus | The solid lump of ice and dust |
| Coma | The fuzzy glowing cloud around the nucleus |
| Tail | Gas and dust blown off it |
Ask a class which way the tail points before telling them. Almost everyone says it streams out behind, like hair in the wind.
It always points away from the Sun, whether the comet is coming or going, because it is the solar wind and the Sun's own light pushing the gas and dust away. On the way out, a comet travels tail first.
Halley's comet returns every 75 or 76 years. It was last seen from Earth in 1986 and is next due in 2061. A comet loses ice and dust on every close pass, so it cannot keep doing this forever.
The shape of the solar system, and where it ends
| Region | Where |
|---|---|
| Inner planets | Out as far as Mars |
| Asteroid belt | Between Mars and Jupiter |
| Outer planets | Jupiter out to Neptune |
| Kuiper Belt | Beyond Neptune. Pluto lives here |
| Oort Cloud | Far further out, and a shell rather than a flat belt. Where long-period comets come from |
You will see the Kuiper Belt described as being outside the solar system, beyond Neptune. Beyond Neptune is right. Outside the solar system is wrong.
The Kuiper Belt is part of the solar system, and so is the Oort Cloud. The solar system does not stop at Neptune. It goes on as far as the Sun's gravity keeps winning, which is thousands of times further out than Neptune is.
Gravity holds all of it together, and it is the Sun everything orbits because the Sun has 99,8 % of the mass and therefore by far the strongest pull.
Why Earth, of all of them, supports life
Earth is the third planet out, and its distance from the Sun is what makes everything else possible.
| What Earth has | Why the distance matters |
|---|---|
| Water as a liquid, a gas and a solid | Earth sits in the narrow band where water can be liquid at all. Closer in it boils away, further out it stays frozen |
| Oxygen, about 21 % of the air | Nitrogen is 78 % and everything else about 1 %. Living things need the oxygen to release energy |
| An atmosphere that holds heat in | It keeps the temperature swing survivable |
| Sunlight to drive the food chain | Plants use it to make their own food, and everything else eats the plants or eats the eaters |
The oxygen was not always there. Early simple life, the kind we now call cyanobacteria, began releasing it long before anything complex existed. Photosynthesis is what keeps the level up today, which is why our photosynthesis practical is really a page about the air you are breathing.
The water cycle runs on the Sun too. The Sun evaporates water, it condenses, it falls, and it runs back to the sea. Earth is a closed system for water: the amount does not change, it only moves.
The practical: walk the solar system
One period, a field, and a 100 m tape measure. A 5 m tape will not do this.
- Cut nine cardboard circles to the model radii in the table below, one per group, and tape each to a stick
- Find a straight line about 600 m long. A field boundary, a long driveway, or the pavement down one side of the school
- Plant the Sun at one end. Every distance is measured from there
- Walk each planet out with the tape and plant it
The scale model table, recomputed
Sizes are at 1 to 1 billion. Distances are at 1 to 8 billion. Those are two different scales, and that matters: see below.
| Body | Real distance from the Sun | Model distance | Model radius |
|---|---|---|---|
| Sun | 0 m | 696 mm | |
| Mercury | 58 million km | 7,25 m | 2,4 mm |
| Venus | 108 million km | 13,5 m | 6,1 mm |
| Earth | 150 million km | 18,75 m | 6,4 mm |
| Mars | 228 million km | 28,5 m | 3,4 mm |
| Jupiter | 778 million km | 97,25 m | 71,5 mm |
| Saturn | 1 427 million km | 178,4 m | 60,3 mm |
| Uranus | 2 871 million km | 358,9 m | 25,6 mm |
| Neptune | 4 498 million km | 562,25 m | 24,8 mm |
Every model distance is the real distance divided by 8 billion. Check any row with a calculator and it comes out.
THE FIGURE THAT IS PRINTED WRONG, AND IT WILL RUIN THE ACTIVITY.
Published versions of this table give Mars a model distance of 26,5 m. At the table's own scale, 228 million km is 28,5 m.
Every other row divides correctly, which is exactly why the error is easy to miss. A class with a tape measure will plant Mars two metres short and nothing on the field will look wrong. Use 28,5 m.
What the class should notice
| What they see | What it means |
|---|---|
| All four inner planets fit in the first 30 metres | And then there is nothing at all for 70 metres |
| Jupiter to Neptune takes up most of the walk | The outer solar system is mostly emptiness |
| Earth is a grain 19 metres from a beach ball | This is the moment the diagram breaks |
| Neptune is out of sight of the Sun | On most fields you cannot see one from the other. That is the result, not a problem |
The honest answer to "is it to scale?"
It is two scales at once, and a class should be told.
The balls are at 1 to 1 billion. The distances are at 1 to 8 billion. So the distances have been squeezed by a factor of eight to make the model fit on a field.
At a single honest scale of 1 to 1 billion:
| Body | Size | Distance from the Sun |
|---|---|---|
| The Sun | 1,4 m across. A large beach ball | |
| Earth | 13 mm. A pea | 150 m |
| Neptune | 50 mm | 4,5 km |
4,5 km is most of the way across a town. That is the real shape of the solar system, and admitting that the field version cheats is more honest and far more memorable than pretending it does not.
If it does not work
| Problem | Cause | Fix |
|---|---|---|
| You run out of field | 562 m needs a long straight line | Halve every distance and tell the class you have, or run it along a pavement |
| Mars sits wrong next to Earth | You used 26,5 m | 28,5 m |
| A group cannot cut its planet out | Mercury at 2,4 mm is smaller than a pencil dot | Say so. Mark it with a pin head and let the impossibility be the lesson |
| The class thinks the model is fully to scale | Two scales in one table | Give them the 4,5 km figure. Then they know what was traded away |
| The markers blow over | Cardboard on sticks | Push them in properly, or weight each one with a stone |
How the 45 marks are made up
| Part | Marks |
|---|---|
| The Sun | 10 |
| The planets, inner and outer | 10 |
| Dwarf planets, asteroids, comets and meteors | 10 |
| Moons, the Kuiper Belt and the shape of it | 8 |
| The scale model | 7 |
Nothing in this topic is prescribed for formal assessment. This grade puts all three assessed pieces in Terms 1, 2 and 3. The topic revision section is 30 marks, counted and checked. Our worksheet is 45 and the split above is ours.
Free worksheet and marking memo
Both free, no sign up, straight to the PDF.
- Learner worksheet, 45 marks, including the scale-model calculation, the Pluto question and the comet tail
- Marking memorandum, with what to accept for each answer and the nine answers that look right and score nothing
Related practicals
- Visible light, Grade 8. The same 8 minute 20 second sum from the other direction, plus the spectrum, colour and the eye
- Beyond the Solar System, Grade 8. The Milky Way, light years, galaxies, black holes and dark matter
- Looking into Space, Grade 8. Telescopes, SALT, the Southern Cross and African star lore
- Electric circuits, Grade 8. The Term 3 neighbour
- Static electricity, Grade 8. Five practicals that need only a balloon
- The Sun and the seasons, Grade 7. The tilt, the solstices, and why we are closest to the Sun in January and it is still summer
- Sources of energy, Grade 7. Where the Sun is first met as an energy source, and the megawatt written as a milliwatt
What you need to run it
The whole practical needs one thing you probably do not already own: a 100 m tape measure at R105. Everything else is cardboard, sticks and a field.
That is the cheapest practical in our whole range, and it produces the one lesson about the solar system that a diagram cannot.
| Item | Price | What it is for |
|---|---|---|
| Measuring tape, PVC, 100 m | R105 | The practical. Buy this one |
| World globe, 30 cm | R400 | Earth in the room while you talk about the rest |
| Solar system wall chart | R85 | The system at a glance |
| The planets wall chart | R85 | Planet by planet |
| Solar system model, lit | R1 700 | A demonstration model that shows the orbits turning |
One honest note on the wall charts. We hold the astronomy charts in ones and twos, not in depth, and this is the month they sell. If you want a set for a corridor rather than a single chart for a classroom, ask us first and we will tell you what we can actually send this week.
For a learner building a project at home, the solar system model project kit at R68 has the parts in one packet, and it is the one page on our site where the parent and the teacher want different things from the same topic.
The models and charts are in astronomy, and the full chart range is in wall charts.