Chapter 1 of 8

All 8 chapters
  1. 1The planets, readunderstood
  2. 2How big and how far, readunderstood
  3. 3Gravity and orbits, readunderstood
  4. 4Tilt and the seasons, readunderstood
  5. 5Seasons around the world, readunderstood
  6. 6Day length and the Sun's height, readunderstood
  7. 7Moons, asteroids and comets, readunderstood
  8. 8Exploring space, readunderstood

S3.12.1 Chapter 1

The planets

Some bright dots in the night sky are planets, not stars. What is a planet?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

  • You picked A. Read on. At the end, the chapter comes back to your guess.

  • You picked B. Read on. At the end, the chapter comes back to your guess.

  • You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

  • Name the eight planets in order from the Sun
  • Compare planets by size, distance and what they are made of

The Sun is a star. A planet is a big ball of rock or gas that goes round a star. Eight planets go round our Sun.

The Sun and everything that goes round it is called the solar system. The Earth is a planet, so you live in the solar system.

Draw it first

Draw the planets in order

  1. The Sun in the middle at the top. Eight rings go round it, with one planet on each ring. Last, the names are written beside the planets, in order from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune. The labels are Sun, Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and not to scale.

    1 Draw the Sun. It is the star in the middle.

  2. The Sun in the middle at the top. Eight rings go round it, with one planet on each ring. Last, the names are written beside the planets, in order from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune. The labels are Sun, Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and not to scale.Sunnot to scale

    2 Draw eight rings round the Sun. Put one planet on each ring.

  3. The Sun in the middle at the top. Eight rings go round it, with one planet on each ring. Last, the names are written beside the planets, in order from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune. The labels are Sun, Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and not to scale.MercuryVenusEarthMarsJupiterSaturnUranusNeptuneSunnot to scale

    3 Write the names in order. Start with the planet nearest the Sun.

In order from the Sun, the planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.

A sentence helps you remember the order. Use the first letters: M, V, E, M, J, S, U, N. Make your own sentence. Your own silly sentence will stay in your head best.

The picture is not to scale. The rings are spaced evenly so that all eight fit. In space the outer planets are far, far apart.

The four planets nearest the Sun are small and made of rock. They are rocky planets. The Earth is one of them. You can stand on a rocky planet.

Jupiter and Saturn are huge balls of gas. They are gas giants. They have no ground to stand on.

Uranus and Neptune are ice giants. They hold a lot of water and other icy stuff, under thick gas. They are not solid balls of ice.

See it three ways

As a picture

A table of the eight planets, nearest the Sun first. planet, type, million km. Mercury, rocky, 58. Venus, rocky, 108. Earth, rocky, 150. Mars, rocky, 228. Jupiter, gas, 778. Saturn, gas, 1430. Uranus, ice, 2870. Neptune, ice, 4500. The last column is the distance from the Sun in million km. The type is rocky, gas or ice, meaning rocky planet, gas giant or ice giant. Labels: planet, type, million km, and every name and number in the table.planettypemillion kmMercuryrocky58Venusrocky108Earthrocky150Marsrocky228Jupitergas778Saturngas1430Uranusice2870Neptuneice4500

In words

The table lists the planets in order from the Sun. The four nearest are rocky. Then come two gas giants and two ice giants. The distances grow bigger and bigger, and the gaps between the outer planets are huge.

The planets are very different in size. Jupiter is the biggest. It is about 11 times as wide as the Earth. Mercury is the smallest.

Two circles drawn at the same scale. A small circle is labelled Earth. A circle about eleven times as wide is labelled Jupiter. Labels: Earth, Jupiter.EarthJupiter
Earth and Jupiter at one scale. Jupiter is drawn 11 times as wide as the Earth.

Distances in space are so large that we count them in millions of kilometres. The Earth is about 150 million km from the Sun. Neptune is about 4 500 million km away.

A bar chart called million km from the Sun. The bar for Earth reaches 150. The bar for Mars reaches 228. The scale goes from 0 to 250 in steps of 50. Labels: million km from the Sun, Earth, Mars, 0, 50, 100, 150, 200, 250.050100150200250EarthMarsmillion km from the Sun
How far Earth and Mars are from the Sun. Read the two numbers for the next example.

Worked example

How much farther from the Sun is Mars than the Earth?

  1. Earth is about 150 million km from the Sun.
  2. Mars is about 228 million km from the Sun.
  3. Take away the smaller from the bigger: 228 − 150 = 78.
  4. Mars is about 78 million km farther from the Sun.

Answer: 78 million km

Try it

Draw Earth and Jupiter at one scale

You need: a big sheet of paper, a pencil, a string about 10 cm long, a ruler or a paper strip marked in cm.

Be safe: Keep the pencil point away from your face and eyes.

  1. Let 1 cm stand for 10 000 km. The Earth is about 12 700 km wide, so it is about 1.3 cm wide.
  2. Draw the Earth as a dot or a tiny circle, 1.3 cm wide.
  3. Jupiter is about 140 000 km wide, so it is about 14 cm wide. Tie the pencil to one end of the string. Mark the string 7 cm from the pencil point.
  4. Hold the mark on the paper with a finger. Pull the string tight and draw a full circle round your finger.
  5. Lay your Earth beside Jupiter. How many Earths would fit across it?

What you will see: About 11 Earths fit across Jupiter. The planet that looks like a tiny dot beside Jupiter is the world you live on.

If you cannot try it, this is what happens.
A large circle labelled Jupiter 14 cm wide. Across its middle sits a line of 11 small circles of the same size. Below is written about 11 Earths fit across. Labels: Jupiter 14 cm wide, about 11 Earths fit across.Jupiter 14 cm wideabout 11 Earths fit across

A wrong idea many people have

All the planets are rocky balls, like the Earth.

Why it feels right: We live on rock, and pictures show every planet as a round ball with a surface.

The test that breaks it: Look at the table. Only four planets are rocky. Jupiter is a gas giant. A ship that tried to land would sink through thick gas. It would find no ground.

What is true: Only Mercury, Venus, Earth and Mars are rocky. The other four are giants, made mostly of gas or of water and icy stuff.

Key words

planet
a big ball of rock or gas that goes round a star.
solar system
the Sun and everything that goes round it.
rocky planets
small planets made of rock. There are four.
gas giants
huge planets made of gas. Jupiter and Saturn.
ice giants
huge planets with a lot of water and icy stuff. Uranus and Neptune.

Back to your guess

The answer is A: Four are small and rocky, two are giant balls of gas, and two are giant and icy.

The four planets nearest the Sun are small and rocky. Jupiter and Saturn are giant balls of gas. Uranus and Neptune are giants with a lot of water and icy stuff. The planets are very different in size.

Check yourself

Try each one before you look. A wrong answer tells you where to go next.

  1. Going away from the Sun, which planet comes right after Mars?

    Tap your answer.

    1. Not this one. Venus is closer to the Sun than Mars. You went towards the Sun, not away from it. Go to S3.12.1. Try another.

    2. Yes, that is right. The order is Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune. Jupiter follows Mars.

    3. Not this one. Saturn is the planet after Jupiter. You skipped one planet. Go to S3.12.1. Try another.

    See the answer

    Jupiter

    The order is Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune. Jupiter follows Mars.

    • If you chose A: Venus is closer to the Sun than Mars. You went towards the Sun, not away from it. Go to S3.12.1.
    • If you chose C: Saturn is the planet after Jupiter. You skipped one planet. Go to S3.12.1.

  2. The picture shows the Sun and the eight planets, with the names left out. Which planet is third from the Sun?

    The Sun at the top and eight rings going round it, with one planet on each ring and no names. The labels are Sun and not to scale.Sunnot to scale
    Tap your answer.

    1. Not this one. Venus is second. If you counted the Sun as number one, every planet comes one place too early. Go to S3.12.1. Try another.

    2. Not this one. Mars is fourth. Count again: Mercury is first, Venus second. Go to S3.12.1. Try another.

    3. Yes, that is right. Mercury is first, Venus is second and the Earth is third.

    See the answer

    Earth

    Mercury is first, Venus is second and the Earth is third.

    • If you chose A: Venus is second. If you counted the Sun as number one, every planet comes one place too early. Go to S3.12.1.
    • If you chose B: Mars is fourth. Count again: Mercury is first, Venus second. Go to S3.12.1.

  3. Which of these is a gas giant?

    Tap your answer.

    1. Not this one. Mars is small and rocky. A gas giant is a huge planet with no ground. Go to S3.12.1. Try another.

    2. Not this one. Neptune is a giant, but it is an ice giant, not a gas giant. It has a lot of water and icy stuff. Go to S3.12.1. Try another.

    3. Not this one. Venus is a rocky planet, about the size of the Earth. Go to S3.12.1. Try another.

    4. Yes, that is right. The two gas giants are Jupiter and Saturn. Neptune and Uranus are ice giants.

    See the answer

    Jupiter

    The two gas giants are Jupiter and Saturn. Neptune and Uranus are ice giants.

    • If you chose A: Mars is small and rocky. A gas giant is a huge planet with no ground. Go to S3.12.1.
    • If you chose B: Neptune is a giant, but it is an ice giant, not a gas giant. It has a lot of water and icy stuff. Go to S3.12.1.
    • If you chose C: Venus is a rocky planet, about the size of the Earth. Go to S3.12.1.

  4. The Earth is about 12 742 km wide. Neptune is about 49 244 km wide. Which is wider, and about how many times?

    Tap your answer.

    1. Yes, that is right. 49 244 ÷ 12 742 is about 4. Round to 49 000 and 12 000 and you get 4 with a little left over.

    2. Not this one. You may think a far planet must be small. Neptune is far away, but it is wider than the Earth. Compare the two numbers. Go to S3.12.1. Try another.

    3. Not this one. Check the sum. 49 244 is about 4 times 12 742, not 40 times. Go to S3.12.1. Try another.

    4. Not this one. The two numbers are different. 49 244 is much bigger than 12 742. Go to S3.12.1. Try another.

    See the answer

    Neptune, about 4 times as wide

    49 244 ÷ 12 742 is about 4. Round to 49 000 and 12 000 and you get 4 with a little left over.

    • If you chose B: You may think a far planet must be small. Neptune is far away, but it is wider than the Earth. Compare the two numbers. Go to S3.12.1.
    • If you chose C: Check the sum. 49 244 is about 4 times 12 742, not 40 times. Go to S3.12.1.
    • If you chose D: The two numbers are different. 49 244 is much bigger than 12 742. Go to S3.12.1.

  5. The Earth is about 150 million km from the Sun. Neptune is about 4 500 million km from the Sun. About how many times as far is Neptune?

    Tap your answer.

    1. Not this one. You dropped a zero. 150 × 3 is only 450. Try 150 × 30. Go to S3.12.1. Try another.

    2. Yes, that is right. 4 500 ÷ 150 = 30. Neptune is 30 times as far from the Sun as the Earth.

    3. Not this one. You added a zero. 150 × 300 is 45 000, far more than 4 500. Go to S3.12.1. Try another.

    4. Not this one. You took one number away from the other. That gives how much farther, not how many times as far. Go to S3.12.1. Try another.

    See the answer

    30 times

    4 500 ÷ 150 = 30. Neptune is 30 times as far from the Sun as the Earth.

    • If you chose A: You dropped a zero. 150 × 3 is only 450. Try 150 × 30. Go to S3.12.1.
    • If you chose C: You added a zero. 150 × 300 is 45 000, far more than 4 500. Go to S3.12.1.
    • If you chose D: You took one number away from the other. That gives how much farther, not how many times as far. Go to S3.12.1.

  6. Ravi reads that Mercury is 58 million km from the Sun and Mars is 228 million km. How much farther from the Sun is Mars?

    See the answer

    170 million km

    228 − 58 = 170. Mars is 170 million km farther.

Keep it

Copy this into your exercise book.

Eight planets go round the Sun. Rocky: Mercury, Venus, Earth, Mars. Gas giants: Jupiter, Saturn. Ice giants: Uranus, Neptune. Jupiter is biggest.

Draw: The Sun, and eight rings round it with the planets named in order.

Reading with a child? A short page for the adult: what this chapter teaches and three questions to ask.

No internet tomorrow? Save the whole book as one file (911 KB). It opens on any phone.