The Solar System

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Planet Earth and Beyond The Solar System The Sun and everything that orbits it

The Solar System is the Sun and everything that travels around it: eight planets and their moons, rocky asteroids, dwarf planets, and many icy and dusty objects far out at its edge.

In this lesson you will learn:

• why the Sun is a star, and where its energy comes from
• the eight planets, their order and their special features
• how to read a table of facts about the planets
• how big the planets are and how far apart they are, using diagrams drawn to scale
• what asteroids, dwarf planets, the Kuiper Belt, the Oort Cloud and comets are
• how gravity keeps everything in orbit
• why Earth is the only planet known to support life

There are quick checks along the way. Try each one before you move on.

1. The Sun

The Sun is a star

The Sun is a star, not a planet.

Like all other stars, it produces large amounts of heat and light, and it does this continuously: day and night, without stopping.

The stars you see at night are also suns. They look like tiny points of light only because they are so much farther away from us than the Sun is.

Hydrogen gas Powerful nuclear reactions Helium gas + heat and light

Deep inside the Sun, powerful nuclear reactions take place all the time. In these reactions, hydrogen gas changes into helium gas.

The reactions release enormous amounts of energy, which leaves the Sun as heat and light. A tiny part of it reaches Earth as sunlight.

This is not burning like a fire. It is a nuclear reaction.

The Sun is enormous. Its diameter is about 1,4 million km, which is about 109 times the diameter of Earth.

In the diagram, Earth and the Sun are drawn to the same scale. Earth is the tiny dot; only part of the Sun fits in the picture.

The Sun is also extremely hot: its surface is about 5 500 °C.

A star Hydrogen changes into helium Heat and light, continuously

Remember:

• The Sun is a star.
• Like all stars, it produces large amounts of heat and light, continuously.
• Its energy comes from powerful nuclear reactions in which hydrogen gas changes into helium gas.

2. The Eight Planets

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

A sentence can help you remember the order: My Very Educated Mother Just Served Us Nachos. The first letter of each word is the first letter of a planet.

Four inner planets: rocky Mercury, Venus, Earth, Mars Four outer planets: gas giants Jupiter, Saturn, Uranus, Neptune

One special feature of a planet is what it is made of: its composition.

• The four inner planets (Mercury, Venus, Earth and Mars) are small and rocky, with solid surfaces.
• The four outer planets (Jupiter, Saturn, Uranus and Neptune) are gas giants: huge planets made mostly of gas, with no solid surface to stand on. Uranus and Neptune are also called ice giants.

Between the two groups lies the asteroid belt. You will meet it later in this lesson.

Table of facts 1: distance and size Planet Distance from the Sun (million km, about) Diameter (km) Mercury 58 4 879 Venus 108 12 104 Earth 150 12 756 Mars 228 6 792 Jupiter 778 142 984 Saturn 1 427 120 536 Uranus 2 871 51 118 Neptune 4 498 49 528

Scientists record the special features of each planet in a table of facts. This table gives each planet's distance from the Sun and its diameter (the distance straight across it, through the centre).

How to read it: find the planet in the first column, then read across its row. Earth is about 150 million km from the Sun, and its diameter is 12 756 km.

Table of facts 2: orbit and temperature Planet Time for one orbit (Earth days or years) Average temperature Mercury 88 days 179 °C Venus 225 days 464 °C Earth 365 days 14 °C Mars 687 days −47 °C Jupiter 12 years −108 °C Saturn 29 years −139 °C Uranus 84 years −197 °C Neptune 165 years −201 °C

The time a planet takes to travel once around the Sun is the time for one orbit. Earth takes about 365 days: one Earth year.

Planets far from the Sun have much longer journeys. Mercury takes only 88 days, but Neptune takes about 165 years.

The last column shows each planet's average temperature. A minus sign means below 0 °C.

Moons known in 2026 Planet Moons known in 2026 Mercury 0 Venus 0 Earth 1 Mars 2 Jupiter 95 Saturn 284 Uranus 29 Neptune 14

A moon is a natural object that orbits a planet. The number of moons is another special feature.

Mercury and Venus have no moons. Earth has 1 (the Moon) and Mars has 2.

The gas giants have many moons, and astronomers keep discovering more. That is why the table says known in 2026: these numbers will probably grow.

Size Distance from the Sun Number of moons known Composition Surface temperature Time for one orbit

Every planet, and every other object in the Solar System, has its own special features:

• its size
• its distance from the Sun
• the number of moons known
• its composition (what it is made of)
• its surface temperature
• the time it takes for one orbit

You can compare planets by reading these features from a table of facts.

3. Sizes & Distances

This diagram shows all eight planets drawn to the same scale, so you can compare their sizes fairly.

• Jupiter is the largest planet. Its diameter is about 11 times Earth's.
• Mercury is the smallest planet.
• Earth and Venus are almost the same size.
• Even Jupiter is small next to the Sun. The curve at the bottom is the edge of the Sun, drawn at the same scale.

Sizes compared with Earth Object Diameter compared with Earth (Earth = 1) Mercury 0,38 Venus 0,95 Earth 1,00 Mars 0,53 Jupiter 11,21 Saturn 9,45 Uranus 4,01 Neptune 3,88 The Sun (a star) about 109

Another way to compare sizes is to compare each diameter with Earth's. Earth counts as 1.

A number smaller than 1 means the planet is smaller than Earth: Mars (0,53) is only about half as wide as Earth. A number bigger than 1 means it is bigger: Jupiter (11,21) is about 11 times as wide.

Now the distances, drawn to scale. Each number tells you how many times farther from the Sun a planet is than Earth. Earth's distance (about 150 million km) counts as 1.

The four inner planets are crowded close to the Sun, while the outer planets are spread very far apart. Neptune is about 30 times farther from the Sun than Earth is.

At that scale the inner planets were squashed together, so here they are zoomed in, still to scale.

Mercury is about 0,39 of Earth's distance from the Sun, Venus about 0,72, and Mars about 1,52.

Sizes and distances need different scales. The planets are tiny compared with the huge spaces between them, so one diagram cannot show both well.

Largest: Jupiter Smallest: Mercury Inner planets close together Outer planets far apart

Remember:

• Jupiter is the largest planet (about 11 times Earth's diameter) and Mercury is the smallest (0,38 times).
• The Sun's diameter is about 109 times Earth's.
• The inner planets are close together near the Sun; the outer planets are very far apart.
• Neptune is about 30 times as far from the Sun as Earth.

4. Other Objects

Asteroids are rocky objects that orbit the Sun. Most of them are in the asteroid belt, between the orbits of Mars and Jupiter.

The largest is Ceres, about 2,77 times as far from the Sun as Earth.

A dwarf planet is round like a planet, but much smaller, and it shares its part of space with many other objects.

Pluto is the best-known dwarf planet. Its diameter is only 2 380 km, smaller than Mercury (4 879 km), the smallest planet. Pluto orbits far out in the Kuiper Belt, about 39,6 times as far from the Sun as Earth.

Beyond Neptune lies the Kuiper Belt: a region of many icy objects orbiting the Sun. Pluto is one of them.

On this diagram the distances are to scale. Neptune is about 30 times as far from the Sun as Earth, and Pluto about 39,6 times.

Much farther out, at the very edge of the Solar System, is the Oort Cloud: a huge collection of distant icy and dusty objects. It is so far away that its size can only be estimated. Scientists think it is between about 3 700 and 420 000 times as wide as the distance from the Sun to Earth.

From time to time, a comet from the Oort Cloud travels in close to the Sun. As it comes close, the Sun heats it and it can grow a glowing tail of gas and dust that we can sometimes see from Earth.

Asteroids: between Mars and Jupiter Dwarf planets, such as Pluto Kuiper Belt: icy objects beyond Neptune Oort Cloud: at the edge

Remember:

• Asteroids are rocky; most orbit between Mars and Jupiter.
• Dwarf planets, such as Pluto, are round but much smaller than planets.
• The Kuiper Belt is a region of icy objects beyond Neptune.
• The Oort Cloud is made of distant icy and dusty objects at the edge of the Solar System.
• Comets from the Oort Cloud come close to the Sun from time to time.

5. Orbits & Gravity

If you could look at the Solar System from the side, it would look like a flat disc or plate. The planets all orbit the Sun in nearly the same flat plane, a bit like marbles rolling around on a dinner plate.

An orbit is the curved path an object follows as it travels around another object.

The Sun spins at the centre of the Solar System, and the planets and all the other objects orbit it in the same direction.

Gravity is a force of attraction between objects. The Sun's gravity pulls every planet, asteroid and comet towards it.

Each planet is also moving forward very fast. The forward motion and the pull of gravity together bend the planet's path into an orbit around the Sun.

Think of swinging a ball on a string around your head: the string pulls the ball inwards, and the ball keeps going round. Let go, and the ball flies off. For the planets, gravity is the string.

Mercury 88 days Earth 365 days Neptune 165 years

Gravity keeps every object in a stable, predictable orbit. Because of this, scientists can predict where each planet will be many years ahead.

The farther a planet is from the Sun, the longer it takes to complete one orbit: Mercury takes 88 days, Earth about 365 days and Neptune about 165 years.

6. Earth & Life

Earth is the third planet from the Sun, about 150 million km away.

It is the only planet known to support life. Four conditions make this possible: the right temperature, water, sunlight and oxygen. The next steps look at each one.

Earth's distance from the Sun gives it the ideal temperature range for life. Its average temperature is about 14 °C.

Compare its neighbours:

• Venus is closer to the Sun and far too hot: about 464 °C. It is even hotter than Mercury (179 °C), although Mercury is closer to the Sun. Venus is covered by a very thick layer of gases that traps heat like a blanket.
• Mars is farther from the Sun and far too cold: about −47 °C.

Solid ice Liquid water Gas water vapour

Living things need water. In Earth's temperature range, water can exist in all three states:

• solid: ice and snow
• liquid: oceans, rivers and rain
• gas: water vapour in the air

Liquid water is especially important for life. Venus is far too hot for liquid water, and on Mars most of the water is frozen as ice.

Sunlight Grass Zebra Lion

Sunlight provides the energy in the food chain.

Green plants, such as grass, use sunlight to make their own food. A zebra that eats the grass gets some of that energy, and a lion that eats the zebra gets some of it next.

Without sunlight, the food chain would have no energy to start with.

Early life forms and algae Oxygen More complex life

Most living things need oxygen, and Earth's air has not always had much of it.

Over a very long time, early life forms and algae produced oxygen. Once there was enough oxygen, more complex life could evolve, including, eventually, animals and humans.

Comparing Earth with its neighbours Venus Earth Mars Order from the Sun 2nd 3rd 4th Distance from the Sun (million km, about) 108 150 228 Average temperature 464 °C 14 °C −47 °C Water Far too hot for liquid water Liquid, gas and solid Mostly frozen as ice Life None known Yes None known

Put the conditions together and you can see why Earth is the only planet known to support life:

• Temperature: Earth's distance from the Sun gives the ideal range.
• Water: it exists as a liquid, a gas and a solid.
• Sunlight: it provides the energy in the food chain.
• Oxygen: early life forms and algae produced enough for complex life.

The Sun is a star Eight planets orbit it Asteroids, dwarf planets, Kuiper Belt, Oort Cloud, comets Flat disc, same direction, held by gravity Earth: 3rd planet, the only one known to support life

The Solar System: full summary

• The Sun is a star. Powerful nuclear reactions change hydrogen gas into helium gas, so it produces large amounts of heat and light, continuously.
• Eight planets orbit it: Mercury, Venus, Earth and Mars (rocky), then Jupiter, Saturn, Uranus and Neptune (gas giants).
• Each object has special features: size, distance from the Sun, moons known, composition, surface temperature and time for one orbit.
• Jupiter is the largest planet and Mercury the smallest. The Sun is about 109 times Earth's diameter.
• Asteroids orbit between Mars and Jupiter; Pluto is a dwarf planet in the Kuiper Belt; the Oort Cloud is at the edge, and its comets sometimes come close to the Sun.
• The Solar System looks like a flat disc. The Sun spins, everything orbits in the same direction, and gravity keeps the orbits stable and predictable.
• Earth is the 3rd planet and the only one known to support life, thanks to the right temperature, water, sunlight and oxygen.

You are ready for the quiz.