The Solar System: A Young Reader's Guide

Welcome to Space

Look up on a clear, dark night and you will see the sky sprinkled with tiny points of light. Most of those lights are stars, far, far away. But our own little corner of space has something very special. We call it the solar system.

The solar system is our family in space. At its heart is a star we call the Sun. Around the Sun travel eight planets, including the one we live on — Earth. Spinning around the planets are moons, and zooming through the gaps are lumps of rock and ice called asteroids and comets.

In this book you will take a journey from the Sun all the way out to the cold, dark edge of the solar system. You will meet every planet, learn why some are hot and some are freezing, and discover surprising facts about each one. Strap in. Our adventure begins at the centre, with the brightest object of all.

The Sun, Our Star

The Sun is a giant ball of glowing hot gas. It is so big that more than a million Earths could fit inside it! Even though it looks small in the sky, that is only because it is very far away — about 150 million kilometres from us.

The Sun is a star, just like the twinkling stars you see at night. It looks different only because it is so much closer. Deep inside the Sun, tiny bits of gas crash together and make huge amounts of heat and light. This light streams out in every direction, and a small slice of it reaches Earth.

Without the Sun, our world would be dark, frozen and dead. The Sun's warmth keeps our planet at just the right temperature for water, plants, animals and people. Its light helps green plants grow, and those plants feed almost every living thing. The Sun is the engine that keeps the whole solar system going.

A quick but important rule: never look straight at the Sun, even when wearing sunglasses. Its light is so strong it can hurt your eyes.

Planets and Orbits

A planet is a large, round object that travels around the Sun. The path each planet follows is called its orbit. The orbits are shaped a bit like stretched circles, and the planets stay on their paths because of an invisible pulling force called gravity. Gravity is the same force that pulls a ball back down to the ground when you throw it up.

The planets do not all move at the same speed or take the same time to go around. The closer a planet is to the Sun, the faster it travels and the shorter its journey. One full trip around the Sun is called a year. Earth takes 365 days to make one orbit — that is exactly one of our years.

The eight planets fall into two groups. The four nearest the Sun — Mercury, Venus, Earth and Mars — are smaller and made of rock. We call them the inner planets. The four farthest away — Jupiter, Saturn, Uranus and Neptune — are enormous balls of gas and ice. We call them the outer planets, or the gas giants. Let's visit them one by one.

The Rocky Inner Planets

Mercury is the closest planet to the Sun and also the smallest. It races around the Sun faster than any other planet. Mercury has no air to trap heat, so it is scorching hot on its sunny side and freezing cold on its dark side. Its grey surface is covered in bowl-shaped dents called craters, made by space rocks crashing into it long ago.

Venus comes next, and it is the hottest planet of all — even hotter than Mercury. That is because Venus is wrapped in a thick blanket of cloud that traps the Sun's heat like a greenhouse. Venus is sometimes called Earth's twin because it is almost the same size, but you certainly would not want to live there.

Earth is our home, the third planet from the Sun. It is the only planet we know of that has liquid water on its surface and living things crawling, swimming, flying and growing all over it. From space, Earth looks like a beautiful blue marble swirled with white clouds and green and brown land. Earth has one Moon, which circles around us.

Mars is the fourth planet and is known as the Red Planet. Its surface is covered in rusty red dust. Mars is colder and smaller than Earth, and it has the tallest volcano in the whole solar system, a giant called Olympus Mons. Scientists have sent robot rovers to drive across Mars and search for signs that water — and maybe tiny life — once existed there.

The Giant Outer Planets

Beyond Mars, the planets become huge.

Jupiter is the biggest planet of all — so big that all the other planets could fit inside it together. Jupiter is a giant ball of gas with colourful swirling stripes. Its most famous feature is the Great Red Spot, a storm bigger than the whole Earth that has been spinning for hundreds of years. Jupiter has dozens of moons.

Saturn is the planet everyone loves, because it wears beautiful rings. These rings are not solid — they are made of billions of pieces of ice and rock, some as tiny as dust and some as big as a house, all circling the planet. Saturn is the second-largest planet and also a gas giant.

Uranus is a cold, blue-green planet. The strangest thing about Uranus is that it is tipped over on its side, so it seems to roll around the Sun like a barrel instead of spinning upright like the others.

Neptune is the farthest planet from the Sun, way out in the cold and dark. It is a deep, beautiful blue and has the fastest, wildest winds in the solar system. A trip around the Sun takes Neptune a very long time — about 165 Earth years!

Moons, Asteroids and Comets

Planets are not the only objects in the solar system.

A moon is an object that orbits a planet, just as a planet orbits the Sun. Earth has one Moon, but some planets have many. Our Moon is the brightest thing in the night sky and the only other place that people have ever walked on.

Between Mars and Jupiter there is a wide gap full of rocky lumps called asteroids. Together they form a ring around the Sun called the asteroid belt. Asteroids come in all sizes, from tiny pebbles to giant rocks hundreds of kilometres across.

A comet is a ball of ice, dust and rock that travels on a long looping path around the Sun. When a comet comes close to the Sun, the heat turns some of its ice into gas, and it grows a glowing tail that can stretch for millions of kilometres. People in the past sometimes thought comets were magical, but now we know they are simply frozen visitors from the edge of the solar system.

What About Pluto?

You may have heard of Pluto. For a long time it was called the ninth planet. But Pluto is very small and very far away, out past Neptune. In 2006, scientists decided to make a clear rule about what counts as a planet, and Pluto did not quite fit. So they gave it a new name: a dwarf planet.

Pluto is still out there, circling the Sun in the cold and dark. It is simply part of a different group now. Scientists have even sent a spacecraft, called New Horizons, to fly past Pluto and take the first close-up photos of it.

How We Explore Space

How do we know all these facts about places we can never walk to? People have been studying the sky for thousands of years. Long ago, all they had were their eyes. Then clever inventors built the telescope, a tube with special glass that makes faraway things look bigger and closer.

Today we also send spacecraft out into the solar system. Some carry astronauts, like the ones who landed on the Moon. Others are robots that travel for years to reach distant planets, taking photographs and measurements and sending them all the way back to Earth. Thanks to brave explorers and clever machines, we learn more about our solar system every single year.

What We Learned

What a journey! We started at the blazing Sun and travelled all the way out past Neptune to the dark edge of the solar system.

We learned that the Sun is a star at the centre, and that eight planets orbit around it. The four inner planets — Mercury, Venus, Earth and Mars — are small and rocky. The four outer planets — Jupiter, Saturn, Uranus and Neptune — are giant balls of gas and ice. We also met moons, the asteroid belt, icy comets and the dwarf planet Pluto.

The solar system is enormous, beautiful and full of wonders, and our planet Earth is a precious home within it. The next time you look up at the night sky, remember that you are looking out into your own home in space.

Want to keep exploring the cosmos? Float deeper into space with Understanding Our Universe, or read the true tale of how humans reached for the stars in A Short History of Flight.