Look at the sky on a clear night and you can see thousands of stars. The universe contains countless millions more stars which are too far away to see. Our own part of this vast universe is called the Solar System. If we could travel from one side of the Solar System to the other it would take many years.
Telescopes enable us to see faraway objects such as galaxies.
The Solar System is made up of the Sun, nine planets, several moons (the number changes as more are discovered) and a band of rocks called the Asteroid Belt. The planets and the Asteroid Belt all travel round the Sun. we say they ‘orbit’ the Sun. They each take a certain time to round the Sun. The time is the planet’s ‘year’. As the planets orbit, they themselves spin. They all spin at different speeds. The time they take to spin round once is called a ‘day’. Many of the planets have moons. While the planets orbit the Sun, the moons orbit the planets.
All the planets are different sizes. The Sun is so big compared to the planets that only a tiny part of it fits on the page!
The planets vary greatly in size and all are a very long way from the Sun. The planets closest to the Sun have the shortest years as they do not have as far to travel. The planets with the shortest days are the ones that spin round fastest.
Scientists think that the Sun and planets grew out of a cloud of dust and gas about 4,600 million years ago. Part of this cloud collapsed and shrank and got very hot. This was the beginning of the Sun. The planets formed from the left-over gas and dust that circled the Sun.
Our Sun is a star. Stars form in enormous groups called ‘galaxies’. Our Sun is part of the Milky Way galaxy. Stars are so far apart that we use special units called ‘light years’ to measure distances between them. Light travels faster than anything else in the universe. But light takes about 80,000 years to cross from one side of the Milky Way to the other! We say the Milky Way measures 80,000 light years across.
Birth of the Solar System
These diagrams show how the Solar System probably began. The Sun formed first at the centre of the cloud. Specks of material bumped into each other and gradually built up into lumps. These grew to form the planets.
Close to the Sun, where it was hottest, rocky planets grew. They had iron at their centres. These are the ‘inner planets’. Further from the Sun, where it was cooler, giant gas planets grew. All the planets moved around the Sun.
This spiral galaxy at the top of the photo is a similar shape to our own galaxy.
The Sun keeps the planets in their orbits. Like other stars, the Sun emits radiation in the form of heat, part of which is visible as light. Energy from the Sun is essential for life on Earth.
The Sun is huge. A hollow ball the size of the Sun could hold about a million Earths! It is also very hot. The ‘surface’ of the Sun reaches about 6,000 degree Celsius. The centre of the Sun is about 13 million degrees centigrade!
The Sun is made of the gases hydrogen and helium. At its centre, hydrogen is constantly being turned into helium. This is a nuclear reaction which releases huge amounts of energy. This energy travels to the surface of the Sun and then into space as radiation.
The surface of the Sun is called the ‘photosphere’. Here there are often dark patches called ‘sunspots’. These are areas of gas that are cooler than the rest of the surface. Although we call them spots, they are many times larger than the Earth. Giant jets of gas shoot out from the Sun. They are called ‘flares’. Sometimes arches of gas loop across the surface. These are known as ‘prominences’.
This picture of the Sun shows a giant loop prominence.
The inner planets – Mercury, Venus, Earth and Mars – are all made of rock. They are quite small compared with some of the other planets. Mercury is closest to the Sun. It is extremely hot during the day but the temperature falls to -175 degree C at night. Mercury is only a little bigger than our Moon.
Although Venus is further from the Sun than Mercury, it is the hottest planet in the Solar System. Venus spins in the opposite direction to all the other planets. It also spins so slowly that its ‘day’ is longer than its ‘year’!
Earth has water, oxygen and moderate temperatures. Because of this it can support life. Mars is about half the diameter of Earth. Temperatures here are never above zero degree C and the only water is ice at the frozen poles.
Venus
On the surface of Venus, below, there are deep cracks and dead volcanoes. The atmosphere is thick carbon dioxide gas and the planet is surrounded by clouds of sulphuric acid.
Mercury
Like the Moon, the surface of Mercury is pitted with crates and covered in dust and stones. There is no air or water. It is bathed in dangerous radiation from the Sun.
Earth
About three-quarters of the Earth’s surface is covered by water. The atmosphere contains the gases oxygen, nitrogen and a small amount of carbon dioxide.
Mars
Mars is made of red rocks. It has craters and dead volcanoes on its surface. The atmosphere is dusty and made of carbon dioxide. There are strong winds which blow up dust storms.
Jupiter and Saturn are the two largest and fastest spinning planets in the Solar System. They are both made mostly of the gases hydrogen and helium. Jupiter is the largest. It weighs two and a half times as much as all the other planets put together. Its outer layer of gas clouds is about 1,000 km thick. Fierce winds blow these clouds and huge streaks of lightning flash between them. Below the cloud layer the gases get denser and denser until they become liquid.
Saturn is made of less dense gases. It could even float on water! The planet looks beautiful as it is surrounded by rings. The rings aren’t solid. They are probably made of pieces of rock and ice which orbit the planet.
Here we see Jupiter with two of its moons. It has at least 39 moons in total.
This picture is made up of photographs of Saturn and several of its moons.
Uranus, Neptune and Pluto are the planets furthest from the Sun. they get very little of the Sun’s radiation so they are all dark and cold places. Uranus and Neptune are large planets made of gas. They look greeny-blue because they contain a gas called methane. Uranus is circled by nine rings, which are smaller than the rings around Saturn. The rings seem to be made of rocks and ice.
Pluto is the greatest mystery of all. It was only discovered in 1930 and is even smaller than our Moon. It is probably made of rock and covered in ice. Pluto is usually the outermost planet. But sometimes its orbit crosses Neptune’s for a period of 20 years, and then Neptune is the outermost planet. This last happened in 1979.
This artwork shows the Voyager 2 space probe as it passes Neptune’s North Pole.
A moon is a ball of rock that orbits a planet. Mercury and Venus are the only planets that do not have moons. Jupiter and Saturn each have at least 16 moons. Our Moon is our closest neighbour in space. Moons may be lumps of material that were left over when the planets formed. Like planets, moons can only be seen when they are lit up by the Sun.
We know more about our own Moon than any other member of the Solar System, apart from the Earth. Astronauts have even landed on the Moon. The Moon is a bare, dead place where nothing lives or grows. Its surface has hills and mountains as well as flat plains and deep craters. There is no water or air on the Moon and it is covered with a layer of fine dust. From the Moon, the sky always looks black and the Earth seems to change shape and go through phases.
Phases of the Moon
Our Moon takes 
days to orbit the Earth. During this time it seems to change shape. This is because only the side of the Moon that faces the Sun is lit up. And as the Moon orbits the Earth we see different amounts of this lit-up side.
When the Moon is between the Sun and the Earth we cannot see it. This is called a ‘New Moon’. A ‘Full moon’ is when the Earth is between the Sun and the Moon. The diagram shows phases of the Moon seen in the Northern Hemisphere.
Seen from the Moon, (foreground), the Earth appears to change shape.
The planets travel round the Sun in nearly circular paths. So they are constantly changing direction. A force called ‘gravity’ makes the planets turn. You will have felt the force of the Earth’s gravity – without it you would float up into space! Gravity is a force which pulls. All the planets exert their gravity. The more massive they are, the stronger is their pull of gravity. The Sun is so massive that its gravity is strong enough to keep all the planets turning around it. Without this force, they would each fly off in a straight line.
The Moon also has gravity. Because it is less massive than the Earth, its pull of gravity is weaker – as the astronauts that landed on the Moon found out!
Force
Like the planets, this ball will keep moving in a circle as long as it travels fast enough. The force that acts along the string pulls the ball and makes it change direction. Although a different kind of force, its effect can be compared to the gravity exerted by the Sun. if the child let go of the string, there would be no pulling force to keep the ball turning. It would fly away in a straight line.
An astronaut leaps from the Moon’s surface while saluting the US flag.
Occasionally we see ‘wanderers’ in our Solar System. A comet is a ball made of rock and ice. This nucleus is surrounded by a cloud of gas called the ‘coma’. When comets move close enough to the Sun. they reflect the Sun’s light and form a tail of gas and dust.
Comets leave bits of dust from their tails behind in space. Some of this dust enters the Earth’s atmosphere where it burns up. We may then see a shower of bright ‘shooting stars’. The scientific name for a shooting star is a ‘meteor’.
Sometimes lumps of rock or metal from space crash to the Earth. These are ‘meteorites’. A large meteorite may make a crater where it lands. Meteorites probably caused the craters on the surface of the Moon, Mercury and Mars – as well as this one in Arizona.
This meteor crater in Arizona is over 1 kilometre wide and 175 metres deep.
Halley’s Comet
Halley’s Comet is a regular visitor to our part of the Solar System. It returns about every 76 years and has been seen throughout the ages. The diagram shows the strange shape of the comet’s orbit – a long oval. The comet is invisible beyond Saturn’s orbit. You can see how the comet’s tail always points away from the Sun.
The planets and stars are far too far away to see clearly, however good your eyesight is. The telescope was invented to help people look at faint, distant objects and see them in more detail. Telescopes collect more light than the human eye. They can also make things look bigger. Stars looked at through a telescope seem to be brighter and closer.
Telescopes use either a lens or a mirror to collect light and focus it. Another lens is used to produce a magnified image. Modern telescopes are housed in giant buildings called ‘observatories’. These are often built at the top of a mountain where there is a clear view of the sky.
The telescope
This telescope uses one convex lens to collect and focus light, and another to magnify the image. Sliding the outer tube changes the distance between the two lenses. This is important as it allows you to see objects that are near and far away.
Looking down on a telescope in the observatory at Kitt Peak, Arizona, USA
Watching the Solar System from the Earth is one way of finding out about it. To learn more, people have used rockets to travel into space. There they have been able to look at parts of the Solar System more clearly. But although the first men landed on the Moon in 1969, no one has visited any of the planets. They are too far away and conditions are dangerous for humans.
However, as you have seen space probes – robot controlled unmanned spacecraft – have been sent far into the Solar System. The space probe Voyager 2 has travelled through the Solar System and is now well beyond the orbit of Pluto. As it travels it takes pictures and sends them back to Earth.
Voyager 2
This space probe gas already travelled across millions of kilometres of space, and has collected information on Jupiter, Saturn, Uranus and Neptune. The craft’s onboard computers are reprogrammed during its flight by electronic signals from Earth. The entire vehicle weighs only 815 kilograms. It carries equipment for 11 scientific experiments which are powered by a nuclear generator.
This is a photograph of the surface of Mars. The surface of Mars – you can see the solar panels of the lander at the bottom of the photo.
Life and death of a star
When a star like our Sun first forms it is different from the way we see it today. At first the Sun was a very hot, blue star. As it grew bigger it cooled down and looked white. The Sun will shine as it is for about 10,000 million years. Eventually it will swell and form a ‘Red Giant’. Some of the stars we see are Red Giants. They are cooler than the Sun. But as they are many times bigger, they appear brighter. A Red Giant slowly cools and shrinks. Its outer layers of gas drift away and a small hot star called a ‘White Dwarf’ is left. This slowly cools and becomes a ‘Black Dwarf’.
Very large stars
Stars much bigger than our Sun also become Red Giants, but they blow up in a huge explosion called a ‘supernova’. They shrink into themselves and form a ‘Black Hole’. Anything nearby gets sucked into a black hole. Even its own light cannot escape!