The Solar System


Our solar system is made up the sun, eight planets, more than 150 moons, as well as comets, asteroids, dwarf planets and other space rocks.

Planets, asteroids and comets orbit the sun. They travel around our sun in an ellipse. It takes Mercury, the nearest planet, only 88 days but Neptune 164 years to travel around the sun once.

Moons orbit planets. Currently, Jupiter has the most moons – over 60. Mercury and Venus don’t have any moons.

The inner planets Mercury, Venus, the Earth and Mars are called terrestrial planets. This means they have a hard surface to stand on. Jupiter, Saturn, Neptune and Uranus are the outer planets. They are also called the gas giants because you can’t stand on them – their surface is made of gas.

There are many theories on how the solar system developed. About 4.5 billion years ago a big cloud of gas and dust probably collapsed. The sun formed in the middle, the densest region. Further away from the sun, gases changed to planets made of rock.

The planets of our solar system
Image : International Astronomical Union/NASA, edited by w:User:SG.,
Public domain, via Wikimedia Commons

The Sun – Centre of Our Solar System

The sun is a star at the centre of our solar system. It is a huge spinning ball of hot gas that lights up the Earth and provides us with heat. Our sun is a medium-sized yellow star that is about 150 million km away from the Earth.

Here are some important facts about our sun:

  • The sun is one of more than 200 billion stars in our galaxy, the Milky Way.
  • The Milky Way is one of billions of galaxies in our universe. There are as many galaxies, as there are stars in the Milky Way
  • The sun is 30,000 light years away from the centre of the Milky Way.
  • The sun is made up of over 70 % hydrogen, 27 % helium and other elements.
  • On the surface, the sun has a temperature of over 5,000 °C and in its core a temperature of a few million degrees.
  • Even though there are billions of stars, only about 6000 are visible to us.
  • Even if the sun dies today, it can send light to Earth for years to come

The sun was formed about 4.5 billion years ago. As a star of the second generation, it doesn’t only burn hydrogen but also other elements, like helium and metals. They were formed when a big explosion took place, which formed our solar system.

At the sun’s centre, nuclear fusion produces great amounts of energy. This energy turns into heat and light, which warms up the solar system and makes it brighter. The sun has enough energy for about 5 billion years. Then it will explode and a cloud of gas will destroy all the planets of the solar system.

The sun’s diameter is about 1.4 million km. It is 10 times larger than the planet Jupiter and over 100 times larger than the Earth. About 1.3 million Earths could fit into the sun. But it is not a big star compared to others in our universe.

The core is the inner part of the sun. It is the place where the star converts hydrogen to helium. The energy travels to the outer parts of the sun as radiation. The photosphere is the surface of the sun. The chromosphere is the lower atmosphere with temperatures of up to 7000° C. The corona is the upper atmosphere. You can only see it during an eclipse. It is one of the hottest parts of the sun with temperatures as high as a million degrees C.

A solar flare happens when the sun’s magnetic fields crash into each other. It causes gas to shoot out of the sun. Sometimes these flares shoot up as high as 100,000 km and can last for hours. Solar flares are not really dangerous to us but they can interfere with radio signals on earth.

The eclipse of the sun is one of nature’s most spectacular special effects. It happens when the earth passes through the moon’s shadow. The moon always has a shadow and if the sun, the earth and the moon are in the right line, the moon’s shadow passes over the earth. When this happens the sun is blocked from your view and it gets as dark as night, but only for a few minutes. This shadow might only be a few hundred kilometres wide and people from all over the world come and see it.

The sun gives us heat, light, our food and the air that we breathe. It powers the atmosphere to give us winds and rain. Even coal and oil come from plants and animals that lived millions of years ago and depended on the sun for life. The sun heats the land, the oceans and our air. Green plants use the sun’s rays to turn carbon dioxide into the oxygen that we breathe.

A solar flare
Image : NASA Goddard Space Flight Center
[CC BY 2.0], via Wikimedia Commons

The Inner Planets


Mercury is the first planet in the solar system, only about 60 million km away from the sun. It takes Mercury about 88 days to go round the sun once and one Mercury day is about 58 Earth days long. Mercury is a small planet – not much larger than our moon. Temperatures go up to 500 °C when it faces the sun and -150° C on its dark side.

The surface of Mercury is very much like that of the moon. Scientists believe that over 3 billion years ago, when the solar system was forming, the planet was constantly hit by asteroids and rocks . For its size, Mercury is a heavy planet. Gravity is about one third of the Earth’s. The core of Mercury is probably still liquid .

Some scientists believe that Mercury is really the moon of another planet that flew off course and began its own orbit as a planet. The orbit of Mercury is egg-shaped – which means it is closer to the sun at some times of the year. The sky is pitch black and all of a sudden the sun begins to rise. Because it is so close to Mercury it looks enormous, a huge blazing ball that is twice as big as it is on Earth.


Venus is the second planet in our solar system. It is sometimes called the Earth’s twin, because it is about as big as our planet. Like the earth, Venus is covered with thick clouds and has an atmosphere. There is almost the same amount of gravity on Venus but the pressure on the planet is about 100 times bigger than on Earth. That means that you would be crushed if you tried to walk on the surface.

Venus is very different from our Earth. The clouds that surround the planet are thick and yellow. This makes it almost impossible to see the surface of Venus. The clouds are not made up of water, like on Earth. They have poisonous gases, mostly sulphur, in them. Below the clouds there is a thick atmosphere of carbon dioxide – so it is impossible for us to breathe on this planet.

Venus is the hottest planet in our solar system. This is because the sunlight that gets into the atmosphere is trapped there and can’t get out. Temperatures can reach over 500 ° Celsius. We call this the greenhouse effect – the same thing happens on our Earth and warms up our atmosphere.

The surface of Venus is flat with a few higher mountains. One of them is even higher than Mount Everest. There are many volcanoes; most of them may still be active. Lava flows for hundreds of kilometers. Venus doesn’t have very many craters because asteroids burn out before they hit the planet.

Venus orbits the sun in the same direction as the Earth. But it rotates in the opposite direction. This means that on Venus the sun rises in the west and sets in the east. Because it rotates so slowly, one day on Venus lasts about 240 Earth days, longer that the time it takes the planet to orbit the sun (224 days).

Some scientists think that, one day; it might be possible to live on Venus. They believe that if we put plants on the planet that produce oxygen there would be an atmosphere that we could breathe. And human beings might be able to start a colony on Venus.

Venus was named after the goddess of love and beauty. People believe this because Venus is the brightest star in the sky. It is sometimes called the morning or evening star.


The Earth is the third planet in our solar system and the fifth largest of all the eight planets. It is about 150 million km away from the sun, and the only planet known to have life. The Earth is not a perfect sphere. It is a bit flatter at its poles and wider at the equator. The planet rotates on its axis once every 24 hours. At the equator, it spins at a speed of 1600 km an hour.

Life on Earth exists because there is oxygen in the atmosphere, the temperatures are moderate and about 75 % of the planet is made up of water. The atmosphere is filled with lots of particles – gas, water vapour, dust, dirt etc.. When light from the sun hits these particles it is scattered in all directions. The blue light reaches our eyes from all directions. The other colors go straight to the surface. That is why the sky is blue. To the astronauts in space, the sky is dark black.

The sun is so powerful that it would burn everything on our Earth and make our planet unliveable. We are lucky to have protection – the ozone layer. It is about 20 – 40 km above the Earth’s surface and it protects us from the dangerous ultraviolet rays of the sun. Without it many of us would have skin cancer and most plants and animals wouldn’t be able to live. In the past years scientists have found out that the ozone layer is becoming thinner and thinner because people are using too many chemicals on our Earth.

The Earth is made up of three parts :
  • The core is the inner part. It is probably made up out of liquid iron and nickel. It is the heaviest part of our planet. Temperatures in this part may be as high as 6000° C.
  • The mantle is the middle part. It is made up out of rock that has been melted.
  • The crust is made up out of cold rock that cooled down millions of years ago.

There is a very powerful magnetic field around our Earth. It is as if there was a great magnet in its core. The magnetic North Pole is not the same as the geographic North Pole. It is the place where a compass needle shows to. Currently, they are about 1200 km away from each other. The magnetic poles of the Earth are moving all the time and never stay in one place.

Our planet is about 4.5 billion years old. At first the Earth was a great ball of gas and dust. Then it lost a lot of its heat and became cooler. The lighter rocks floated to the surface and the heavier ones sank down to the core. Later on, gas and liquid rock from the inner part of the Earth came to the surface in the form of volcanic eruptions.

Water vapour cooled down and turned into water – today’s oceans. The gases, mostly nitrogen and carbon dioxide were kept in the atmosphere by the Earth’s gravity.

Layers of the Earth
Image (altered and modified) : SurachitCC BY-SA 3.0,
via Wikimedia Commons


Mars is the fourth planet in our solar system. It has two moons – Phobos and Deimos. Some astronomers think that the moons are like asteroids that were captured by Mars very early in its history.

If you look at Mars without a telescope you can see a reddish object that is sometimes very bright. When it is closest to the earth – at about 55 million km – Mars is, after Venus, the brightest object in the sky. Through a telescope, Mars has bright orange and darker areas. Some of the colors change with Martian seasons.

The Martian atmosphere is mostly made up of carbon dioxide with small amounts of nitrogen, oxygen and water. It is very thin and high yellowish clouds can often be seen. Scientists think they are made up of dust that is carried around by high winds on the planet.

The temperatures on Mars depend on the season and the time of day. But most of the time it is very cold. Summer temperatures may get as high as 17° C – but on average they don’t get much higher than -30° C. The coldest parts of the planet are down to – 125 °. That’s why the carbon dioxide freezes , turns white and forms the polar ice caps . In the winter they cover almost half of the planet and in the summer they get smaller as the sunlight melts them. They are about 300 km wide at the South Pole and more than 1,000 km wide at the North Pole. They are probably made up of frozen water and gases and may be up to 2.5 km thick.

Some of the surface of Mars consists of the biggest volcanoes in the solar system. One of them , Olympus Mons , is 25 km high and 600 km wide. There is no sign that volcanoes are active today. There are also many craters because in its early days Mars was hit very hard by asteroids and comets. Canyons run for over thousands of kilometers across the surface .

Most of the knowledge we have about Mars has come from spaceships that have been sent to visit the planet since 1964. The first views of Mars came from spacecraft that flew by the planet at a larger distance. The first spacecraft to orbit the planet was Mariner 9 . It studied the atmosphere and the surface for almost a year and gave scientists the first pictures of its two moons. In 1976, two Viking spacecraft landed on Mars and took a look at the surface of the planet. In 1997, Mars Pathfinder, set down on the surface and a 10kg heavy vehicle, called Sojourner, moved around the planet and examined rocks.

The idea that life could, or even does, exist on Mars has a long history. In 1877 the Italian astronomer Schiaparelli said he saw a system of canals all over Mars. But close-up pictures from Mars have shown that no such canals exist. The early Viking spaceships that landed on Mars in 1976 proved that there is no organic material on the planet.

First Mars rover
Image : By NASA – , Public Domain
via Wikipedia Commons

The Outer Planets


Jupiter is the fifth planet from the sun and the largest in our solar system. It has 1 400 times the volume of our Earth, but is only 300 times as heavy because the planet must be made up of gas rather than rocks or metal.

It takes Jupiter almost 12 years to orbit the sun. But it rotates on its own axis very quickly – it completes one full turn every 10 hours. If you look at Jupiter closely, you can see stripes , probably clouds that are created by fast-moving winds.

We don’t know very much about Jupiter because not very many spaceships have visited it. In 1979 two American Voyager spacecraft flew past Jupiter and gave us lots of new information. Today we know that most of the planet consists of gases – hydrogen and helium – and does not have a hard core , like the Earth. In 1994 a big comet crashed into Jupiter and stirred up the planet’s atmosphere. Scientists could find out what kind of gases Jupiter’s atmosphere is made up of.

In 1989 NASA launched an unmanned spacecraft to Jupiter – Galileo. After 6 years, Galileo reached the planet and went into orbit. It sent a small probe through the clouds of Jupiter to find out more about the atmosphere. It turned out to be very dense and filled with sulphur and other poisonous gases, impossible to breathe .

One of Jupiter’s main features is a great red spot on the planet, probably a big storm about the size of our earth.

Jupiter has four large moons and many other smaller ones – over 60 moons have been found so far. Galileo discovered the four biggest moons in the 17th century. They are also called the Galilean moons. Ganymede, the biggest moon in the solar system is even larger than Mercury and would be an own planet if it didn’t travel around Jupiter. Callisto is as big as Mercury. Both these moons have an icy surface. Io is a rocky, volcanic moon from which lava and sulphur come out. It is about as big as our moon and the innermost of Jupiter’s moons. Europa is the smallest of the Galilean moons. It has a very smooth surface and a lot of lines and dots on it that may be frozen rivers or seas. Maybe there is even water underneath the surface of Europa.

Scientists discovered that, not only Saturn, but also Jupiter has a system of rings. They do not reflect the light from the sun because they are made of dark dust and pieces of rock. That’s why they are not visible.


Saturn is the sixth planet from the sun and the second-largest in our solar system. It is different from the other planets because of its rings, which were first seen by the Italian astronomer Galileo in 1610.

You can see Saturn without using a telescope, but you need one if you want to see its rings. Saturn has a diameter about 10 times larger than the Earth and about 760 Earths could fit into the planet.

Because it spins so quickly, Saturn looks a bit flat, with a longer diameter through the equator than through the poles. Saturn is a very light planet – the only one that would float in a big body of water.

One Saturnian day lasts about 10 hours and it takes the planet almost 30 years to orbit the sun once. Because it moves so quickly around its axis there are strong winds that sweep the whole planet. At the equator they probably have a speed of up to 1700 km an hour. Because it is very far away from the sun, temperatures on the surface are abut -175 ° C.

Saturn’s rings are the most fascinating feature about the planet. They are extremely wide, but very flat. They stretch to a distance of over 130,000 km from the planet’s centre, but most of them are only very few meters thick. There are probably over 100,000 separate rings – made of icy rock and frozen gases. This makes them shine in the sunlight.

More than 50 moons have been discovered around Saturn. Some of them are only 20 km wide, others are bigger than our moon. Saturn’s largest moon is Titan- even larger than Mercury. Not very much is known about this moon because it has a very thick orange-colored atmosphere made up of nitrogen and other gases. Underneath thick clouds there might be some form of water on Titan.

In 1997 NASA launched a spacecraft from Cape Canaveral, Florida with the aim of reaching Saturn. After a 7-year trip Cassini went into orbit around Saturn and sent a small probe to the surface of Saturn’s biggest moon, Titan. In the past few years it has sent important data about Titan back to Earth. It also found out that liquid methane rains down on the surface of Titan, forming rivers and lakes of hydrocarbon.

Artist’s image of Cassini spacecraft in orbit around Saturn
Image : NASA/JPL, Public domain, via Wikimedia Commons


Uranus is the seventh planet from the sun. It is sixth in size and just visible to the human eye. It was discovered by accident by the British astronomer William Herschel in the 18th century.

Uranus has a diameter of over 50,000 km – about 4 times that of the Earth and it is 3 billion km away from the sun. It takes Uranus 84 years for one single orbit around the sun and 17 hours for one rotation around its axis. The unusual thing about Uranus is that its poles are pointed directly at the sun. This means that it orbits the sun on its side. Each pole gets 42 years of sunlight and then 42 years of darkness.

Uranus belongs to the “gas giants”. Its atmosphere consists mostly of hydrogen and helium and a bit of methane, which gives the planet a bluish-green color. The surface of Uranus is probably made up of frozen gas. Underneath this crust, there is a layer of poisonous water. The core is ice and rock.

In 1977 an American astronomer discovered that Uranus also has a system of rings. 10 of the 17 moons were discovered when Voyager 2 flew by the planet in 1986.


When Neptune was discovered in 1846 astronomers thought it was a star. It is the eighth planet from the sun. It does not shine so brightly, so it is only visible when you use a telescope. It appears as a green – bluish disc, like Uranus.

It takes Neptune, which is almost 4.5 billion km away from Earth, almost 165 years to travel around the sun once . Neptune’s day is shorter than an Earth day – only 16 hours.

Neptune has a few dark spots. Scientists think that these spots are tremendous hurricanes that travel across the frozen planet. Strong and icy winds of up to 1000 km an hour blow on this planet. They are the fastest winds ever measured in our solar system. Neptune’s atmosphere can change very quickly. When Voyager 2 flew past the planet in 1989 the dark spots were gone.

Like the other giant planets, Neptune is a ball of gas. The atmosphere is made up of frozen methane , which gives the planet its blue color. The planet has 8 known satellites. The biggest moon is Triton – about the same size as our own moon. It has active ice volcanoes. When they erupt, they shoot frozen nitrogen and gas about 20 km high.

Dwarf Planets

Dwarf planets are like the solar system’s eight planets, but smaller. They orbit the sun but are not moons. Today, scientists have classified 5 dwarf planets including Pluto, which was originally the ninth planet until 2006.

Pluto was discovered in 1930 by an American astronomer. It is so far away that astronomers can hardly see it, even with the most powerful telescopes.

Pluto is only 2,200 km in diameter, even smaller than our moon. It revolves around the sun once every 248 years. Sometimes Pluto crosses the path of Neptune and for about 20 years Neptune is farther from the sun than Pluto.

When it gets nearer to the sun, Pluto has more of an atmosphere. The frozen gas melts a bit and even clouds sometimes form. When it moves away from the sun again the atmosphere freezes and falls back to the surface.

The gravity on Pluto is only about 8 % of the gravity on Earth. If you weigh 70 kg on earth, you would weigh only about 5 kg on Pluto. It is very, very cold and the sun can only be seen as a very tiny spot. Temperatures can go down to – 240 ° C. Pluto’s only moon , Charon, is almost the size of Pluto itself.

Ceres is a dwarf planet between Mars and Jupiter, in the asteroid belt. It has a rocky surface with no atmosphere. Ceres was the first asteroid that was discovered.

Eris is even larger than Pluto. It orbits the sun at a distance of almost 15 billion kilometers. Like Pluto Eris has a surface made up of methane ice.

Size comparison
Image (modified) : NASA, Public Domain
via Wikimedia Commons

The Moon

The moon is the Earth’s natural satellite and the object in space that is closest to us. It has a diameter of about 3,400 km – only about one quarter of the Earth’s. Gravity on the moon is only one sixth of the Earth’s. You would be very light on the moon. A person who has a weight of 60 kg would weigh only ten kg on the moon.

The moon moves around the earth at an average distance of about 400,000 km and at a speed of 3,700 km an hour. It completes one orbit around the Earth every 27 days and 8 hours. It takes the moon the same time to rotate around its own axis. That means that the same side of the moon is pointed towards us all the time. Whenever you look at the moon you will always see the same face. Nobody, other than astronauts, has seen the other – the dark side – of the moon.

The moon has no atmosphere. There is nothing to breathe, not even poisonous gases and there is no weather to change its surface. Temperatures on the moon are very extreme. They range from 125 ° C when the sun shines on the lunar surface down to -175 °C on the dark side of the moon.

We can see the moon in different phases as it travels around the Earth. Half of the moon is always in the sunlight just as half of the earth has day while the other half has night. The phases of the moon depend on how much of the sunlit half can be seen from the Earth.

  • Phase 1 : New Moon – The face of the moon is completely in shadow.
  • Phase 2 : Half Moon – Half of the sunlit side of the moon can be seen from the Earth.
  • Phase 3 : Full Moon – The moon shows its fully lit surface.
  • Phase 4 : Half Moon – The moon appears as a half-circle again .

The moon’s phases
Image :Orion 8CC BY-SA 3.0,
via Wikimedia Commons

Ancient observers of the moon saw dark areas, which they thought were great oceans. They gave them the Latin word “mare”, which means sea. Today we know that the moon has great flat areas (mare) that are thousands of km wide, but it also has craters and mountains that are very high. The craters come from the impact of asteroids early in the moon’s history. Not very much has changed the surface of the moon for the last 3 billion years. The largest crater is almost 300 km wide and 4,000 m deep. The highest mountains are almost as high as the Himalayas on Earth.

Before astronauts landed on the moon, there were many theories about how the moon was formed. But today, after scientists have studied moon rocks for almost 50 years, most of them think there is only one theory left.

Over 4 billion years ago, the Earth was hit by a large object, maybe the size of the planet Mars. The crash blasted parts of the Earth and what was left of this planet into an Earth orbit. As time went on all of the parts that were flung into space joined together to form the moon.

Our tides ( high tide and low tide) happen because the moon pulls at the Earth. When the moon is directly over a certain place on the earth the water rises higher than it normally does, sometimes up to 10 meters . In other places, the water level is lower than it usually is. High and low tides usually take place twice a day.

The moon is the only object in space that human beings have visited. In 1969, the American Apollo 11 spacecraft landed on the moon and two astronauts – Neil Armstrong and “Buzz” Aldrin – became the first people to set foot on the moon. Between 1969 and 1972 there were six landings on the moon. But in the last 40 years nobody has visited it.

Buzz Aldrin and the American flag – Apollo 11
Image: NASA / Neil Armstrong, Public Domain


A comet is an object in space that revolves around the sun. It has a long tail of light, but only when it passes near to the sun. A long time ago, people thought that comets were only strange changes of sunlight in the atmosphere, but Isaac Newton proved that they were like planets with orbits.

A comet consists of a core, called the nucleus and a surrounding cloud made up of ice , dust and other gases , like hydrogen, oxygen and carbon. Some scientists call it a dirty snowball. A comet may be very big, like the planet Jupiter or even the sun, but the nucleus itself is mostly very small.

When a comet approaches the sun, the heat of the sun melts the icy parts and it becomes brighter and brighter. The comet gets one or more tails that may be so big, that they can extend millions of km into space. This tail always points away from the sun even when the comet shoots back into space. These tails are mostly made up of gases and dust. When comets move away from the sun less gas and dust are lost and the tails disappear. Some comets with small orbits have short tails, so that they are almost invisible.

Not all comets appear with the same brightness to us on earth. Astronomers have recorded 1,400 comets up to now and less than half of them have had tails that we could see. Comets have elliptical orbits and the time they take to orbit the sun has been calculated by astronomers. Some comets – like Donati’s comet – have a period of 3 or 4 years, others have periods of 2,000 years and more. Some comets have orbits that even take them out of our solar system.

Halley’s Comet is the best-known comet in our solar system. It appears every 76 years and is bright enough to be seen without a telescope.

Comets were once believed to come from outside the solar system. Today, scientists think that they originate in the outer, colder parts of our solar system from rocks and ice that made up the planets.

Comets have often appeared at the time of important events. Whenever comets appeared, people thought they would crash into our Earth and destroy all life. A collision of a comet would have catastrophic effects on our Earth, but such a crash is not likely. In 1994 a comet – Shoemaker-Levy 9 – broke into many small pieces as it came very near to the planet Jupiter. Some of these fragments – up to 2 km wide – crashed on the surface of the planet .

Comet Hale-Bopp flies across the sky in Croatia
Image : Philipp Salzgeber [CC BY-SA 2.0 AT]


Asteroids are small pieces of rock that move around the sun between Mars and Jupiter. Over 200 asteroids have a diameter of more than 100 km and thousands of smaller ones exist. The largest asteroid that man knows of is Ceres, with a diameter of 1,100 km.

It is not clear how asteroids developed. Scientists today think that they may be in a place where a planet should have formed. But such a planet couldn’t form because of the power of Jupiter. Another theory says that the asteroids were a big planet that was destroyed in a collision a long time ago.

There are also other asteroid belts in the solar system. New asteroids have been found between Saturn and Uranus. Over 400 asteroids have been found near the Earth and Venus. NASA sends probes to asteroids when they are close to Earth, in order to discover what they are made of.

Asteroids that are on a course that may hit the Earth are called meteorites. Most of them burn up in our atmosphere but some of them are big enough to hit the Earth. About 65 million years ago, a large meteorite crashed into the Earth. Scientists believe that a lot of dust and rock was stirred up and made our planet very dark – so sunlight couldn’t get in. It became colder and colder and the dinosaurs probably died out this way.



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