The Inner Planets


Table of Contents :



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.



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.


Related Topics



  • active = they can explode at any time
  • amount =quantity
  • asteroid = rocks that travel through space very quickly. Some of them are very big and they can crash into other planets
  • astronomer = a person who studies the sky and the stars
  • atmosphere = the gases that are around a planet
  • axis = the line around which an object turns or spins
  • beauty = being beautiful
  • billion = a thousand million
  • blazing = fiery
  • breathe = if you get air into you lungs from your nose or mouth
  • bright = light,shining
  • cancer = very serious disease in which cells in your body start to grow in an uncontrolled way
  • canyon = mountain valley that has very steep slopes
  • capture = here:to pull towards
  • carbon dioxide = the gas that is produced when animals or people breathe out
  • close-up = pictures taken from very near
  • compass = an instrument that shows where directions are; its needle always points north
  • consist of = made up of
  • constantly = always
  • core = inner part, the centre of a planet
  • cover = spread over
  • crush = to press something so hard that it breaks
  • currently = right now
  • dark side = the side that we cannot see
  • depend on =to be affected by
  • distance =the space between two things
  • dust = very small particles of dirt
  • enormous = very, very big
  • equator = line that goes around the middle of the Earth
  • eruption = explosion
  • examine = to take a close look at and study
  • face = look towards
  • flat =even, without any hills or mountains
  • float = drift, come up
  • freeze -froze- frozen = when water turns to ice
  • goddess = a female god
  • gravity = the power that pulls you to the ground
  • greenhouse effect = sunlight gets into the atmosphere and when it reaches the surface it turns into heat. But the heat cannot get out, so it becomes warmer and warmer.
  • heavy = opposite of light
  • huge = very big
  • impossible = not possible, unthinkable
  • iron = hard metal used to make steel
  • knowledge = the facts we know about something
  • lava = the material that comes out of volcanoes. Sometimes it flows very slowly
  • liquid = fluid, watery
  • magnetic field = the area around the Earth that has magnetic power
  • Martian = from Mars
  • melt = if you make a hard object hot it can turn into liquid or when ice turns to water
  • moderate =not too hot and not too cold
  • needle = a small thin piece of metal
  • nitrogen = a gas that has no colour and no smell; it forms most of the Earth's atmosphere
  • off course = away from its normal path
  • on average = normally
  • once = one time
  • orbit = to move around an object in a circle
  • organic = material that comes from something living
  • oxygen = gas that is in the air and that we need to breathe
  • particle = very very small pieces of something
  • pitch black = very very dark; with absolutely no light
  • poisonous = something in the air that is dangerous and can kill you
  • polar ice caps = the ice that forms near the poles of a planet
  • pressure = the weight of an object on the surface
  • protection = something that protects and defends us
  • reach = get to
  • reddish = red
  • rotate = to go around
  • scatter = spread
  • scientist = a person who is trained in science
  • season = time of year
  • sink-sank = to go down slowly
  • size = how big something is
  • spacecraft = spaceship
  • speed =how fast something is
  • sphere = form of a ball
  • spin = rotate, to go around
  • straight = directly
  • sulphur =a yellow chemical substance that burns and smells bad
  • surface = the hard top part of a planet
  • surround = around
  • telescope = an instrument that makes small, faraway objects look bigger
  • trap = it cannot escape
  • twin = two things that look the same
  • ultraviolet ray = the light that makes your skin darker
  • unliveable = you cannot live on it
  • vehicle = an object with four wheels, like a car
  • view = sight; what we see
  • water vapour = water in the form of small drops in the air
  • wide =broad – not narrow