Eight planets orbit the Sun

Image by Daniel Roberts on Pixabay 

1 The People and the Stars

Vincent van Gogh painted the picture "The Starry Night," circled

Image: public domain

People have been observing the starry sky for as long as anyone can remember. Even astronomers in ancient times noticed, for example, that most stars appeared as fixed points in the sky. Their positions in relation to one another did not change. They were called “fixed stars”. Five others, however, which appeared especially bright, kept changing position. They were called “wandering stars”.

They were given the names of the Roman gods Mercury, Venus, Mars, Jupiter and Saturn. Today we know that these “wandering stars” are not stars at all, but celestial bodies that orbit our Sun. We call them planets.

We also know that there are not just five of them, but eight, and that one of them is Earth. Uranus and Neptune were added to the planets orbiting the Sun that were already known in antiquity.

Find out:

1. What are the names of the eight planets that orbit our Sun?

2 What are planets?

The four inner planets of the Sun are much smaller than the “gas giants”

Image: NASA (PD)

Today, we also know about important differences between planets and stars. Stars burn the gases they are made of, are extremely hot, and radiate light and heat into space. Planets, on the other hand, shine because they are illuminated by the light of a star. They reflect this light.

Planets orbit a sun, while suns themselves move around the centre of a galaxy. Like suns, planets are spherical bodies. Some are orbited by moons. 

Our solar system has eight planets. Compared with the Sun, Mercury, Venus, Earth and Mars seem tiny. They are made mostly of solid materials such as iron and rock. The four outer planets—Jupiter, Saturn, Uranus and Neptune—are larger than the others, but still much smaller than the Sun. They are made largely of gas, mainly hydrogen and helium. They are sometimes called “gas giants”.

Several dwarf planets, a number of comets and millions of meteoroids also orbit the Sun. They are especially common in a belt between Mars and Jupiter. Until a few years ago, Pluto was also counted among the planets. It is now classified as a dwarf planet.

Find out:

2. How does a planet differ from a star?

3. What are planets made of?

4. Which former planet is now classified as a dwarf planet?

3 Planets Formed from the Solar Nebula

The Sun and its planets formed in a protoplanetary cloud

Image: NASA (PD)

Scientists explain how the planets formed as follows: It happened about 4.6 billion years ago. Back then, where the Sun and its planets are today, there was a huge cloud of dust and hydrogen. This “solar nebula” gradually contracted under the force of gravity.

The Sun formed at the centre. Pressure and temperature inside the Sun rose so dramatically that, eventually, hydrogen nuclei fused together.

This fusion is called nuclear fusion. Since then, hydrogen has been continuously converted into helium. In the process, unimaginable amounts of light and heat are radiated into space.

The rest of the solar nebula began to rotate around this centre. Collisions and the force of gravity first formed small dust grains, which attracted more and more matter. Many eventually grew into large clumps. But only eight of them became large enough to turn into planets, attracting all the material around them.

They orbit the Sun. Their orbits all lie on the same plane, as if they were moving in circles of different sizes around the center of an invisible plate.

Find out:

5. How were the planets formed?

4 Mercury

The planet Mercury

Image by WikiImages on Pixabay

Mercury, the smallest planet, is closest to the Sun. It moves in an elliptical orbit that looks more like an elongated O than a circle. Its distance from the Sun varies between 46 and 70 million kilometres, averaging 55 million kilometres.

Mercury is only 4,880 kilometres in diameter. It orbits the Sun once every 88 Earth days. It rotates very slowly, taking almost 59 Earth days to complete one rotation. Temperatures reach up to 425°C on the sunlit side, while on the night side they can drop below -170°C.

Mercury’s interior is made of iron, and its outer layer of rock. Its surface is covered in craters. The planet has only a very, very thin atmosphere, made up of hydrogen and helium. There is no water or life on Mercury.

Find out:

6. How long does Mercury take to orbit the Sun?

7. How hot does it get during the day and at night on Mercury?

8. What is Mercury made of?

5 Venus

The planet Venus

Image by WikiImages on Pixabay

Venus appears in the evening and morning sky as the brightest of all the stars. It is slightly smaller than Earth, with a diameter of 12,104 kilometers. It is about 108 million kilometers from the Sun. It takes 225 Earth days to orbit the Sun. Venus also rotates very slowly on its axis, completing one rotation every 243 days.

Compared with Earth’s rotation, Venus rotates in the opposite direction. This means the Sun rises in the west and sets in the east. Venus has an iron core, surrounded by layers of molten rock. This planet is wrapped in a dense cloud cover of carbon dioxide and sulfuric acid, which makes it appear especially bright.

Carbon dioxide causes a strong greenhouse effect. The surface of Venus is 475°C hot. Its dense cloud cover prevents Venus from being observed directly from Earth. It also means that this planet is equally hot during the day and at night. Life is not possible at these temperatures.

Find out:

9. Why can Venus only be explored from Earth to a limited extent?

10. How long does Venus take to orbit the Sun?

11. How hot is it on Venus?

6 Soil

This picture of Earth was taken during the flight of Apollo 17 to the Moon on December 7, 1972

Image: NASA

Seen from space, Earth appears blue. Much of its surface is covered by water, which reflects mainly the blue parts of the light.

Earth is the only planet in the solar system where life exists. It is made up mainly of three layers: the core, the mantle, and the crust. Earth's interior consists of a core of iron and nickel. Above it is the mantle, made of hot, viscous magma. In some places, magma rises to the surface through volcanoes. The continents and oceans are parts of the thin Earth's crust. It is made of solid rock, only about 30 to 50 kilometers thick, and very uneven. Different plates in the Earth's crust push against one another. The pressure from these plates formed the mountains. When they shift against each other, earthquakes occur.

The highest mountains on Earth are over 8 kilometers high. In the lower-lying parts of Earth’s crust, water has collected to form oceans that can be up to 11 kilometers deep. Earth is surrounded by a thin layer of air, made up mostly of nitrogen and oxygen. This layer of air keeps temperature differences between day and night small.

The Earth has a diameter of just under 12,800 kilometers. It spins on its axis. We call the time it takes to complete one rotation a day. It has one moon, and together they orbit the sun in 365 days. Its average distance from the sun is 150 million kilometers.

Find out:

12. Why does Earth appear blue from space?

13. What layers is the Earth made up of?

14. Which gases make up most of Earth’s atmosphere?

15. How long does it take Earth to orbit the Sun?

7 Mars

This is what Mars looks like. This image was taken by the Perseverance rover.

Image: NASA

It is known as the Red Planet. Its surface is covered with rock containing a great deal of iron, which gives it a reddish sheen. Ice can be found beneath a layer of frozen carbon dioxide at both poles. It is thought that Mars once had a lot of water, but it probably escaped into space.

Mars is only about half the size of Earth. Its diameter is 6,792 kilometers. It is 228 million kilometers from the Sun. It takes almost 2 years to orbit the Sun—exactly 687 Earth days. It rotates once on its axis every 24 hours and 40 minutes.

Mars’s atmosphere consists mainly of carbon dioxide. There is only a small amount of oxygen. The surface of this planet is covered in craters. It also has volcanoes. The largest is Mons Olympus, which is 500 kilometres wide and 25 kilometres high.

Mars has two moons, Phobos and Deimos. Both are very small and irregularly shaped. Their average diameters are just 25 kilometers and 13 kilometers, respectively. Temperatures on Mars can reach 25°C. At night, they can drop to as low as -133°C.

There are plans to send people to Mars sometime in the next few years. The outbound flight alone, however, will take about six months. Several probes have already landed on the planet, most recently one on 18 February 2021. There are already many images of Mars.

Find out:

16. Why is Mars called the "Red Planet"?

17. What is the highest mountain on Mars? 

18. How long does Mars take to orbit the Sun?

8 Jupiter

The planet Jupiter with the “Great Red Spot” in the southern hemisphere

Image: NASA (PD)

Jupiter is the largest planet in the solar system. Its diameter is 142,980 kilometres, about one tenth of the Sun’s diameter. It is twice as heavy as all the other planets combined. The Sun, however, is still a thousand times more massive than Jupiter.

It spins on its axis at great speed, completing one rotation in just 9 hours and 50 minutes. Jupiter is 797 million kilometres from the Sun. It takes 11 years and 316 days to orbit the Sun.

Jupiter is made up mainly of the gases hydrogen and helium. You cannot land on it, because a spacecraft would sink deep into it. The gases at its surface are constantly churning violently. The Great Red Spot has been observed in the planet’s southern hemisphere for at least three centuries. It is a gigantic storm.

At least 92 moons orbit Jupiter. The largest are called Io, Europa, Ganymede, and Callisto. They were discovered by Galileo Galilei in 1610. Their movements provided evidence that not all celestial bodies orbit Earth. Io has numerous active volcanoes, while Europa is almost entirely covered in ice.

Find out:

19. What is Jupiter made of?

20. What is the name of the giant storm on Jupiter’s southern hemisphere?

21. How many moons orbit Jupiter?

22. How long does Jupiter take to orbit the Sun?

9 Saturn

Saturn in natural color, photographed by the Cassini spacecraft from a distance of 6.3 million km

Image: NASA/JPL/Space Science Institute

Saturn is the second-largest planet in the solar system. Its diameter is 120,536 kilometers, and its average distance from the Sun is 1,376 million kilometers.

This means it is almost twice as far from the Sun as Jupiter. Even so, at certain times it can be seen in the night sky with the naked eye.

Saturn spins on its axis in just over 10 hours. Because it spins so quickly, it is much flatter at the poles. It takes 29 years and 233 days to orbit the Sun once.

Saturn is one of the four gas giants in the Solar System. It is made mostly of hydrogen and helium. At its centre is a core of rock and ice. Saturn is surrounded by many thousands of rings. They are made up of numerous large and small chunks of rock.

Saturn is orbited by at least 274 moons. The largest is Titan, with a diameter of 5,150 kilometres.

Find out:

23. What is Saturn made of?

24. What are Saturn’s rings made of?

25. How many moons orbit it?

26. How long does Saturn take to orbit the Sun?

10 Uranus

The planet Uranus is made up of hydrogen and helium

Image: NASA (PD)

Uranus was not discovered until 1781. This planet is about nineteen times farther from the Sun than Earth. Its average distance from the Sun is 2,870 million kilometers, and its diameter is 51,200 kilometers.

Uranus takes 17 hours and 44 minutes to rotate once on its axis. It takes this planet 84 years and 3 days to orbit the Sun.

Uranus consists almost entirely of the gases hydrogen and helium. Deep inside, there is probably a small core of rock and ice. It is bitterly cold on Uranus, with temperatures of -220°C.

Because of the methane gas in its atmosphere, Uranus looks bluish-green from a distance. Like Saturn, Uranus also has several rings. At least 27 moons orbit this planet, too.

Find out:

27. What is Uranus made of?

28. What temperatures are found on this planet?

29. How long does Uranus take to orbit the Sun?

11 Neptune

The planet Neptune takes more than 165 years to orbit the Sun once

Image: NASA (PD)

This planet was not discovered until 1846. Neptune is almost 4.5 billion kilometers from the Sun. It takes 165 years and 5 months to orbit the Sun once. So since its discovery, it has completed just one orbit around the Sun.

However, it takes only 15 hours, 57 minutes and 59 seconds to rotate once on its axis. It is similar in size to Uranus and is made of similar materials. Its diameter is 49,532 kilometres. Neptune is also probably made of rock and ice, but mostly of hydrogen, helium and methane. Methane in the atmosphere gives it its blue-green colour. Neptune has 14 moons, the largest of which is Triton.

Temperatures on Neptune reach minus 215 degrees, and extremely violent storms rage there at speeds of 2,000 km/h.

Find out:

30. Which planet is Neptune similar to?

31. How long does Neptune take to orbit the Sun?

32. How cold is it on Neptune? What wind speeds occur on this planet?