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 .
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.
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
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.
Size of Uranus compared to the Earth
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.