The inner planets are Mercury, Venus, the Earth and Mars. They are all rock planets with a metal core. Mercury and Venus can only be seen occasionally from Earth. Mercury is a desert planet, little bigger than the Moon of the Earth. Its surface is covered in craters of which the largest is the Caloris basin.
Venus is about the same size as Earth, but cannot support life. The surface is covered by a thick cloud of sulfuric acid. Temperatures are a little under 500┬░ Celsius. Besides large plains, there are two highlands called Ishtar Terra and Aphrodite Terra.
It was long suspected that Mars had life, but this was not confirmed. It has been called the red planet because its surface is covered in iron oxide. The Olympus Mons, a huge extinct volcano, is one of its most prominent features. Mars has two moons: Phobos and Deimos. They have been called "potato moons" because of their irregular shapes.
The outer planets are Jupiter, Saturn, Uranus, Neptune and Pluto. All except Pluto are gaseous planets.
Jupiter is the biggest planet. It consists mainly of hydrogen and helium and is shrouded in turbulent clouds. A huge storm system, the Great Red Spot, is especially prominent. The four Galilean moons Io, Europa, Ganymede and Callisto were discovered by Galileo Galilei. There are also numerous smaller moons.
Saturn has a beautiful ring system made of particles of rock and ice. It is accompanied by numerous moons, the largest of which is Titan.
Uranus has a dense atmosphere of hydrogen and helium. The methane in the atmosphere gives the planet a blue-green appearance. Seventeen moons of various sizes orbit Uranus, most of them first discovered by the Voyager 2 space probe.
Neptune is a blue planet with cloud bands. The Great Dark Spot, a storm system larger than the Earth, is one of its most prominent features. Triton is by far the largest of Neptune's moons.
Pluto is the outermost planet in our solar system. Because of its unusual orbit, it sometimes crosses the path of Neptune, making the latter the outermost planet for a time. Pluto's moon is called Charon and is almost as big as the planet itself. For this reason the planet and its moon are referred to as a double planet.
