The Planets
Mercury
Mercury is the closest planet to the Sun in the Solar System. Without the protection of atmosphere, Mercury's day can be as hot as 400 degrees Celsius and the night as cold as -170 degrees Celsius. It is also the smallest planet in the Solar System.
Venus
Covered by thick clouds, Venus undergoes the extreme greenhouse effect, making it the hottest planet in the Solar System. The atmospheric pressure at the surface is 92 times that of Earth. Unlike Earth and most other planets in the Solar System, Venus's rotation is clockwise.
Earth
Earth is the largest of the four inner rocky planets of the Solar System. With the ocean covering 71 percent of the surface, Earth is called the Blue Planet. The 23.5 degree tilt of the axis causes the seasons. The Moon is the only natural satellite of Earth.
Mars
With a volume that is about 15% of Earth's, Mars is the second smallest planet in the Solar System. Its surface is rich in iron oxide, which makes the planet appear red. Mars's global magnetic field disappeared about four billion years ago. Without the shield of the magnetic field, most of the atmosphere was removed by the solar wind.
Jupiter
Jupiter is the most massive planet in the Solar System. It is called a gas giant because it is mainly composed of hydrogen and helium and has no solid ground. More than 60 moons have been found around Jupiter. The four largest moons, the Galilean moons, were discovered by Galileo Galilei in 1610.
Saturn
Although Saturn is not the only planet with rings, it has a very extensive ring system that can be observed from Earth. As one of the two gas giants, Saturn is the second largest planet in the Solar System. The low density and fast rotation cause Saturn to bulge at the equator and flatten at the poles.
Uranus
The most noticeable feature of Uranus is probably the axis that is tilted over 90 degrees. The blue-green color of Uranus is caused by methane contained in the clouds. As one of the two ice giants in the Solar System, Uranus contains more ice, methane and ammonia than the two gas giants, Jupiter and Saturn.
Neptune
Neptune was discovered in the 19th century through mathematical prediction rather than observation. 4.5 billion kilometers distant from the Sun, Neptune takes 165 years to make one orbit around the Sun. Its diameter is about four times that of Earth. The Great Dark Spot on the surface is a huge storm that is about the same size as Earth.
The Sun
Our star has been shining for more than four billion years and will continue burning for another five or six billion years. Most of its mass consists of hydrogen and helium. It is so large that one million Earths can fit into it. Inside the core of the Sun, energy is generated by the process of nuclear fusion and then reaches our planet in the form of light and heat that we feel everyday.