When the ancient Greeks studied the heavens they observed points of light which seemed to move back and forth against the background of apparently fixed stars.
These moving lights shone steadily and did not twinkle like the stars.The Greeks called these heavenly bodies planets, meaning 'wanderers'. We know now that the planets are those bodies, like the Earth, which revolve around a star, the Sun. Planets do not give off light of their own, but get their light from the Sun. Including the Earth, there are nine planets in our solar system that is revolving around our Sun. In sequence, moving away from the Sun, they are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto.
The first four are known as the Inner Planets and are mainly solid. Mercury is the smallest of the planets; Venus can be seen as the Evening Star in the western sky; Mars seems red even to the naked eye. The others are known as the Outer Planets and the first four are composed mainly of gases but Pluto is believed to be solid. Jupiter is the largest of all the planets, about 1,000 times as big as Earth, and has twelve satellites; Saturn is surrounded by three beautiful rings; Uranus has five satellites and Neptune two; Pluto was only discovered in 1930.
Other stars in space probably have planets also but, as planets give off no light, we cannot detect them with our present instruments.
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