The Solar System contains the 4 outer, gas giant planets of Jupiter, Saturn, Uranus and Neptune, which are basically gigantic balls of gas and liquid. There are also the 4 inner, terrestrial planets of Mercury, Venus, Earth and Mars, which are balls of rock and metal. Mercury, Venus, Mars, Jupiter, and Saturn are easily seen from the Earth and have been known to man for a very long time. Uranus was discovered by William Herschel in 1781, and Neptune in 1846 by John Couch Adams. Pluto was discovered in 1930 by Clyde Tombaugh, but has since been relegated to a ‘dwarf planet’.
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