The Sun itself dominates our Solar System, making up 99.8% of its mass. Eight major planets circle the Sun, divided into two groups: the small rocky planets of the inner solar system, and the massive gas giants beyond the asteroid belt.
Mercury is the smallest planet in our Solar System (about 4,900 km in diameter), and is closest to the Sun. Venus is hidden under a blanket of clouds, and is a hellish world of crushing atmospheric pressure, high temperature and acid rain. Earth is the largest of the rocky planets (12,742 km in diameter), and has the greatest density of any planet in the Solar System. Mars has a diameter of 6,799 km, about half that of the Earth, with a surface temperature range of -87 to -5°C. A day on Mars is about 40 minutes longer than an Earth day, and it takes 687 days to orbit the Sun. Jupiter is massive, with an equatorial diameter of 141,000 km. Its gaseous envelope has complex cloud bands that are roiled by massive storms. Saturn, famous for its complex ring system, is the sixth planet from the Sun and has an equatorial diameter of 120,536 km. Uranus is the first planet to be discovered with the aid of a telescope (in 1781 by Sir William Herschel), and is an unusual world, tipped on its side by an ancient impact. Neptune, the outer most of the major planets, was discovered by telescope (in 1846, by Urbain Le Verrier and Johann Galle) as the result of mathematical calculations.
Pluto, also discovered mathematically in 1930 (by Clyde Tombaugh) was once considered a planet, but is now categorised as a dwarf planet.
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