You'll find symbols that represent the Solar System bodies in many astronomy books and charts. They were developed in the Middle Ages when astronomy was closely linked to astrology. More recently, planets like Uranus have been allocated symbols on their discovery. The IAU recognises the older symbols and gives approval for any new symbols proposed, like the recent one for Ceres. Eris has so far failed to receive a symbol, despite several proposals.
The symbol for the Sun is supposed to represent the solar disc with a central sunspot. Mercury, the Roman god of trade, is depicted as the god's head. Venus is the Roman goddess of love, and the symbol represents her hand mirror. Earth is shown as a globe bisected by meridian lines into four quarters, while the Moon is a crescent. Mars's symbol shows the shield and spear of the god of war.
Of the outer planets, Jupiter, named after the head of the Roman gods, is represented by a stylised lightning bolt. Saturn was named after the Roman god Saturnus, and the symbol depicts an ancient sickle. Uranus was the personification of heaven in Greek mythology and the symbol for Uranus is a combination of those for the Sun and Mars - the Sun represents light, and Mars represents power. Neptune, named after the Roman god of the sea, has a symbol that shows the god's weapon: a trident. The dwarf planet Pluto is a monogram of the initials of Percival Lowell, who predicted the planet's discovery.