What are Astronomical Planet Symbols used to Represent the Planets?

When astronomy was closely linked to astrology, astronomical planet symbols were used to represent various celestial objects.

The planets' related metals and corresponding days were represented by the symbols in alchemy and calendars, respectively. Although their current shapes are a development from the 16th century, the use of these symbols dates back to Classical Greco-Roman astronomy.

You'll find symbols representing the Solar System bodies in many old 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 for their discovery. The IAU recognises the older symbols and approves any new symbols proposed, like the recent one for Ceres. Eris has so far failed to receive a symbol despite several proposals.

However, the International Astronomical Union (IAU) discourages the use of these symbols in modern journal articles, and their style manual proposes one- and two-letter abbreviations for the names of the planets for cases where planetary symbols might be used, such as in the headings of tables. The exception is the Sun ☉ which is still used, especially when comparing stars with our Sun; for example, astronomers will say that a star is 2M_☉ which means it has 2x the mass of our Sun.

What are the Planet Symbols?

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.