The trident is the symbol for Neptune, the god of the sea who carries a three-pronged fork. Symbols for the planets and zodiac constellations were developed for use in astrology and astronomy.
The five visible planets in ancient times were named after Babylonian gods.The Greeks in turn named them after their gods, and the Romans translated the names to correspond with the Roman deities, which are the names we are familiar with today. The three outer planets were named in modern times following the same tradition.
Heinrich D’Arrest and Johann Balle discovered the planet in 1945, and the name Neptune, which had previously been a name considered for Uranus, was given to the newly discovered planet. According to Roman mythology, Neptune was Jupiter’s brother and Saturn’s son.