The symbols for the planets, Moon, and Sun (along with the symbols for the zodiac constellations) were developed for use in both astronomy and astrology. The symbol for Pluto is a monogram made up of the P and L in Pluto and also the initials of the astronomer, Percival Lowell, who predicted its discovery.
The ancient Babylonians named the five planets (or "wandering stars") that were visible to the naked eye after their gods. The ancient Greeks followed suit, and later the ancient Romans translated these names into those of the Roman deities who corresponded to the Greek gods. These are the names we know today. The outer three planets were named in modern times, but according to the same tradition and manner as the ancient names.
Pluto was discovered in 1930 by the American astronomer Clyde Tombaugh. The name "Lowell" was first suggested (after Percival Lowell). However, this would have broken with the established priniciple of naming planets after mythological deities. Pluto was suggested by Venetia Burney, an 11-year-old Oxford schoolgirl. The first two letters were the initials of Percival Lowell, and the god of the underworld, Pluto, was the brother of both Poseidon (Neptune) and Zeus (Jupiter). The name, like Uranus, breaks with the tradition of Roman names, because there was no Roman equivalent to the Greek. It is appropriate that the god of the underworld was chosen for this dwarf planet that was the outermost object known in the solar system at the time, and so was the most remote and in "eternal night".
Image Credit: Lunar and Planetary Institute
Credit: Lunar and Planetary Institute