134340 Pluto

This dwarf planet is famous for once having been considered the ninth planet in our Solar System. As far as minor planets go, it was among the first couple thousand to be discovered, and the first whose orbit lies partially beyond Neptune. Because it was reclassified in 1993, it received a much larger designation number than it would have originally garnered. The minor planet is named after Pluto, the Roman god of the underworld. His Greek counterpart is Hades, and his wife was Proserpina.

This minor planet was discovered in 1930 by Clyde Tombaugh, as part of a "Planet X" observational project started by Percival Lowell at Lowell Observatory. There was an incredible number of names proposed by the general public for this so-called planet. Constance Lowell, widow of Percival, suggested Zeus, Percival, and Constance — all objectively bad names. At least a thousand other names were suggested, including Cronus, Odin, Persephone, Erebos, Atlas, Prometheus, and more. The one that Vesto Slipher, director of Lowell Observatory, selected was the suggestion of 11-year-old Venetia Burney, for the Roman god of wealth and the underworld. He is also known for his skill of invisibility, much like this barely-perceptible ex-planet.

There is a common astronomical symbol for this minor planet, devised by Slipher: a monogram composed of the letters P and L, representing both the name Pluto and honoring Percival Lowell. It is represented by the Unicode symbol U+2647, ♇. There is also an astronomical symbol, created by I know not who, which resembles the symbol for Neptune and combines a circle, representing Pluto's helm of invisibility, with an arc on a staff, representing his bident. With apologies to Dr. Silpher, I lean towards the more evocative astrological symbol, but the "PL" monogram finds representation in the symbology in the symbols for both 1884 Lowell and 6235 Burney.

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The Plutonian system has five moons. The largest of these is Pluto I Charon, named after the ferryman of the underworld rivers Styx and Acheron in Greek myth. His duty was to take the deceased to their afterlife, for the price of a single coin, which in Greek funerary tradition was placed in or on the mouth of the deceased upon burial. The symbol for this moon is a lunar crescent, sharing the same broad curve shape as Pluto, with the same small circle above. Together, they represent Charon's boat, and the payment needed to ride on it.

Interestingly, the Plutonian system is more of a Pluto-Charon binary system, as the epicenter of orbit for these two bodies exists outside both of their surfaces. Pluto just happens to be the larger one.


The next two moons to be discovered were Pluto II Nix, which shares a referent with the asteroid 3908 Nyx, the Greek primordial goddess of the night, and Pluto III Hydra, a nine-headed serpent monster also from Greek myth. The mythological Hydra does not have any specific connection to the underworld or to Hades, but it is a sly reference to Pluto's former ninth planet status. The goddess Nyx doesn't really either, other than being Hades' great aunt, but at least she is on theme with darkness and stuff. Nyx and Hydra together form an acronymic reference to the New Horizons mission, through which both of them were discovered.


The symbol for Nix is as Pluto, but with a Nyxian/Chaotian starburst instead of a circle, in part representing her primordial nature. The symbol for Hydra is as Pluto, but replacing the vertical line and cross is a curved body, like the constellation Hydra, connecting to the circle.

The dwarf planet moon Pluto IV Kerberos is named after Cerberus, sharing a referent with the asteroid 1865 Cerberus, the giant hound with three (usually) heads that belonged to Hades. Heracles stole him once. The symbol for this satellite is a broad Plutonian curve, with three tangent circles resting above it, one on top of the other two, representing the hound's multiple heads.


The dwarf planet moon Pluto V Styx is named after the largest and longest river in the underworld, which surrounds all of Hades' domain and mingles with all of the other rivers. It is also perhaps the name of a forgotten deity, a river goddess that swore allegiance to Zeus in the Titanomachy. Its waters granted near-invulnerability to Achilles, and ancient Greeks were said to swear oaths in the name of this river. The symbol for this satellite is as Pluto, but with a wave crossing the vertical staff instead of the usual cross.


Interestingly, Kerberos and Styx were originally granted the names Vulcan and Romulus, respectively, according to an official online poll. These were ostensibly named after the Roman god of the forge and the mythical founder of Rome, again respectively, but were really references to fictional planets from the Star Trek series. The names were denied by the IAU, because Vulcan was already the name of a now-debunked planet within the orbit of Mercury, and there is already an asteroidal moon by the name of 87 Sylvia I Romulus.

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