Pluto is the largest and second-most-massive known dwarf planet in the Solar System, and the ninth-largest and tenth-most-massive known object directly orbiting the Sun. It is the largest known trans-Neptunian object by volume but is less massive than Eris. Like other Kuiper belt objects, Pluto is primarily made of ice and rock and is relatively small—about one-sixth the mass of the Moon and one-third its volume. It has a moderately eccentric and inclined orbit during which it ranges from 30 to 49 astronomical units or AU (4.4–7.4 billion km) from the Sun. Light from the Sun takes about 5.5 hours to reach Pluto at its average distance (39.5 AU). Pluto has five known moons: Charon (the largest, with a diameter just over half that of Pluto), Styx, Nix, Kerberos, and Hydra. Pluto and Charon are sometimes considered a binary system because the barycenter of their orbits does not lie within either body.
On July 14, 2015, the New Horizons spacecraft became the first spacecraft to fly by Pluto. During its brief flyby, New Horizons made detailed measurements and observations of Pluto and its moons. In September 2016, astronomers announced that the reddish-brown cap of the north pole of Charon is composed of tholins, organic macromolecules that may be ingredients for the emergence of life, and produced from methane, nitrogen and other gases released from the atmosphere and transferred about 19,000 km (12,000 mi) to the orbiting moon.
The cosmic voice of Pluto
In the 1840s, Urbain Le Verrier used Newtonian mechanics to predict the position of the then-undiscovered planet Neptune after analyzing perturbations in the orbit of Uranus. Subsequent observations of Neptune in the late 19th century led astronomers to speculate that Uranus’s orbit was being disturbed by another planet besides Neptune. In 1906, Percival Lowell—a wealthy Bostonian who had founded Lowell Observatory in Flagstaff, Arizona, in 1894—started an extensive project in search of a possible ninth planet, which he termed “Planet X“. By 1909, Lowell and William H. Pickering had suggested several possible celestial coordinates for such a planet. Lowell and his observatory conducted his search until his death in 1916, but to no avail. Unknown to Lowell, his surveys had captured two faint images of Pluto on March 19 and April 7, 1915, but they were not recognized for what they were. There are fourteen other known precovery observations, with the oldest made by the Yerkes Observatory on August 20, 1909.
The name Pluto, after the god of the underworld, was proposed by Venetia Burney (1918–2009), an eleven-year-old schoolgirl in Oxford, England, who was interested in classical mythology. She suggested it in a conversation with her grandfather Falconer Madan, a former librarian at the University of Oxford’s Bodleian Library, who passed the name to astronomy professor Herbert Hall Turner, who cabled it to colleagues in the United States. The final choice of name was helped in part by the fact that the first two letters of Pluto are the initials of Percival Lowell.Symbol was then created as a monogram constructed from the letters “PL“. Astrological symbol resembles that of Neptune, but has a circle in place of the middle prong of the trident.
Origin and identity had long puzzled astronomers. One early hypothesis was that Pluto was an escaped moon of Neptune, knocked out of orbit by its largest current moon, Triton. This idea was eventually rejected after dynamical studies showed it to be impossible because Pluto never approaches Neptune in its orbit.
Orpheus before Pluto and Proserpina (1605), by Jan Brueghel the Elder
Plouton was one of several euphemistic names for Hades, described in the Iliad as the god most hateful to mortals. Plato says that people prefer the name Plouton, “giver of wealth,” because the name of Hades is fear-provoking. The name was understood as referring to “the boundless riches of the earth, both the crops on its surface—he was originally a god of the land—and the mines hidden within it.” What is sometimes taken as “confusion” of the two gods Plouton and Ploutos (“Wealth”) held or acquired a theological significance in antiquity. As a lord of abundance or riches, Pluto expresses the aspect of the underworld god that was positive, symbolized in art by the “horn of plenty” (cornucopia), by means of which Plouton is distinguished from the gloomier Hades.
The names of both Hades and Pluto appear also in the Greek Magical Papyri and curse tablets, with Hades typically referring to the underworld as a place, and Pluto regularly invoked as the partner of Persephone. Five Latin curse tablets from Rome, dating to the mid-1st century BC, promise Persephone and Pluto an offering of “dates, figs, and a black pig” if the curse is fulfilled by the desired deadline. The pig was a characteristic animal sacrifice to chthonic deities, whose victims were almost always black or dark in color. A set of curse tablets written in Doric Greek and found in a tomb addresses a Pasianax, “Lord to All,” sometimes taken as a title of Pluto, but more recently thought to be a magical name for the corpse.
Pasianax is found elsewhere as an epithet of Zeus, or in the tablets may invoke a daimon like Abrasax.
I know that even below the earth, if there is indeed a reward for the worthy ones, the first and foremost honors, nurse, shall be yours, next to Persephone and Pluto.