Is Pluto’s moon Charon actually a dwarf planet?

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Charon is the largest of the five known moons, or natural satellites, of the dwarf planet Pluto.

With a diameter of 1,212 kilometres (slightly more than half the diameter of Pluto), Charon is the 12th-largest natural satellite in the solar system. It is calculated to be composed of 55 per cent rock and 45 per cent ice, compared to Pluto’s 70 per cent rock and 30 per cent ice. It orbits Pluto at an average distance of 19,570 km.

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While Charon is currently listed as a satellite or moon by the International Astronomical Union (IAU), consideration is being given to it perhaps being classified as a dwarf planet in its own right, due to its mass being approximately 12.2 per cent that of Pluto’s, and the fact that neither object truly orbits one another.

The two bodies slowly rotate around a common center of mass, referred to as a barycentre, which lies 960 km above the surface of Pluto. Consequently, a proposal was once put forward (and later abandoned) that the Pluto-Charon system should be classified as a “dwarf-double planet” — a binary (two) satellite system where both objects are planets or planetary-mass objects, whose barycentre is exterior to both planetary bodies.

How are Pluto and Charon different than other planets and moons?

Pluto and Charon are gravitationally or tidally locked with one another.

Typically, it is only the satellite that is tidally locked to the larger body, as with the Earth-moon system, whereby the moon always shows the same face towards Earth (although not vice versa).

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Where both the difference in mass and the distance between the two bodies are relatively small, both objects may be tidally locked to the other, as with Pluto and Charon, where the same side or face of each body always faces towards the other.

How was Charon spotted?

Charon was discovered on June 22, 1978, by American astronomer, James Walter Christy, at the US Naval Observatory in Washington, DC, USA. It was initially going to be called “Vulcan” after the Roman god of fire. However, the IAU rejected that name, as it was not the name of an Underworld deity, and had already been suggested for a hypothetical planet orbiting between Mercury and the sun.

Instead, on June 24, 1978, the IAU approved the name suggested by Christy: Charon, the boatman who transported the souls of the dead into Hades, the realm of the dead in Greek religion and mythology.

Charon is what is referred to in mythology as a “psychopomp” (from the Greek meaning “guide of souls”): a creature, spirit, angel, ghost, or deity in many religions that escorts or transports the souls of the newly deceased from the real world to the afterlife.

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Hades was an earlier name given to the Greek god of the Underworld. Ancient Roman religion and mythology would eventually equate the Etruscan chthonic deities — gods or spirits, typically associated with death or fertility, who inhabited the Underworld or existed in or under the ground — Dis Pater and Orcus with Hades, and subsequently blend all three into Pluto, the ruler of the Underworld, keeping Hades as the name of the realm over which he (Pluto) ruled.

In keeping with the tradition of naming the solar system’s planets after ancient Roman (as opposed to ancient Greek) deities, the “ninth planet” discovered (later demoted to “dwarf planet”) was named Pluto.

A look at the mythology behind Charon

Charon was the boatman who ferried the souls of the newly deceased across the River Acheron from the land of the living to the Underworld.

While ancient Greek literary sources place Charon on the River Acheron, Roman writers place him on the River Styx. In his famous Aeneid, the Roman poet Virgil (70-19 BC) identifies Charon with both rivers. According to Greek mythology, there are actually five rivers that flow within the realm of Hades — Styx, Lethe, Acheron, Phlegethon, and Cocytus — each of which had a unique role in how Hades functioned, as well as a unique character name to reflect an emotion associated with death.

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The River Styx (Hatred or Abhorrence) is the principal river of Hades, circling the Underworld seven times; named for Styx, the nymph of the river, it flows out of the world river, Oceanus, and is typically the river associated with the Underworld and the boatman Charon.

Upon entering the Underworld, the newly dead are required to drink from the waters of the River Lethe (Oblivion or Forgetfulness) so they would forget their former earthly existence. The River Acheron (Woe or Misery) is, in some Greek tales, the principal river of the Underworld, displacing the River Styx as the river across which the souls of the deceased must cross to enter Hades.

The River Phlegethon (Fire) is said to extend as far as Tartarus, the lowest level of Hades, Tartarus, where the landscape is covered in the flames of funeral pyres, and the dead are judged. The River Cocytus (Wailing) is a river of cries and lamentations, along the banks of which the souls of those who could not afford Charon’s fee to cross into Hades, or who had not received the proper funerary rites, were doomed to wander for eternity.

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While the River Acheron is often indicated as the divide between the real world and the Underworld, many literary references and paintings show Charon ferrying the dead across the River Styx into Hades. Before Charon ferried the dead across into Hades, they were obliged to pay him a fee in the form of a coin for his services.

Archaeology has confirmed that such coins, referred to as “Charon obols” — coins placed on or in the mouth, or sometimes on the eyes, of the dead before burial as a payment for the deceased’s passage into the afterlife — were a common feature of many ancient funerary rites around the world.

In Latin, such a burial coin is called a viaticum or “sustenance for the journey” (into the afterlife). In ancient Greece, Charon’s fee was typically a coin of gold or silver, or, among the less-wealthy, copper or bronze.

The coin was a symbolic seal to protect the deceased’s soul and to prevent it from returning (to haunt the living). Those who could not afford Charon’s fee were doomed to roam the banks of the River Cocytus for eternity, never to be granted access to Hades.

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How did Charon form?

Computer simulations in 2005 suggest that Charon could have formed as the result of a hypothetical violent collision, when a large Kuiper Belt object struck Pluto at a high speed, destroying itself while blasting a huge amount of Pluto’s outer mantle into space, which subsequently coalesced into Charon.

This is similar to the mechanism that astronomers initially believed formed Earth’s moon. However, such a hypothetical collision should have resulted in an icier Charon and a rockier Pluto than what exists today.

A recently published paper by C. Adeene Denton et al., titled “Capture of an ancient Charon around Pluto”, proposes an alternate hypothesis. Far from being a violent collision, which would have destroyed them both, the paper suggests the two icy objects, both likely residents of the Kuiper Belt, may have had a much softer collision, slowly coming together in a “kiss and capture” mechanism, at first slowly spinning around one another like two dancers in a “pas de deux” until they bumped into each other.

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Such a soft impact may have boiled off any volatile gases, such as methane, but would not have been enough to destroy either body.

Pluto and the proto-moon Charon may then have remained loosely attached as a single object in something called a “snowman effect” (where the snowball sections adhere to one another), until their combined spin eventually caused them to separate at a later date into the binary planet-moon system observed today, but remain gravitationally bound to one another, rotating around a common center of mass or barycenter.

This new hypothesis may help explain a lot of observations of other recently discovered two-lobed objects in the Kuiper Belt, such as the famous New Horizon mission’s observations, in 2015, of the Kuiper Belt object known as Arrokoth.

It’s believed that Arrokoth, a two-lobed body connected by a thin “neck” of material, may have, like Pluto and Charon, initially been two separate bodies, orbiting each other in synchronicity, before colliding at a very slow speed and forming a binary “snowman” system. The discovery of Arrokoth and other similar two-lobed objects would suggest that the “kiss and capture” phenomenon may be a fairly common feature of many objects within the Kuiper Belt.

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Does Pluto have other moons?

Pluto’s four other circumbinary, irregularly-shaped moons are theorized to be debris generated when Pluto and Charon separated.

The Styx, approximately seven kms on its longest axis, is the smallest of the five, but the closest moon to Pluto; it was discovered by the Hubble Space Telescope on July 11, 2012, and is named for the River Styx, across which the boatman Charon ferried the souls of the deceased to Hades.

Kerberos, 12 kms in length, the second smallest of Pluto’s five moons, is named for the three-headed, serpent-tailed dog, Cerberus, that devoured anyone attempting to escape from Hades, was discovered on July 20, 2011. The IAU chose the Greek spelling “Kerberos” because Cerberos was already the name of the asteroid 1865 Cerberus.

The two largest moons, Nyx (or Nix) and Hydra, 42 kms and 55 kms respectively along their longest axes, were discovered in 2005, and named in 2006. Nyx was the Greek goddess of the night, while Hydra was a multi-head serpent that would be one of the Greek hero Heracles’ (the Roman, Hercules) famous Twelve Labors.

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This week’s sky

Mercury (magnitude -0.5, in Sagittarius – the Archer), soon to pass behind the sun (superior conjunction), is not observable this coming week, as it is one degree below the southeast horizon at dawn.

Venus (mag. -4.5, in Aquarius – the Water-bearer) becomes visible around 5:30 p.m., 33 degrees above the southwest horizon as dusk fades to dark, before sinking towards the horizon and setting by about 9:15 p.m. On the evening of Feb. 1, Venus will sit just 2.3 degrees to the right of the three-day-old, 13 per cent-lit, waxing crescent moon in the southwest sky after sunset.

Mars (mag. -1.3, in Gemini – the Twins) becomes accessible 20 degrees above the eastern horizon around 5:30 p.m., reaching 69 degrees above the southern horizon around 11:30 p.m., and remaining observable until about 6:25 a.m., when it sinks below seven degrees above the northwest horizon.

Jupiter (mag. -2.6, in Taurus – the Bull) becomes accessible in the evening sky 47 degrees above the eastern horizon around 5:30 p.m., reaching 65 degrees above the southern horizon by about 8:20 p.m., and remaining observable until about 3:05 a.m., when it sinks below seven degrees above the northwest horizon.

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Saturn (mag. +1.1, in Aquarius – the Water-bearer) becomes visible about 5:55 p.m., 23 degrees above the southwest horizon, before sinking towards the horizon and setting around 10:30 p.m.

Uranus (mag. +5.7, in Aries – the Ram) becomes accessible 60 degrees above the southeast horizon around 6:25 p.m., reaching a height of 62 degrees above the southern horizon shortly after 7 p.m., and remaining observable until around 12:15 a.m., when it drops below 21 degrees above the western horizon. On Jan. 30, Uranus ends its retrograde motion across the sky.

Neptune (mag. +7.9, in Pisces – the Fish) becomes visible shortly around 6:25 p.m., 28 degrees above the southwest horizon, before sinking towards the horizon and setting around 9:30 p.m.

Correction

My apologies. In last week’s article, I meant to display each planet’s gravitational force compared to that of the sun as percentage figures, and not as fractions. I also mistakenly used 293 instead of 293.9 in my calculations. Here are the correct figures, rounded up.

The other planets in the solar system have the following gravitational forces (with each planet’s force as a percentage of the sun’s): Mercury 3.9 N/kg (1.33 per cent); Venus 8.8 N/kg (3.0 per cent); Mars 3.7 N/kg (1.26 per cent); Jupiter 24.7 N/kg (8.40per cent); Saturn 10.7 N/kg (3.64 per cent); Uranus 9.0 N/kg (3.06 per cent); and Neptune 11.7 N/kg (4.0 per cent).

Until next week, clear skies.

Events:
Jan. 29 – New Moon
Jan. 30 – Uranus ends its retrograde motion across the sky
Feb. 1 – Venus 2.3 degrees to right of waxing crescent moon in the southwest post-sunset

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