The Planets and Their Moons
A Strange Question
My mind works in mysterious ways. For no reason at all, the question came to me out of the blue: “Why doesn't our moon have a name, such as the moons of Jupiter do?” Ever since childhood, I've been interested in astronomy, but on a very informal level. For one, I lack the math skills to have gone into the field.
Back to my initial query; naturally, Google came to the rescue. Our moon does have a name: it's “Luna.” But that discovery was unsatisfying, because it's Latin, and the translation simply means “moon.” Back to square one. Our Earth's moon, therefore, does not truly have an actual name.
However, that spurred further research into the other planets and their moon or moons. It was a very interesting journey.
What about all the other planets, though? Do they all have moons? Some do, and some do not. Of all the planets having moons, the one most people have likely heard of is Jupiter, with its four largest probably at hand to the tongues of many (in no particular order); Io, Ganymede, Callisto, and Europa.
Read on, and the mystery and wonder of our solar system unfolds some tidbits I never knew, and chances are, neither did most people outside of the professional astronomical community.
Mercury and Venus
They must have been hiding in the proverbial closet when moons were handed out, for neither of these planets have any moons at all.
Just two moons orbit Mars: Phobos and Deimos. Named for the twin sons of Ares, who went to war with him. Ares is the god of war in Greek mythology.
Phobos means panic and fear, and Deimos means terror and dread. In Roman mythology, Ares is known as Mars, hence the relationship to the names of the moons of Mars.
Grab your coffee and settle in for a long and interesting exposition of the true number and nature of the moons of this gas giant.
The “big four” we've already named. But, hold onto your seat, and be sure you've already swallowed your drink. Jupiter actually has...79 moons! Whoa! More than its share, don't you think? The strong gravitational pull of this planet surely has much to do with its having pulled in so many orbiting satellites.
The first four were discovered in 1614 by none other than Gallieo Galliei. However, he did not name them. That happened a year later, by one Simon Marius.
Then, in 1892, E.E. Barnard discovered Amalthea. With 20th century improvements in telescopes, came new discoveries, in rather rapid succession: Himalia in 1904, Elara in 1906, Pasiphael in 1908, Sinope in 1914, Lysithea and Carme in 1938, Ananke in 1951, and Leda in 1974.
By the time the Voyager spacecraft made its flyby, thirteen more had been found. Themisto, discovered in 1975, was not included because up until the year 2000, not enough observations had been made to confirm its existence.
Voyager, in 1979, discovered Metis, Adrastea, and Thebe.
From October 1999 through February of 2003, a whopping 34 more bodies were found orbiting Jupiter which could classify as moons! All of them are in irregular orbits, some of which are retrograde (opposite the orbit of the planet). Some or maybe most of the retrograde moons are likely to be captured asteroids or comet nuclei.
By 2015, yet an additional 15 more, and 2 more in 2016, for a total of 69 moons had been found! In 2018, the International Astronomical Union (IAU) confirmed the prior discovery by Sheppard of Carnegie Institute, of 10 more, bringing the total count to 79.
Originally, the moons were numbered with Roman numerals, referencing the planet around which they orbited, e.g., “Jupiter V,” etc. In 1975, the IAU, through its subsidiary, the Task Group For Outer Solar System Nomenclature, adopted a formal naming practice, which was to name newly discovered moons of Jupiter after lovers and favorites of the god Zeus, with the exception of Dia, a lover of Jupiter, and Valetudo, after a great-granddaughter of Jupiter.
Names also reflect the type of orbiting body and their orbiting style. Those ending in “a” or “o” are for prograde (orbiting with the direction of the planet), and irregular satellites; names ending in “e” are for retrograde irregulars.
The most recently discovered moons, Jupiter LI through LXII have not been named as of this writing.
Second only to Jupiter, Saturn, famous for its rings, has 62 moons. Of these, Titan is the largest, being bigger than both Mercury and Pluto. After Titan, the next 7 among the largest are: Hyperion, Iapetus, Mimas, Rhea, Tethys, and Enceladus.
Of all these many moons, 53 have names; but of those, only 13 have diameters larger than 50km. Interestingly, Saturn's famous rings are also in orbital motion around the planet. 24 are considered regular satellites.
In 1847, John Herschel named the seven moons then known after Titans, Titanesses, and Giants who were the siblings of Cronus from Greco-Roman mythology. When these names were used up, the naming protocol shifted to names from other mythologies, those being the Inuit, Norse, and Gallic.
Hyperion and Titan are locked into a mutually resonant orbit at about the same distance from the planet.
Farther out from Saturn are 38 irregulars, which range in size from 4km to 213km. They may well be captured minor planets or debris from such captured bodies that collided.
The names of these are grouped by their orbital habits into the Inuit, Norse, and Galllic mythologies. The largest of these is Phoebe, the 9th moon, discovered in 1899 by Pickering. Fast forward to the 20th century, and in 1966, a 10th sphere was discovered by one Audouin Dollfus. He named it Janus. Later, Epimetheus was discovered, and it was noted to share the orbit of Janus. These last two are the only known co-orbitals in the entire solar system.
The Voyager flyby added Atlas, Prometheus, and Pandora. In 2004, the Cassini spacecraft's photos added Methone, Pallene, Polydeuces (the 2nd trojan moon of Dione—a moon with moons of its own!), and Daphnis.
In 2005, astronomers at the Mauna Kea Observatory in Hawaii found 12 more small outer moons.
In 2006, nine more irregular moons were found, and in April of 2007, Tarqeq (S/2007 S1) was found, and in May of the same year, S/2007 S2 and 3 were discovered.
In 2007, Anthe was added, and Aegaeon in 2009. This latest is classified as a “moonlet,” and orbits within Saturn's “G” ring...yes, the rings are labeled by alphabetical characters.
Since their original discovery, some have been “lost” due to their orbits not having been well known enough to allow pinpointing their current locations.
Shy of the two giants with a myriad of moons, Uranus sports a mere 27. Its moons are named for characters in plays by William Shakespeare and from Alexander Pope's “Rape of the Lock”! We have Umbriel, Titania, Oberon, Ariel, Cressida, Puck, Desdemona, and Miranda.
Titania, Oberon, Ariel, Umbriel, and Miranda are large enough to have planetary mass; they could possibly be called dwarf planets, were they in direct orbit around the Sun.
The innermost moons of Uranus seem to be small dark bodies that share common characteristics and origin with the planet's rings. Yep! Other planets besides Saturn have rings! All the remaining moons were discovered in 1985 between the Voyager flyby and advanced Earth-based telescopes.
In 1999 Perdita was discovered by study of old photos from the Voyager mission. Since 1997, 9 distant irregular moons from ground-based observation have been found, and two more small, inner moons, Cupid and Mab were found by the Hubble telescope in 2016. There is Juliet, but I did not find a corresponding Romeo.
In the shrinking moons game, Neptune has but 14. The largest, Triton, is nearly as big as our Earth's moon, and was found just 17 days after the planet's discovery. Its name did not come into popular use until the 20th century, even though it was first suggested by one Camille Flammarion from his 1880 book, “Astronomie Populaire.”
The names of Neptune's moons come from Greek and Roman water gods, mainly children of Poseidon (the Greek version of Neptune), lovers of Poseidon, and minor Greek water deities. These are, in no particular order: Triton, Proteus, Despina, Thalassa, Larissa, Halimede, Galatea, Neso, Naiad, Sao, Lao media, Psamathe, and Nereid.
A later discovery was Hippocamp, after a mythological creature said to be half horse and half fish! What imaginations they had! However, this moon was not named until very recently; in February of 2019—the same month of this article!
Neptune also has rings; the two innermost moons, Naiad and Thalassa are associated with these rings.
The planet was discovered in the 1930s by Clyde Tombaugh, and is in the Kuiper Belt, described as “a ring of bodies beyond Neptune.”
Poor declassified Pluto! Demoted to the status of a dwarf planet. There has been much argument about this, both within and outside the scientific astronomical community. The debate rages on; as of this writing, its status has not been changed since that action was taken.
The particular criteria which were established for a body to be considered as a planet are these:
A planet must be massive enough that its gravity would pull itself into a “hydrostatic equilibrium”: round if it wasn't rotating, but a spherical shape if it was.
A planet had to orbit the Sun and no other object: Earth could be a planet, but not the Moon.
And finally, it had to clear its orbit, meaning there could be no other comparably large masses at the same orbital distance: Mars is a planet, but asteroids and Kuiper belt objects were all out.
But I digress. Pluto has 5 moons, down to a detectable limit of 1km in diameter. In order of distance from the planet, they are: Charon, Styx, Nix, Keberos, and Hydra.
Charon is the largest, and is in a locked orbit such that they could be considered to be a double-dwarf planet, Pluto-Charon.
All of the names are from Greek mythology with ties to Hades (the Greek equivalent of Pluto), god of the underworld. Kerberos is Cerberus in Roman mythology.
What Else is Out There?
There are many dwarf planets, but two fairly recent discoveries are Eris, (with its moon, Dysnomia, is 27% more massive than Pluto), and Haiimea, (with its two moons Hi'iaka and Nemekea), were discovered in 2005 at the Keck Observatory, and are named after Hawaiian goddesses.
Humans have been stargazing since the beginning of our time on Earth, and will continue to do so. We will travel to the stars. Not in my lifetime, maybe not in our childrens' lifetimes. But that day will come.
First came science fiction, then came science fact, often mimicking or or even fulfilling the fantasies of paper and screen.
There are even those who believe we are descendants of 'star beings.' Who am I to say they are wrong? I keep an open mind on those matters, but with my feet firmly on Terra Firma, and one eye to the stars.
© 2019 Liz Elias