Astronomy: Moons

A popular style of quiz question is, "Which planet has moons (or satellites) named ...?" This page aims to help with what can be a very confusing subject.

The number of known moons has risen steadily over the years, especially since the dawn of what used to be called "the space age". Last time I checked (November 2022), Jupiter had 80 known moons, Saturn had 83, Uranus 27 and Neptune 14. Mars has two, both of which were discovered in 1877; Pluto, now a dwarf planet, has five. Jupiter and Saturn had each gained one since my previous check (in 2020).

These numbers came from GoAstronomy.com, which was the best summary of Solar System moons that I could find.

The latest moon of Jupiter to be discovered was Ersa (in 2018, according to Wikipedia). It's named after the Greek goddess of dew. Saturn's new moon has yet to be given a proper name, but is known as S/2019 S 1. It was discovered in 2019 and announced in 2021.

GoAstronomy also lists eight other dwarf planets (apart from Pluto) that have moons: Haumea (2), Orcus, Salacia, Quaoar, Makemake, Varda, Gonggong and Eris (one each). In 2020 it also listed seven minor planets in the asteroid belt that have eleven moons between them, but I couldn't find this list in 2022.

That's a grand total of 231 (including the eleven in the asteroid belt, and Earth's solitary moon).

The good news for quizzers, however, is that there are only about ten that you really need to know much about – although there are about fifteen more that it might be useful to know the names of.

Before we move on ... some other fascinating facts I learnt from GoAstronomy:

•	Mercury is too close to the Sun to hold on to any moons
•	Venus may have had a moon in the distant past, which impacted the planet after colliding with another object
•	Earth has several quasi–satellites – asteroids 2020 CD3 and 2020 HO3 being the closest with the most stable temporary orbits
•	Saturn also has "hundreds to thousands" of moonlets embedded in its ring system
•	Triton (the largest of Neptune's 14 moons) is believed to be a dwarf planet from the Kuiper Belt, captured by Neptune
•	Ceres, the only dwarf planet in the asteroid belt, has no moons ("surprisingly")

The figures (below) are the mean diameters of the respective moons.

Mars

Mars has two known moons:

	Phobos		Deimos
	22 km		12.5 km

Both Phobos and Deimos were discovered in 1877 by the American astronomer Asaph Hall. They are named after the sons of Ares, the Greek god of war (Mars to the Romans), who accompanied their father into battle. Their mother was Aphrodite (Venus to the Romans). Phobos and Deimos were, respectively, the personifications of panic (or fear) and terror (or dread).

Note that Phobos and Deimos are by some margin the smallest of the moons covered on this page, except for the smallest moons of Pluto. (All four of the giant planets have lots of other moons that are bigger than these two, but are never going to come up in quizzes.)

Jupiter

Jupiter's six biggest moons, in descending order of size, are:

	Ganymede		Callisto		Io		Europa		Himalia		Amalthea
	5,262 km		4,821 km		3,643 km		3,122 km		170 km		167 km

The four largest satellites of Jupiter were first observed in 1610 by Galileo, who quickly realised what he had discovered. He wanted to name them the Medicean Stars, in honour of his patron, Cosimo II de' Medici – the heir to the Grand Duke of Tuscany – but this never really caught on. It was Johannes Kepler who first suggested that names from Greek mythology should be used, and the German astronomer Simon Marius (who had originally claimed to have found the Galilean satellites) came up with the names by which they are known today.

Those names came from various 'love interests' of Zeus (the Greek equivalent of Jupiter). Ganymede was the (male) cup–bearer to the gods, beloved of Zeus; Callisto was a nymph; Io was a priestess of Hera; Europa was a Phoenician noblewoman who became the queen of Crete.

According to Wikipedia, it was only in the mid–20th century that these names came into "common use". Until that time they were known as Jupiter I (Io), Jupiter II (Europa), Jupiter III (Ganymede) and Jupiter IV (Callisto) – according to their respective distances from the planet.

The smallest of the four so–called Galilean satellites is Europa. The diameter of Jupiter's fifth biggest satellite, Himalia (one of the four that orbit closer to the planet than the Galileans) is only slightly more than 1/20th that of Europa.

Saturn

Saturn's eight biggest moons, in descending order of size, are:

Titan

Rhea

Iapetus

Dione

Tethys

Enceladus

Mimas

Janus

5,150 km

1,527 km

1,470 km

1,123 km

1,062 km

504 km

396 km

179 km

The principal satellites of Saturn are named after the Titans – including Titan itself (discovered by Huygens, in 1655), Iapetus, Rhea, Tethys, Dione (Cassini, 1671), Mimas, Enceladus (Herschel, 1789), Hyperion (1848), Phoebe (Pickering, 1899). Note however that Mimas and Enceladus were not actually Titans but Gigantes (see Titans in Mythology). Calypso (discovered in 1980) was also not a Titan; she was a nymph.

Uranus

The six biggest moons of Uranus are, in descending order of size:

	Titania		Oberon		Umbriel		Ariel		Miranda		Puck
	1,577 km		1,523 km		1,169 km		1,158 km		472 km		162 km

The two largest (Oberon and Titania ) were discovered in 1787 by William Herschel; the next two (Umbriel and Ariel) in 1851 by William Lassell, and the fifth largest (Miranda) in 1948 by Gerard Kuiper. Only these five were known until the visit of the Voyager 2 spacecraft in 1985–6. Puck (the biggest of the ones discovered by Voyager 2) orbits just inside the "big five".

The satellites of Uranus are all named after characters from Shakespeare – except for two or three that are named after characters from Alexander Pope.

Umbriel and Ariel are named after characters from Pope's The Rape of the Lock. A third moon – Belinda, which was one of the ones discovered by Voyager 2 – is also named after a character from The Rape of the Lock; Belinda is the unfortunate victim of the 'rape'.

Rather confusingly, Shakespeare also has a character named Ariel, in The Tempest.

Neptune

The four biggest moons of Neptune are, in descending order of size:

	Triton		Proteus		Nereid		Larissa
	2,705 km		420 km		340 km		195 km

The principal satellites of Neptune are named after water nymphs from Greek mythology.

The largest one, Triton, was discovered in 1846, just 17 days after Neptune itself. Next to be discovered – just over 100 years later, in 1949 – was Nereid, with a diameter of just over one–eighth that of Triton. Larissa was discovered in 1981, and in 1989 five more were discovered – the largest of which was Proteus (second biggest of them all!).

Triton's orbit is retrograde to the rotation of Neptune, and is steeply inclined relative to the planet's equator. In both of these respects, Triton is unique among the larger moons of the Solar System.

Pluto

Pluto, although no longer classified as a planet, has five known satellites:

	Charon		Hydra		Nyx		Kerberos		Styx
	1,207 km		50 km		50 km		12.5 km		10.5 km

Charon is the only satellite in the Solar System that's bigger, in comparison to its parent body, than our own Moon. It's so big that the barycenter (the centre of gravity, in layman's terms) of the Pluto–Charon system lies outside Pluto. Charon's mean diameter of 1,207 km is just over half that of Pluto (2,368 km).

In Greek mythology, Charon was the ferryman who carried the souls of the dead across the rivers Styx and Acheron to Hades (which was ruled by Pluto).

Hydra and Nyx were discovered by the Hubble telescope in 2005. Hydra is so called because the mythological creature of that name had nine heads, and Pluto was, at the time of its discovery, still the ninth planet (in both size and orbital radius). Nyx is named after the personification of night in Greek mythology.

Two more satellites were discovered in 2011 and 2012 respectively. Kerberos is named after Cerberus, the three–headed dog that guarded the endrance to Hades; and Styx is named after the river already referred to.

Size Order

When I was researching this page, I noticed that if you sort the biggest satellites into descending order of size, they fall into several well–defined groups:

	Ganymede		Titan		Callisto
	Jupiter		Saturn		Jupiter
	5,262 km		5,150 km		4,821 km

	Io		The Moon		Europa		Triton
	Jupiter		Earth		Jupiter		Neptune
	3,643 km		3,475 km		3,122 km		2,705 km

Titania

Rhea

Oberon

Iapetus

Umbriel

Ariel

Dione

Tethys

Uranus

Saturn

Uranus

Saturn

Uranus

Uranus

Saturn

Saturn

1,577 km

1,527 km

1,523 km

1,470 km

1,169 km

1,158 km

1,123 km

1,062 km

	Enceladus		Miranda		Proteus		Mimas		Nereid
	Saturn		Uranus		Neptune		Saturn		Neptune
	504 km		472 km		420 km		396 km		340 km

	Larissa		Janus		Himalia		Amalthea		Puck
	Neptune		Saturn		Jupiter		Jupiter		Uranus
	195 km		179 km		170 km		167 km		162 km

Notice how the biggest gap in the first group is 329 km, then the gap between the smallest satellite in the first group and the largest in the next group is nearly 1,200 km. Similarly, the biggest gap in the second group is 417 km, then there's a gap of 1,128 km to the biggest in the third group.

The following diagram illustrates this:

This diagram shows the relative diameters of all of the bodies that we've been discussing on this page, plus the minor planets, the biggest asteroids, and the planet Mercury (all of which are shown on the top row). The next smallest body in the Solar System (the smallest one not shown on the diagram) is Mars, with a mean diameter of 6,780 km – very nearly 40% bigger than Mercury, and well off to the right of the diagram on this scale.

The red dots are where there are two bodies that are too similar in size to show separately on this diagram.

The white vertical bands represent the ranges of sizes where there are satellites, and the grey bands represent the ranges where there aren't. My point is that the grey bands are similar in size to the white ones. This suggests that the sizes of satellites in the Solar System may not be random.

I'm not sure if this is significant scientifically; but by breaking the list into groups like this it helps me to remember the order.

© Haydn Thompson 2017–23