Neptune is the outermost planet of our Solar System since Pluto's demotion in 2006. The planet was named after the Roman god of the sea; in Greek legend Neptune is known as Poseidon. Like the other outer planets, Neptune has many moons - we know of 14, ranging in size from almost as large as the Earth's Moon, down to the smallest at 18km diameter. Their names reflect a connection with the sea god; they honour mythical aquatic characters. This Entry provides some scientific information about the moons of Neptune and some fascinating stories involving their namesakes.

Neptune I: Triton

Triton was discovered on 10 October, 1846, by English astronomer William Lassell (1799 - 1880); one of the rings of the planet Neptune is named in his honour. Triton is extremely interesting to astronomers because of the mystery surrounding it. Triton orbits in retrograde motion, meaning it goes around Neptune in the opposite way to the planet's spin direction. There can be only one reason for this; Triton is a captured object rather than a natural satellite. This in itself is not unusual; both of Mars' moons Phobos and Deimos are captured asteroids. Triton, however, is much larger than the usual space captive; at 1,353.4km radius it's not far off the size of Earth's Moon. Astronomers don't know how such a large object became captured, but they can offer their best guesses. One idea is that Triton crashed into one of Neptune's original moons; this slowed Triton down so much that it was trapped by Neptune's gravity. The resulting debris would have created some of the other moons and formed one of the planet's rings. Another interesting feature of Triton is the presence of an atmosphere¹, likely caused by Triton's active geysers.

Triton the Greek God

Triton is the son of Poseidon and Amphitrite. He is portrayed with the top half of a human male, but his bottom half is a fish tail. He is depicted with a conch or triton shell and a trident similar to his father's. Disney used the classic image of Triton in their animated film The Little Mermaid; he was a king and the father of the title character, the feisty mermaid Ariel. Triton blew into his shell to announce the imminent arrival of his father the sea god. In nature a triton is a giant sea snail with a very attractive protective shell.

Neptune II: Nereid

Nereid was the second moon detected around the ice giant, primarily because it has a highly elliptical orbit ranging from less than a million miles to over six million miles. It was discovered in 1949 by Dutch astronomer Gerard Kuiper (1905 - 73), after whom the Kuiper Belt was named. Interestingly, Nereid orbits Neptune in just over 360 days which is almost the same amount of time the Earth takes on its annual journey around the Sun.

Nereids

The Nereids, after whom Neptune's second moon was named, were in legend the 50 sea nymph sisters who attended Poseidon. They were the friendly daughters of Nereus and Doris who rode dolphins and hippocamps (seahorses). Some of the Nereids aided sailors in peril, unlike Sirens who attracted humans with hypnotic song across the waters then watched as their ships were dashed upon rocks. Some stories about Nereids appear in the Iliad and the Aeneid.

Leader of the Nereids, Thetis, was the mother of the demigod Achilles. When he was born, Thetis tried to make Achilles immortal by immersing him in the River Styx, the magical waterway dividing the world of the living from that of the dead. Unfortunately the heel by which his mother was holding him was not covered by the water so it was left vulnerable. Achilles was eventually killed by an arrow which penetrated that very spot. Today the expression 'Achilles' heel' is used as a metaphor for a weakness or soft spot, possibly leading to a downfall.

Neptune III: Naiad

Naiad is the closest moon to Neptune. It was discovered in September 1989 by the Voyager 2 team analysing data sent back to Earth. Unfortunately, Naiad has a subsynchronous² orbit around Neptune, which means that it is doomed. Gravitational tidal forces are dragging it down and its orbit will eventually decay, though this probably won't happen in our lifetime.

Naiads

Naiads are the freshwater nymphs of Greek legend. There are many stories about them collectively and individually, usually involving sex. One Naiad was shunned by her fellows when she left their waterworld and married a human. Another blinded her unfaithful lover. Some Naiads abducted Hylas, one of the fabled Argonauts, and spirited him away to their underwater home. Hylas was a special friend of Heracles, who was so distraught when his lover disappeared that he left the Argo to search for him, to no avail.

Neptune IV: Thalassa

Thalassa was discovered in September 1989 by the Voyager 2 team. Thalassa's orbit is subsynchronous so it is doomed along with Naiad.

Thalassa

Thalassa means 'sea' in Greek. There are ancient stories concerning the sea goddess Thalassa. In one she was the parent of 'fish children' with the sea god Pontus (the progenitor of Poseidon), and their descendants became all the marine life we know today - whales, octopuses, electric eels, dolphins, candiru and all things piscatorial. In another legend Thalassa was one of Zeus' lovers and they begat the half-human/half-fish race of undersea merpeople.

In Aesop's Fables a shipwrecked man berates the sea which takes the form of a woman (Thalassa) who replies: 'Blame not me, my good sir, but the Anemoi (winds), for I am by my own nature as calm and firm even as this earth; but the winds suddenly falling on me create these waves, and lash me into fury.' There are images of Thalassa in Roman mosaics as the top half of a woman in the sea with crab claws sprouting from her head and seaweed draped across her breasts.

Neptune V: Despina

Despina was discovered in July 1989 by the Voyager 2 team, but it was two decades later when Professor Ted Stryk realised that Voyager 2 had captured Despina transiting Neptune, something which had not been seen before. Despina's orbit is subsynchronous so it is doomed along with Naiad and Thalassa.

Despina

Despina means 'lady' in Greek and it is a popular given name. In Greek mythology there was a daughter of Poseidon and Demeter whose name was a secret to all who were not initiated into her cult, but for the sake of clarity we'll call her Despina. Her conception was unorthodox, but nothing out of the ordinary for a Greek myth: Poseidon fancied Demeter, goddess of the harvest, who, in order to escape his attentions, morphed into a mare. Not to be outfoxed, Poseidon turned himself into a stallion who pursued, caught and had his wicked way with Demeter. The resulting twin pregnancy brought forth an immortal foal, Arion, and the goddess Despina. Due to her secret nature, statues of Despina were created with heavy veils, which must have been a boon to sculpture students not yet honed in human anatomy.

Neptune VI: Galatea

Galatea was discovered in July 1989 by the Voyager 2 team. Galatea is the shepherd moon for the Adams Ring, named after the Cornish³ mathematician and astronomer John Couch Adams (1819 – 1892), surrounding Neptune. Adams predicted the existence of Neptune and calculated its orbit in 1845, based on a detailed study of its effects on Uranus. Unfortunately, he passed on his reckonings to the Astronomer Royal George Biddell Airy, who didn't capitalise upon the information, so Urbain Le Verrier and Johann Gottfried Galle claimed the planet's discovery in 1846. Astronomers have counted the number of gravitational grooves carved out of the Adams Ring and it numbers 42, which is an incredible coincidence involving someone named Adams! Galatea is a doomed moon like Naiad, Thalassa and Despina. As it is gradually drawn towards Neptune, the stress will probably tear Galatea apart, possibly creating a ring of its own.

Galatea

There is a Greek myth about a Galatea, a marble statue who came to life for her sculptor Pygmalion, but Neptune's moon wasn't named after her. A Nereid called Galatea had two suitors, a Cyclops named Polyphemus and Acis, a shepherd. Acis thought he didn't stand a chance with the beautiful Galatea so he played a lament upon his flute. Galatea heard the music and sought out the performer. When they met, she fell in love and the couple embraced. Unfortunately for them, Polyphemus had followed Galatea so he witnessed their lovemaking. Stung by jealousy, the enraged Cyclops picked up a rock and smashed Acis' head with it, killing him instantly. Galatea was so devastated by the loss of her lover that she created the River Acis from his blood.

Neptune VII: Larissa

Larissa was discovered by a team headed by H Reitsema in May 1981 when it eclipsed a background star. It orbits Neptune in 13 hours 20 minutes. Larissa will share the fate of its sister moons Naiad, Thalassa, Despina and Galatea one day, as it also has a subsynchronous orbit.

Larissa

The ancient Greeks thought the first man was Pelasgus. As Larissa was one of his children, she could possibly lay claim to being the first daughter. In another story Larissa was a nymph from Thessaly in Greece, who bore Poseidon three sons - Achaios, Pelasgus and Phthios. Larissa was so highly thought of in her hometown that her face appeared on some local coinage, circa 400-320 BC.

Neptune VIII: Proteus

Proteus is another moon discovered by the Voyager 2 team. This is the second-largest Neptunian moon after Triton, measuring approximately 436km by 416km by 400km. Proteus is cube-shaped, although it's not much below the mass required for its gravity to have moulded it into a sphere just like our own lovely Moon.

Proteus

Proteus was named after the Greek sea god who was a son of Poseidon and brother to Triton. Proteus had the ability to shapeshift and could also foretell the future. In one legend, Proteus assisted Apollo and Cyrene's apiarist son Aristæus, whose bees had all died off. When Aristæus had done Proteus' bidding, his reward was a new swarm of bees which were disease-free.

In another story, Proteus helped King Peleus win the love of the Nereid Thetis. Their wedding feast was hosted by no less a personage than Zeus himself, with entertainment provided by Apollo⁴. Unfortunately the nuptials were gatecrashed by Eris, the goddess of discord, who sparked a chain of events which led directly to the Trojan War.

Neptune IX: Halimede

Halimede was discovered in 2002 by MJ Holman, JJ Kavelaars and their team. It takes just under 1,880 days to orbit Neptune.

Halimede

Halimede is a Nereid, described as 'lady of the brine'. Brine in this instance could mean naturally occurring salt water, or a saline solution of more than 5%. Created brine is used as a food preservative, so a modern-day Halimede could conceivably be the goddess of tinned tuna. Brine also has a connection with temperature; Daniel Gabriel Fahrenheit set 0° on his scale at the point where brine froze.

Neptune X: Psamathe

Psamathe was discovered by SS Sheppard, DC Jewitt and J Kleyna in 2003. It is so far distant from Neptune (almost 47 million km) that one complete revolution takes over 24 Earth years. Even though that's an enormous orbit, Psamathe isn't a record-breaker; the title of our Solar System's widest-orbiting moon goes to Neptune's 13th, Neso (see below).

Psamathe

Psamathe is the Nereid goddess of sandy beaches. She was married to Proteus the Greek sea god, but King Aeacus of Aegina, a demigod son of Zeus, wanted her. Pursued by him on one of her beaches, Psamathe changed herself into the form of a seal in an attempt to put him off. But his desire for Psamathe was so strong that when King Aeacus caught the seal he had sex with her. Psamathe conceived and gave birth to Phocus. When he grew up, Phocus became renowned for his physical prowess and he was competing in the Olympic Games when he was accidentally killed by one of his half-brothers. Psamathe was so distraught at the loss of her only child that she commanded a wolf to attack her son's killer. Psamathe's sister Thetis, who mourned six sons herself, turned the wolf to stone to prevent another tragedy.

Neptune XI: Sao

Sao was discovered in 2002 by MJ Holman, JJ Kavelaars and their team. It is affected by Lidov–Kozai mechanism, which means its orbit is disturbed by other satellites in the Neptunian system.

Sao

Sao is the Nereid who watches over sailors and keeps their boats and ships safe. Presumably scouring sea lanes for icebergs wasn't on her checklist.

Neptune XII: Laomedeia

Laomedeia was discovered in 2002 by MJ Holman, JJ Kavelaars and their team. Note to Hitchhiker's fans, Laomedeia measures 42km in diameter!

Laomedeia

Laomedeia is one of the Nereids, the 'leader of the folk', according to Hesiod's Theogony. Maritime music, sea shanties, lyrical ballads and any folk songs with a nautical theme would no doubt be within Laomedeia's remit. On the last night of the annual BBC Proms concerts Sir Henry Wood's Fantasia on British Sea Songs is celebrated. Always very popular is the 'Sailor's Hornpipe', especially when it speeds up. During HM Queen Elizabeth II's Diamond Jubilee celebrations in 2012, members of the British Royal Family were filmed performing their own moves to 'Sailor's Hornpipe' during the Thames River Pageant.

Neptune XIII: Neso

Neso was discovered in 2003 by MJ Holman, JJ Kavelaars and their team. Neso has the widest orbit of any known moon, positioned at 48,387,000km⁵ distance from its parent planet. At that range it takes 9,374 days (over 25 years) to complete one circuit around Neptune.

Neso

Neso, another one of the Nereids, is the goddess protector of islands. As there are thousands of Greek islands alone, Neso must be one busy girl!

Neptune XIV

As if to prove that our view of the Solar System is in a constant state of flux, the discovery of Neptune's 14th moon, designated S/2004 N 1, was announced in July 2013 after this Entry had been written. The as-yet-unnamed moon, which had not been spotted by Voyager 2, orbits Neptune between Larissa and Proteus in just under one Earth day. Neptune's smallest moon to date at less than 20km across was detected by Senior Research Scientist Mark Showalter and his team at the SETI Institute⁶ after studying old images taken by the Hubble Space Telescope between 2004 and 2009. Showalter is also responsible for analysing the Hubble's data involved in the discovery of the Plutonian moons Kerberos and Styx.

Future Discoveries

As we have seen, Neptune has 14 moons – at this moment in time – eventually the orbits of the innermost moons will decay and they will no longer dance in attendance to the ice giant world. We don't know if all Neptune's moons are known; there may be a few (or lots) more lurking out there which will require naming in the future. The second Neptunian moon detected was named Nereid after the 50 sea nymph daughters of Nereus and Doris, and six subsequent ones honoured individual Nereids. This means there are plenty of unused, available names for nomenclature of future discoveries, including the remarkable Thetis, goddess of water, whose character has been portrayed on film by luminaries such as Julie Christie and Dame Maggie Smith.