Everyone knows that Martians come from Mars. That's straightforward enough. Beyond that, things get fiddly. Just what do you call someone from Jupiter, Venus or Pluto? What of the asteroid belt? Even in professional astronomy circles, people argue over the correct use of demonyms and adjectives for astronomical bodies. Much of the terminology used today originates from alchemical, astrological and classical works, and the growth of science fiction has only made it more confused. Of course, nowadays we know the likelihood of finding little green men on Mars or Mercury is slim to none, but the use of adjectival forms is necessary to describe features on the planets or their satellites. Plus, there's still fiction - everyone loves a good, old-fashioned Martian invasion.
Firstly, we need to look at where the names of the planets come from. The eight major planets, barring the Earth, take their names from Roman gods, with which they were identified by early astronomers. The Roman gods are mostly equivalent to the Greek pantheon, the Romans being dab hands at assimilating the traditions of other cultures, so each planet has an equivalent Greek name, which, unsurprisingly, is how they are known in Greece. In classical astronomy, the sun and moon were considered to be planets, since the exact definition of a planet was a long way off (and still hasn't really been settled on). The Romans knew of five actual planets beyond the Earth: Mercury, Venus, Mars, Jupiter and Saturn, all of which are, under the right conditions, visible to the naked eye. Uranus was identified as a planet in 1781 by astronomer Sir William Herschel, having previously been misidentified as a star. Neptune was discovered in 1846 byUrbain le Verrier, although it may have been discovered independently by John Couch Adams three years earlier (in fact, Galileo had first seen the planet over two hundred years earlier, but again, had misidentified it as a star). Pluto, which is, of course, no longer considered a planet, wasn't discovered until 1930, by Clyde Tombaugh. Rather wonderfully, a portion of his ashes are currently en route to Pluto on the New Horizons spacecraft.
The upshot of this is that the adjectival forms of planets have to follow classical rules, which sometimes throws up some oddities. Also, the traditions of astrology - which, for centuries, was part of astronomy - and alchemy, have granted supposed properties to the planets. In alchemy, each classical planet had a symbol, and each of the spheres was thought to be related to a human attribute. The result is that the adjectival forms of planet names have meanings in themselves in modern English.
Mercury, the innermost and smallest planet of the solar system, is named for the fleet-footed messenger of the gods; the original, Roman version being Mercurius, which is sometimes used in astrology. His Greek equivalent is Hermes. The adjectival form of Mercury is mercurial, which can mean swift, quick-witted, volatile or changeable. Capitalised, Mercurial is used to describe objects and features related to the planet Mercury, although Mercurian is becoming more commonly used. An inhabitant of Mercury would be a Mercurian. An alternative demonym or adjective would be Hermean, from Hermes; a related term is hermetic, once used in alchemy, but now used to mean sealed away (as in 'hermetically sealed'). Mercury is, of course also the name of a metallic element; all the classical planets were alchemically linked to a particular metal, but only mercury has adopted the name of its planet (the traditional names being hydrargyrum or quicksilver). Mercurial can, therefore, also refer to compounds of the element mercury.
Venus is where things start to get problematic. Named for the goddess of love and sex, Venus is the only major planet named after a female figure (although many minor planets are too). The adjectival form of Venus is venereal, which does not have a pleasant connotation. The Greek equivalent, Aphrodite, is rather pretty, but again produces an adjective with unfortunate connotations: aphrodisial or aphrodisiac. The correct term for an inhabitant of Venus would, therefore, be a Venerean, but the connotations of venereal disease means that this has, unsurprisingly, fallen out of favour. The most common word used today is Venusian (occasionally Venutian), but this is linguistically problematic, kind of like using 'Marsian' for something from Mars. My preferred term is Cytherian/Cytherean, a word originally related to the Greek island of Kytheros, which had traditional links to the goddess Aphrodite. It's a very attractive word, in my view. Sci-fi stories featuring inhabitants of Venus sometimes use both Venusian and Cytherian as names for separate cultures or species on the planet.
Venus is both the Morning and Evening Star, and in Latin, had separate names for both phases. It was Hesperus, or Vesper, in the evening, and Phosphorus or Eosphorus in the morning. A such, other terms you might like to use for Venusians are Hesperian and Phosphorian.
Nobody likes being called a Venerean.
The Earth is, of course, named after that dusty stuff we stand on, rather than any mighty god or goddess. The Latin term for the earth was tellus. The goddess of the Earth was Tellus, or Terra, whose name lives on in romance languages as the term for the Earth (such as the Spanish Terra or French le terre). The Greek goddess was Gaia or Gaea, again, still in common use as an alternative name for the Earth, or its biosphere as an entity, in the so-called Gaia Hypothesis.
The correct adjective to apply to the Earth is terrestrial, used for both things of the earth, as in the land, and the Earth, as in the planet. From this we get the term extraterrestrial, for things beyond the Earth. Science fiction has never been happy with this term. Too mundane, I guess (mundane also meaning 'of the world,' as it happens, from the alternative Latin term mundus). Why not Terrestrian for an inhabitant of the Earth? Old-fashioned sci-fi popularised the terms Earthling and Earthman, which today are considered old hat and are usually used humorously. Futurama suggested Earthican. The most common term for inhabitants of Earth in sci-fi is Terran, which is very occasionally used scientifically; indeed, Terra is commonly used as an alternative name for the Earth in English language sci-fi. Confusingly, terra is also used for landmasses on other worlds in astronomy. An alternative term that I favour is Tellurian, from Tellus, which was used by E.E. 'Doc' Smith in his Lensman series, and by Robert Holmes in his Doctor Who work. Tellurium is a semi-metallic element sometimes called sylvanium; tellurian can also refer to compounds and ores of this element. Arthur C. Clarke, in 3001: The Final Oddyssey and other works, stuck with terrestrials for Earth-born humans, often with 'Terries' as a mildly pejorative nickname used by offworlders.
SOME MARTIANS, YESTERDAY.
Mars, being nice and red and fiery looking, was named for the Roman god of war. As everyone knows, someone or something from Mars is Martian, from the same etymology as martial, meaning warlike or pertaining to soldiery (as in 'martial law'). Martial is still sometimes used in astronomy in the place of Martian. The Greek equivalent of Mars was Ares, which is unfortunately sometimes confused with Aries, the Ram in astrology. The alternative term for Martian is therefore Arean, although sci-fi works have sometimes chucked in the wonky term 'Aretian.'
By Jove! Jupiter, or Iupiter, the king of the gods, was also known as Jove, and hence the adjectival term for Jupiter is jovial, which of course means jolly, or good-spirited. The term used in astronomy today is Jovian, although some sci-fi writers will insist on using 'Jupiterian' or somesuch rubbish. Jovian is also used to refer to the gas giant planets as a group. The Greek name for Jupiter was Zeus, so one might argue for Zeutian as an equivalent.
Stone Men of Saturn
Saturn, or Sarturnus, was an agricultural god of Rome, but was also identified with the powerful Greek figure Kronos (latinised as Cronus). Kronos was, understandably, conflated with Chronos, the god of time, and so Saturn became identified as such a deity as well. The great harvest festival of Saturnalia was a major Roman holiday, and we get the word Saturday from Saturn as well. The traditions of Saturn lived on in our image of Father Time, who traditionally carries a sickle or scythe - a conflation of the agricultural and temporal nature of the god.
The adjectival form can be saturnial, however saturnine is far more common. Saturnine is the opposite of jovial, meaning ill-tempered, sullen or melancholy (that's me then). It can also mean containing, or poisoned by, lead. Beings or objects from Saturn are Saturnian, although Saturnine can be used. The Greek equivalent, Kronos, also gives the possible alternative Kronian. The Doctor Who episode Vampires of Venice gave us aliens from a planet called Saturnyne, while Star Trek has pinched Kronos as the name for the Klingon homeworld (although any true Trekkie would spell it Q'onoS).
The first of the 'new' planets, in the 'second zone,' Uranus was very nearly named Georgium Sidus, or 'George's Star,' after King George III. Unlike the other planets, Uranus is named for a Greek god, and is the latinised form of Ouranos (the Roman equivalent is in fact Caelus or Coelus). The term for things from Uranus is Uranian. We should be thankful it's not Uranal.
There's still plenty of arguing over how one should pronounce Uranus. The traditional pronunciation is 'Your anus,' responsible for generations of schoolboy giggling. The modern compromise, most popular in America, is 'Yura-nuss.' In Futurama, we learn that by the 31st century it shall be renamed 'Urectum,' thereby ending all that silliness once and for all. In my view, we should probably go back to calling it George.
Neptune, now the farthest planet from the sun, is named for the Roman god of earthquakes and the oceans, the equivalent of the Greek Poseidon. Neptune had previously been suggested for the planet that became Uranus. Other suggestions for the name of the new planet included Oceanus and Janus, the latter of which wold have fit rather nicely, since the two-faced god of doorways would suit a world on the edge of the planetary sphere. Neptunian is the correct term, although Neptunial is very occasionally used.
The Sun and Moon, capitalised or not as to your taste, round off the seven classical planets, and are thus part of the same astrological and alchemical system as the true planets. Having numerous names in different languages and cultures, there are several different adjectivals and demonyms for each.
The Latin for sun is sol, which survives as the term in several romance languages. The major Roman sun god was Sol Invictus; the Greek was Helios. The common adjectival term is, of course, solar, however an alternative term is heliacal (not to be confused with helical). An inhabitant of the Sun - as unlikely as this might seem - is often known as a Solarian, or sometimes a Helian, in sci-fi works.
The Roman goddess of the Moon was Luna, the name being used as a proper noun for the Moon in science fiction, to distinguish it from the innumerable other moons of the solar system. The common adjective is lunar. The Greek equivalent of Luna was Selene, more rarely called Cynthia, and so alternative terms are Selenian and Cynthian. Demonyms for inhabitants of the Moon have include Lunarians, Cynthians and Selenians. H.G. Wells's The First Men in the Moon featured a race of insectoid Selenites, ruled by a Grand Lunar. Strax from Doctor Who prefers the term 'Moonites.'
The Selenites abuse Professor Cavor
Mars' two moons, Phobos and Deimos (fear and panic), give the demonyms and adjectives Phobian and Deimosian. Jupiter has, at present count, sixty-seven satellites, edging it past Saturn at sixty-two (but this is bound to change at some point, they do tend to fight over it). The four major moons of Jupiter, which might well be regarded as planets, or at least dwarf planets, if they orbited the Sun directly, are termed the Galilean satellites, after their discoverer, Galileo Galilei. They are Io, Europa, Ganymede and Callisto, named for lovers of Zeus/Jupiter. Their adjectivals are Ionian, Europan, Ganymedean and Callistoan, although Callistonian has been used as an alternative for the latter. Ionian, of course, already refers to a region of Europe and a sea, and Europan is very close to European - indeed Europe is named for Europa (or possibly vice versa). The moons of Jupiter are named for Zeus' lovers and daughters, except for seventeen recently discovered runts, which have yet to be named. My favourite is Megaclite (and yes, that would be Megaclitean).
Saturn's moons are traditionally named for Titans, the giant gods that preceded the Olympians in Greek myth, although the sheer number of them has led to other cultures' giants being used for names. Other than the sixty-two proper moons, there are over a hundred 'moonlets' embedded in Saturn's mighty rings, and the rings themselves are comprised of countless particles, so arguably Saturn has about a million moons. The largest, Titan, is world in itself, and the adjectival is Titanian. Some moons' names cause some anomalies. Pan leads to Pandean, not so bad there, but Celtic, American and Norse names leads to some weirderies: Tarqeq, named for the Inuit moon god, leads to Tarquipian (and an inhabitant of Tarqeq might be a Tarquip!)
The Uranian moons are named for characters from the works of Shakespeare (and Alexander Pope, although only two have come from this source, Belinda and Umbriel, although Ariel could come from either. The greatest of the Uranian moons are Oberon and Titania, the adjectivals for which are Oberonian and Titanian. Yes, this last one is spelled the same as those from Titan, but pronounced differently (like Dominican and Dominican). The adjectival of Puck is the lovely Puckian, although I quite fancy Puckese.Neptune also has a bunch of moons, named for water deities, the greatest of which is Triton, adjectival Tritonian.
A Plutonian sky
DWARF PLANETS, ASTEROIDS AND PLANETESSIMALS
I have decided not to get into what constitutes a planet here, in concern for this piece going completely off on a tangent. There are, currently, five official dwarf planets, with many more distant bodies that may get classified as such once more accurate measurements of size have been made. The most famous, of course, is Pluto, former planet and icy bastion of the 'third zone.' Pluto is named for the Roman god of the underworld, who was himself begat by a conflagration of the Greek underworld god Hades and the Roman god of wealth Pluton (from which we get such terms as plutocracy). The adjectival and demonym for Pluto is Plutonian, although Plutovian has cropped up in sci-fi. An alternative, from the Greek, is Hadean, which has a nice ring to it (unlike Pluto). Pluto's major moon, Charon, gives us Charonian, and we have the minor moons Nix, Hydra, Kerberos and Styx, giving us Nictian, Hydrian, Kerberian and Stygian.
The remaining dwarf planets are Ceres, Haumea, Makemake and Eris. Makemake stands out as obviously not Greco-Roman, named for a god of the Rapanui people of Easter Island. The adjectival is Makemakean. Haumea is also not Greco-Roman, but Hawaiian, but at least sounds like it fits in; naturally, the term is Haumean. Eris, the distant icy rock that caused the whole planet/dwarf planet debacle in the first place, was aptly named for the Greek goddess of strife and discord; her adjectival is Eridian.
Ceres compared with the Earth and Moon.
Ceres is the both the largest asteroid and the smallest dwarf planet, as yet. Named for the Roman goddess of the harvest, she was originally considered a planet, until further asteroid discoveries led to a rethink (so the Pluto/Eris debacle is nothing new). Being rooted in Latin means that the adjectival and demonym is Cererian, sometimes spelled Cererean. Sci-fi usually simplifies this to Cerean. Other large asteroids include Pallas, Juno and Vesta, giving us Palladian, Junonian and Vestian.
There are thousands of asteroid and minor planets in the solar system, with names as wonderful as Rhadamanthus and Agamemnon (Rhadamanthyan and Agamemnonian, naturally). Asteroids can have moons; Rhea has two satellites, Romulus and Remus, and so Romulan is a perfectly acceptable astronomical term. Once the names of gods started running dry, the asteroid belt became something of a free-for-all when it came to nomenclature. What you call something from 8353 Megryan is anyone's guess.