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 by Urbain 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
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
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.
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| Nobody likes being called a Venerean. |
Earth
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.
