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It appears to be a fairly common planetary naming scheme in science fiction: Take the common name (or its bayer designation) of star and append the planetary ordinal in the form of a Roman or Arabic numeral.

Examples

It's not based in reality

I asked an astronomer and he assured me, no astronomer ever proposed such a naming system.

Real exoplanets today are named using an entirely different naming scheme.

Who came up with it?

It is very common in Star Trek, but it exitsted in earlier science fiction novels and short stories. What is the earliest occurrence of this naming scheme? When was it first described? When was it first implied?

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    My instinct is that it derives from the naming convention for gas giant moons which prominently features the [primary] [Roman numeral] format. But that doesn't say much about how it came to be used in science fiction. – Cadence Sep 9 at 18:36
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    I've got another from 1951: "The Masquerade on Dicantropus" by Jack Vance. "... Dicantropus being a relay point for ULR messages between Clave II and Polaris." and "...en route to a contract on Thuban XIV." – DavidW Sep 9 at 19:49
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    I'd like to point out that naming "real exoplanets" is a relatively recent endeavor. The earliest discovery was in 1992, but space-based sci-fi has been around quite a bit longer than that. – Ellesedil Sep 9 at 23:54
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    A possible reason why it is not applied to exoplanets is that we are not able to count the planets orbiting a star, it is difficult to detect them at all. So if we detect an Exoplanet we have no idea if it should be I or something like IX. I would argue that if we were able to see all the planets around a start then this planetary naming scheme would likely be adapted as it seems both convenient and practical. – Erik Sep 10 at 14:35
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    As an addendum, even the use of the name "Sol" to refer to the Sun around which Earth orbits is a bit of a weird quirk common to sci-fi but not real life astronomy. – Jim Cullen Sep 11 at 6:17
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The naming convention has been in common usage forever in science fiction. E.E. Smith from the first Galactic Patrol serial in 1937 referred to planets such as Velantia III, Rigel IV, and Palain VII, and Earth was specifically referred to as Sol Three from time to time in the series.

The following examples are from Galactic Patrol:

"For instance, Kinnison here once had a highly adventurous interview with a lady of Aldebaran II and her friends."

Astounding, September 1937, page 11

...

"Kimball Kinnison of Sol Three calling Mentor of Arisia. Is it permitted that I approach your planet?"

...

"Kinnison of Tellus, greetings. Tregonsee of Rigel IV calling from Trenco space-port. Have you ever landed on this planet before?"

...

"Lensman of Trenco Space-port--Tregonsee or his relief? Lensman Kinnison of Sol III asking permission to land."

...

"....the fifth dove into the deepest ocean of Corvina II, in the depths of which all rays are useless."

...

Out from Radelix and into deep space shot the speedster, bearing the Gray Lensman toward Boyssia II, where the Boskonian base was situated.

...

"He was Lageston of Mercator V--a good man, too. What is your pressure now?"

...

Addendum

One should note that at no point does Smith explain that "Sol III" means "third planet from the star Sol" or Aldebaran II means "second planet from the star Aldebaran". This implies one of two things: either Smith used it for the very first time and expected the readers to figure out what it meant without explanation--which given Smith's writing style would seem to be unlikely since he tends to explain everything--or the style of naming planets in that way was already in science fiction at the time so as not to require explanation.

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    Ah, that beats me; the earliest I can easily find in my collection is Asimov's Foundation, the relevant parts of which were written in 1944. – DavidW Sep 9 at 20:29
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    This answer could be improved by providing a specific quote with citation from the series mentioned. – Harabeck Sep 9 at 20:41
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    "In the Lensman Series, begun in 1937, Kimball Kinnison visited planets with names such as Velantia III, Rigel IV, and Palain VII." - Me. – Keith Morrison Sep 9 at 20:59
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    Note that Triplanetary (as published in 1948) contains some similar names, but they did not appear in the original 1934 serial; the relevant chapter appears to have been a later addition. I have not yet checked if these names are original to the the 1937 serial of Galactic Patrol. – DavidW Sep 9 at 21:28
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    Heh. Astounding, September 1937, page 11: "For instance, Kinnison here once had a highly adventurous interview with a lady of Aldebaran II and her friends." – DavidW Sep 9 at 22:37
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This is a naming scheme that has been used for moons since they were discovered. For example, since their discovery all the way to the 20th century, Jupiter's moons were known simply as Jupiter I, Jupiter II, Jupiter III and Jupiter IV. As new moons were discovered, this practice was followed. Even for the four most easily observable moons of Jupiter, the naming scheme surived for about three hundred years; many of the others only got their name in 1970s or later.

Remember that before heliocentric (or geocentric) models came to a good description of the solar system, people didn't even know that planets had an ordering. They were simply "wanderers" - stars that moved in relation to the "fixed" stars, and in predictable (though sometimes complicated) patterns. So they were named long before the ordering was discovered, and they were often given divine qualities (which still survive today in e.g. Astrology). If planets were somehow only discovered in the 17th century, like the moons of Jupiter, it's quite possible we would have simply called them Sol I, II, III...

The main problem with this is that the ordering can change - or rather, we might discover a orbiting body between two previously discovered bodies. There are some workarounds around this, but either it means you need to change the names of already discovered satellites, or you preserve the order of discovery rather than the orbital order.

The formal naming scheme came from IAU in 1975. Since then, newly discovered satellites of Jupiter are supposed to be named after the lovers and favourites of Jupiter/Zeus. A 2004 definition expanded this to their descendants. Since the names no longer have any implicit ordering, you avoid the confusion of whether Jupiter III is the third moon, or the third moon that has been discovered, or the third largest moon, or having to name the moon between Jupiter III and IV something like Jupiter IIIb.

If you're exploring the universe fast with your flashy new FTL drive, it makes sense to use placeholder names. A numbering scheme would work until you have a better name, which would usually follow an actual colonisation or mining operation. In much sci-fi, planets are known under multiple names - some are official designations, some are local names, others are well-known nicknames. So a planet known as Rigel IV might also be known as "Jerryworld" to its inhabitants, but they would still use the "official" name when communicating with outsiders. At least if you preserve the numbering to mean order, it's easier to maintain interstellar maps - presumably, in such advanced interstellar civilizations, the ordering would only change due to astronomical cataclysms, which are far rarer than human naming :) If you never heard of Earth, which helps you find it in your star map - the name "Earth", or "Sol III"?

As for the somewhat popular names like Terra or Luna, I expect they're meant to symbolise how small one planet is in the galaxy, much less the universe. Latin has long been used as the international language of science (even in medieval times), so it's not really a poor choice of one name all of Earth could agree on. Having 200 widely used names for one planet might be rather inconvenient for a galactic civilization.

  • Most Geocentric diagrams of the planets do have an ordering. The general belief was that the planets that were furthest from the Earth were the ones that took the longest to complete their circuits around the celestial sphere. As such, pretty much every geocentric diagram ordered the planets from outermost to innermost as Saturn, Jupiter, Mars, Sun, Venus, Mercury, Moon. (Though sometimes they would put Venus and Mercury in spheres that surrounded the Sun.) – notovny Sep 10 at 13:23
  • @notovny You're right, but I'm thinking older than that; the planets were named even before the (today known) geocentric models existed. Still, it's not a major point - this point it's just wild guesswork. – Luaan Sep 10 at 14:07
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    @Luaan I believe that Simon Marius suggested the names Io, Europa, Ganymede, and Callisto for the Galilean moons of Jupiter soon after they were discovered in 1610. They didn't become official for centuries, but they were remembered enough for science fiction writers to consistently use them as early as the 1930s or earlier. Also the Roman numerals of moons were usually given in the order of discovery, not the order of planetary distance, so Amalthea, Jupiter V, is closer to Jupiter than Io, Jupiter I. – M. A. Golding Sep 10 at 15:47
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    The obvious example of a problem with this scheme is with Neptune and Pluto (back when it was considered a planet). From 1979 to 1999 Pluto was closer to the sun than Neptune. Pluto's relatively eccentric orbit means that happens about 10% of the time. – T.E.D. Sep 10 at 18:07
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    "If you never heard of Earth, which helps you find it in your star map - the name "Earth", or "Sol III"?" My star map has a search box with auto-completion, Earth works better there. – Jose Antonio Dura Olmos Sep 10 at 18:56
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It was used to designate Earth in Eric Frank Russell's "Hobbyist". That was 1947.

The story involves a scout pilot stranded on a distant world. In his search for fuel he finds a building with a huge number of creatures in cases, apparently in suspended animation, but in fact waiting to have the Breath of Life breathed into them.

The story closes

Something near to the Gods had scribbled its notes - - . “Biped, erect, pink, homo intelligens type P.739, planted on Sol III, Condensation arm BDB – moderately successful. - - - Flapwing, large, hook-beaked, vari-coloured, periquito macao type K.8 planted on Sol III, Condensation arm BDB – moderately successful". But the sparkling Hobbyist had already forgotten its notes. H was breathing his essence upon a jewelled moth.

The second lifeform is a macaw, which all pilots of one-man scoutships are required to take for companionship, so that they don’t go stir-crazy from their solitude.

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I've found a literary description of the naming scheme in a comic Antares Episode 1 – October 2007 ISBN 978-1-84918-097-9:

Clarifications concerning the naming of the planet

The names of extra-solar planets are derived by using the name of the star they orbit followed by a number indicating the planet's position in order of distance from said star. Thus, planet Aldebaran-4 is the fourth planet of the star Aldebaran. Aldebaran-4 being the only habitable planet of the system, common usage has led to dropping the number 4 to call it simply Aldebaran. thus confusing the name of the star and that of the planet.

And in Building a universe by Russ Morrissey – 2016. They refer to it as A planet's classification.

Populated planets tend to have names, as do those with strategic value. With billions upon billions of planets in just one galaxy, however, most planets simply have a classification. A planet's classification is the star's name appended with a roman numeral designation for its position out from its host star. This will give you a name like Gamma Phoenicis VI or Chi Orionis III.

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