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What is the earliest science-fiction story featuring double planets?

I am defining a double planet as two bodies of planetary or dwarf-planetary size that orbit a common barycenter, and the barycenter is outside of both planets.

Pluto and Charon would qualify as a double-planet by this definition, but please do not answer with a story about/on Pluto before 1978 because nobody knew about Charon before then. I am looking for stories where the author intended to write about a double planet, so unless an author predicted the Pluto-Charon system within a story, that story would not count.

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    I am shocked – shocked! – to see mentions of Rocheworld (1982), The Ragged Astronauts (1986), First Cycle (1982), but not The Dispossessed (1974). – Anton Sherwood Jul 4 at 19:41
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    @AntonSherwood I completely forgot about that! Post it as an answer! – DavidW Jul 4 at 22:42
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    To clarify: Are you looking for stories that specify an external-barycenter orbit of each other, or accepting stories that only implied it with "Two similar planets orbiting together and sharing an atmosphere/close enough to 'connect' in some manner"? – TheLuckless Jul 4 at 23:04
  • @TheLuckless Not expecting stories where the planets share an atmosphere, but interesting to see answers with those. I was originally just looking for stories where the planets have an external barycenter. – LincolnMan Jul 5 at 19:32
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Added July 5, 2019:

A story set on the two worlds of a double planet in another solar system (with no Human characters), somewhat earlier than other examples in other answers. "Get Out of My Sky" by James Blish, Astounding Science Fiction, January, February 1957. http://www.isfdb.org/cgi-bin/title.cgi?979511

And:

Stories of Artur Bloyd By E. Mayne Hull and A.E. Van Vogt published in the early 1940s and collected in Planets for Sale, 1954. http://www.isfdb.org/cgi-bin/title.cgi?1207812

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A Cthulhu Mythos fantasy/Science fiction story, "Through the Gates of the Silver Key" by H.P. Lovecraft and E. Hoffmann Price, Weird Tales July 1934, mentions "Kythamil the double planet that once revolved around Arcturus".

https://lovecraft.fandom.com/wiki/Kythanil3

https://en.wikipedia.org/wiki/Through_the_Gates_of_the_Silver_Key4

http://www.isfdb.org/cgi-bin/title.cgi?690335

http://www.hplovecraft.com/writings/texts/fiction/tgsk.aspx6

But I think I remember at least one even earlier example. A highly improbable double planet where the two planets were connected, or were close enough to share an atmosphere, or possibly a ring shaped planet, was the setting of a story by Homer Eon Flint (1888-1924). That was one of Flint's stories about Dr. Kinney: "The Lord of Death" (1919), "The Queen of Life" (1919), "The Emancipatrix" (1921), & "The Devolutionist" (1921).

http://www.isfdb.org/cgi-bin/ea.cgi?1677

Since "The Lord of death" was set on Mercury and the "The Queen of Life" on Venus, the double planet story would have to be either "The Emancipatrix", or "The Devolutionist".

https://www.gutenberg.org/ebooks/author/15928

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How about "Rocheworld" by Robert Forward? (It was also published as "The Flight of the Dragonfly.") It came out in serial form in 1982, and as a book in 1984.

It is about the exploration of a double planet system. The system orbits Barnard's star. The two planets are "Roche" and "Eau" (rock and water, respectively.) Roche is a big, dry rock. Eau is a watery planet.

The two are close enough that they "share" atmosphere. Both are rather egg shaped - distorted by the proximity of the two planets.

The effects of the double world provide several plot points, and there's intelligent life on Eau to make things interesting.


I read it not long after it came out, and I remember thinking "meh, I've read better." Lots of interesting things, but it didn't seem to really do much with them.

But, I have odd taste in books so that probably doesn't count for much.

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    Thanks for your answer and I upvoted it, but because I had to convert the question to ask for earliest example. If somebody else comes up with an earlier story, that will disqualify your answer. – LincolnMan Jul 4 at 14:59
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There’s the Land & Overland series by Bob Shaw, set on double planets so close together that they share an atmosphere. The first volume is The Ragged Astronauts (1986).

There's also anything set on Earth in the very far future, even though it's unlikely to mention Earth's status as one of a double planet pair explicitly. The Moon is slowly receding from the Earth, and in a few hundred million years it will be sufficiently distant that the barycentre of the Earth/Moon system lies outside the Earth and it counts as a double planet by your definition. For example, the end of The Time Machine by H.G. Wells

  • Thanks for your answer and my edit to the question disqualifies your answer, because I had to convert the question to ask for earliest example. The other answer mentions a story that was published sooner. :-( – LincolnMan Jul 4 at 15:01
  • @LincolnMan The earliest example would be the end of The Time Machine, which is set in an era where (although Wells didn’t mention it, or even know it) the Earth-Moon system will have become a double planet. – Mike Scott Jul 4 at 15:03
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    @MikeScott the Earth-Moon barycentre will still be inside the Earth in 802,701 AD, which is where the main action is set. – Spencer Jul 4 at 15:18
  • @MikeScott The Time Machine would not qualify even if Wells knew about the recession of the Moon's orbit because that story occurs only 802,701 years in the future and the Moon would need to recede for millions (billions?) of years before the barycenter was outside the Earth. – LincolnMan Jul 4 at 15:19
  • Some quick back-of-the envelop calculations says that the Moon needs to have an average orbital distance of approximately 525000km for the Earth-Moon barycentre to rise above the average radius (6380km) of the Earth; the 141000km increase in orbital distance, at the current rate of recession of approx. 4cm/y would require in excess of 3Gy. – DavidW Jul 4 at 15:25
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1940: "The Dwindling Sphere", a short story by Willard Hawkins, first published in Astounding Science-Fiction, March 1940, available at the Internet Archive. The double planet is the Earth-Moon system, not because the Moon has gotten farther away (as suggested in Mike Scott's answer), but because the Earth has gotten smaller:

On one occasion it was the legend that, instead of being twin planets, our Earth and Luna were at one time of differing sizes, and that Luna revolved around the Earth as some of the distant moons revolve around their primaries.

This theory has been thoroughly discredited. It is true that there is a reduction of the Earth's mass every time we scrape its surface to produce according to our needs; but it is incredible that the Earth could ever have been several times the size of its companion planet, as these imaginative theorists would have us believe.

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First Cycle, a posthumous work by H. Beam Piper that was completed by Michael Kurland, appeared in 1982. In this novel, the two worlds are roughly equal in size, but one accumulated most of the water during planetary formation and is covered by ocean with a few islands and a small continent, while the other is mostly desert with a few oases and favored areas. On the desert world, a cooperative bipedal species evolves, while on the ocean world, a suspicious bipedal species develops with a theocracy. The two cultures clash, of course. The book has an intriguing premise but was left partly in outline form at Piper's death. It is not one of Piper's best.

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This is a bit of a cheat because barycenter is a difficult word to apply to Pythagorean cosmologies. It may also be unfair to class Pythagorean cosmologies as science fiction.

In the 5th c. BC, Philolaus' model of the Universe had an Earth and a counter-Earth, Antichthon, both revolving around an unseen "Central fire". The "barycenter" of this arrangement is in or near the central fire, an object exterior to both Earth and Antichthon.

(Now an only slightly lesser cheat...)

Twin Earths, a science fiction comic strip that ran from 1952 to 1963 included a counter-Earth. The barycenter of the Earth-counter-Earth subsystem of the Solar system is exterior to both planets (in fact is interior to the Sun).

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    Twin Earths wouldn't count, since the two are individually gravity bound to the Sun, and not to each other. – DavidW Jul 4 at 22:01
  • @DavidW : Agreed. However... The barycenter of the Earth-counter-Earth (sub-)system is exterior to both planets and it is the barycenter that is specified in the Question. As a consequence of symmetry, both Earth and counter-Earth orbit "a common barycenter". I expect any double planet of the type you are thinking of is also embedded in the much larger gravity well of a star. The question doesn't constrain the binding relationship with the star, only the location of the barycenter of the two planets. – Eric Towers Jul 4 at 22:05
  • Barycenter: "the center of mass of two or more bodies that orbit one another." Since Earth-counter-Earth are not orbiting one another - they both orbit the Sun - the pair cannot be considered to have a barycenter by the accepted definition. ("It is the point around which two celestial bodies orbit each other.") – DavidW Jul 4 at 22:14
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    @DavidW : Barycenter: "The center of mass of two or more bodies, usually bodies orbiting around each other, such as the Earth and the Moon." "usually" != "always". Also, the Moon does not orbit around the Earth -- in heliocentric coordinates, the Moon never crosses its own orbit -- in fact, the region (containing the Sun) bounded by the Moon's orbit is convex. – Eric Towers Jul 4 at 22:19
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    @DavidW : Physics : "The center of a mass; often specifically, the point at which the gravitational forces exerted by two objects are equal." I'll go with the physicists definition of barycenter which is ((non-)inertial) frame independent. – Eric Towers Jul 5 at 1:28
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Gene Wolf's 'The fifth head of Cerberus' from 1972.

From Wikipedia:-

Two colony worlds, 20 light-years from Earth, the double planets of Sainte Anne and Sainte Croix, originally settled by French-speaking colonists, but lost by them in a war with an unnamed enemy.

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