A persistent image in speculative fiction is that of a world that is both close to our own and yet strangely alien to it. Many such fictional worlds have been positioned on the opposite side of our own Sun, eternally hidden from our sight by its luminance - but why are these counter-Earths so fascinating?

 Antichthon 

The idea that a twin world to Earth might lie forever out of our view can be traced back to the Greek natural philosophy concept of Antichthon (literally opposite, or against, Earth). While the term could be used to describe the parts of the Earth’s surface which were unknown or beyond reach, such as the antipodes, it was applied to other worlds. The philosopher Philolaus and others of the Pythagorean school proposed that Earth orbited a “central fire”. Antichthon in this conception was a second world which also orbited the same point, but was perpetually out of view of Earth’s inhabitants. This world may have been introduced in Philolaus’ cosmological model to explain some lunar eclipses. However regardless of its practicalities, such a planet was called for by a philosophical system which required certain symmetries and harmonies in its picture of the world, for example favouring ten objects orbiting the central fire rather than nine. However, as a site forever-unobservable, this world also served as a potential location for mythical creatures and aspects of Greek thought which were not present in the real world.

Jumping forward in time, the idea reemerged in the late nineteenth century, with the counter-Earth now positioned on the opposite side of the Sun - on Earth’s orbit but half a turn ahead or behind it. An Earth-mass planet, equidistant from the Sun, the theory went, would orbit in precisely the same period as the Earth itself, and so would remain perpetually behind the Sun from our point of view and never be seen. Early stories based on this premise included the 1896 novel From World to World by D L Stump (which, as far as I can tell, involved a counter-Earth Utopia but has never been republished or made publicly accessible).

It was rapidly realised, of course, that while this might work in an ideal solar system consisting only of the Sun, Earth and Counter-Earth, the other planets act as perturbers, constantly exerting small tugs on any object and gradually modifying its orbit. While the effect in any given year would be very small, over the billions of years of the Solar System’s lifetime, the planetary balance would be disrupted. Venus and Jupiter in particular would make a precise counter-Earth orbit unstable, and would in turn have their motions affected by any planet at such a location - as would smaller bodies such as comets which actually pass behind the Sun and would encounter any planet lurking there. These motions can be measured with a great deal of precision. As a result, we can now rule out the existence of a counter-Earth with a high degree of certainty, but this hasn’t stopped it appearing in countless science fictions.

Parallels 

An interesting example which influenced a lot of later work can be found in Edgar Wallace’s novella Planetoid 127. Published in 1929, it imagined a young couple visiting a rich but reclusive professor. It soon becomes clear that the professor is in contact with a world, Neo [1], which is not only counter to Earth in its orbit but also matches it in geography, biology and even human society:

There are necessarily certain differences in their methods of government, but these differences are not vital. In Neo, men are taught the use of arms, and receive their guerdon of citizenship (which I presume is the vote) only on production of a certificate of proficiency. But in the main their lives run parallel with ours. The very character of their streets, their systems of transportation, even their prison system, are replicas of those on this earth.” … “Men and women were doing in that world exactly as we were doing in ours. There were Stock Exchanges and street cars, railways, aeroplanes, as though twin worlds had produced twin identities; twin inspirations. (Planetoid 127, Wallace, 1929)

As a result, each individual on Earth has a precise counterpart on the other world and even their stock markets vary in unison. Crucially, however, key events on the two worlds occur with a 1007 second (about 17 minute) separation in time, with other variations of event timing up to a few days in either direction. This allows the occupants of one world to take advantage of advance information from the other in areas such as speculation or decision making.

A similar premise of parallel evolution not only in species but also societies and individuals underlies other texts.

A parallel Earth in the counter-world position can be found in the feature film Doppelganger (aka Journey to the Far Side of the Sun, dir. Parrish, 1969), written by Gerry and Sylvia Anderson. Here astronauts working for the NASA and the European Space Exploration Council (EUROSEC) are sent to the newly discovered world. Recovered after a crash at their destination, they find themselves back at their own base, surrounded by familiar colleagues and family, and in trouble, assumed to have aborted the mission. As gradually becomes clear, instead they have crossed paths with an identical mission, sent in the opposite direction by an identical organisation and crewed by identical astronauts. Rather than crashing on their own world, they have come across a perfect doppelgänger of the Earth. As the astronaut Ross explains (with reference to a display of the counter-weighted worlds.

What I’m going to tell you is bizarre. Weird. But it’s the only theory that fits the facts. I propose a complete duplication of matter. A situation where every single atom, every molecule here is duplicated here, except that it’s in reverse.

In fact, the one key aspect in which the worlds are not mirrored becomes crucial to the final denouement of the film.

Rather than such a perfect doppelgänger, most counter-Earth stories assume small differences. Published in 1960, Ben Barzman’s novel Twinkle, Twinkle, Little Star (a.k.a. Echo X, a.k.a. Out of this World) never explicitly places its sister world on the other side of the Sun, although it’s clearly some distance away in space, and communications are described as taking twice the time light takes to travel from the Sun, which is a fair indication. This interesting novel begins with a lengthy biography of the narrator and his wartime experiences before science fictional elements are introduced - but this proves crucial to the plot, since so much of the interaction with the alternate world becomes caught up in the characters' life history and how it played out on the other Earth. It gradually becomes clear that the other world had been identical to ours until about 25 years before the then-present, when it escaped the horrors of the Second World War.

However, while an interplanetary plot unfolds, the main theme of the novel is one of lost opportunities: what might have been achieved by those whose lives were cut short or distorted by warfare, or indeed by racial prejudice. Barzman argues that those lost in conflict might have advanced science and society towards a near-Utopian ideal, against which our world compares poorly. But, while it’s undeniable that conflict and prejudice prevent a great many people from reaching their potential, Barzman does not address the evidence that warfare is also a tremendous driver of technological investment and innovation. While technology might have advanced differently in its absence, it’s questionable whether it would have advanced faster, as Barzman implies. Interestingly, Barzman’s lead scientist character is an attractive young woman who is repeatedly described as being hard to imagine as a scientific genius. Barzman never addresses this point directly, but given the overt discussion of potential lost to the war, prejudice and poverty, it’s interesting to speculate about an unspoken subtext of the potential research breakthroughs also lost due to the enforcement of gender roles and stereotypes.

It’s a little hard to imagine worlds sufficiently identical to have evolved in perfect parallel for four and a half billion years, only to diverge in the few decades before contact - particularly since a number of the cast of mirrored characters only met in our world due to wartime experiences, but still seem acquainted in the other. The idea of recent divergence, such as that seen in Echo-X, makes more sense in the context of parallel worlds theory than counter-weight Earths. Parallel universes arise (in fiction and theoretical physics) from alternate choices being made at various points in history, and so must exist in their thousands, millions or infinities. Those that show infinitesimal changes would recombine with their alternate very rapidly, while those that diverged most recently would be “closest” in some extra-dimensional sense and so most easily reached. By contrast with this quantum-physics-inspired scenario, a classical counter-weight Earth could have had no contact with ours since its formation. Hence a close parallel would require every chance occurrence, every radioactive decay, cosmic ray strike, genetic mutation or animal decision made over billions of years to be identical in its effect, if it were not to create massively divergent histories. For this to somehow occur, only for the divergence to become significant mere decades before the narrative is set, is implausible to say the least.

Also worth mentioning in this context are the counter-Earths in which the broad thrust of species and even societal evolution are similar while important differences exist. The radio anthology series 2000 Plus was written and produced for the Mutual Broadcasting Company by Sherman H. Dryer - broadcasting original science fiction dramas rather than dramatisations of magazine short stories as was the case for the otherwise-similar radio anthologies Dimension X or X-Minus-One. In the episode "Worlds Apart" (1950), an exploration spaceship headed out on the first trip for Neptune finds itself pulled off course and crash-lands unexpectedly on a world its crew takes for Earth. It rapidly becomes clear that this is in fact another Earth-like world, called Vesta, that from our perspective is perpetually hidden behind the Sun. This world has humanoids similar enough to Earth-humans to initially pass for them, and the ship lands in a country which speaks English, but political units such as the United States or the city of Chicago are unknown. Again, this calls for a surprising convergence in some ways - in particular linguistically, although this can be overlooked as dramatic necessity, and in technological level - but allows for a perhaps slightly more realistic case of convergent evolution than one in which individual lives and personalities are mirrored.

What If Worlds

Other counter-Earth worlds in science fiction demonstrate far more obvious examples of divergence from the world we know. Krypton was described as occupying a counter-Earth orbit in the early Superman comics, although it would need to be much smaller and denser than our planet in order to have its famously high gravity. By contrast, the television series Lexx (1997-2002) describes a close binary-planet pair known as Fire and Water in a counter-Earth orbit. In this latter case, the total mass of the planet pair would need to match that of Earth, rather than the mass of each component. On a less serious note, animated television series Dinosaucers (1987) places the planet Reptilon in a counter-Earth orbit, and imagines a world in which dinosaurs never went extinct, allowing for entertaining interactions between human and evolved dinosaur-descendants. 

The original home world of recurrent aggressors the Cybermen in Doctor Who is a planet called Mondas. In their introductory story, "The Tenth Planet" (1965), their planet approaches Earth, and is described as having been on a wandering orbit which would appear to be highly elliptical. However like many long-running, multimedia science fiction franchises, Doctor Who has a complex continuity, and there is conflicting information regarding the origin of Mondas, with some tie-in material suggesting it was once a binary planet orbiting a common centre of mass with Earth, and dislodged by the Moon, while other material suggests it originated in a counter-Earth orbit around the Sun. Either way, it had since wandered to “the edge of space” and back. This world had identical plate tectonics to Earth, with the same configuration of continents, and the Mondasians were described as effectively identical to human but had begun to replace their organs by cybernetics as a way of prolonging longevity, evolving into the Cybermen. Again, we’re looking here at a parallel world, structurally and biologically very similar, but one which diverged sufficiently long before the narrative is set for society - and human bio-integrity - to evolve in a radically different way.

Also in this tradition is the planet Antigeos in Paul Capon’s trilogy of novels The Other Side of the Sun (1950), The Other Half of the Planet (1952) and Down to Earth (1954). These describe a small space expedition led by a maverick scientist and sponsored by a newspaper and an independent sponsor. Before even naming the planet, the scientist muses on the nature of the public support, particularly in the light of the astronomical pessimism regarding Mars or Venus settlements, which was already clear in 1950:

People believed in Planet Z because they wanted so desperately to believe. As soon as they heard of the new planet they saw in its horizons the vision of a promised land, their thoughts reached across space towards it, and away from our own battered, tangled, crowded and war-weary world... (The Other Side of the Sun, pg 41)

The expedition sets out to find a counterweight planet that has been glimpsed a few times in difficult, near-Sun observations which are dismissed by the scientific community. Originally intending to merely swing past the new world in their craft, the explorers instead crash land upon it and experience serious hardship in making a return. The civilisation on one half of the planet is utopian and peaceful, demonstrating the power of such worlds to mirror our aspirations. The other half is harsher and its civilisation less developed, with a separation between the two halves enforced by catastrophic tidal forces which make a crossing between continents perilous. Those tides are largely due to Antigeos’ peculiar moons. The Antigeos series has the distinction of being amongst the early examples of science fiction adapted as radio dramas in the UK by the BBC, although no known recordings of the 1951 broadcasts of The Other Side of the Sun exist.

Antigeos is described as a little smaller than Earth and orbiting perpendicular to its spin axis, but with two moons which are each the equal to our own Luna in its sky. One is a similar distance to our Moon from the planet, and itself has a moonlet. The other is a little smaller but closer. As the characters note, calculating the eclipses in such a system must be rather a headache. I couldn’t resist doing a little calculation here (see extra bonus page for this blog!Link opens in a new window) and indeed the amount of physical information provided in the text is surprisingly high and suggests a presumed scientific-literacy in its readership which might be surprising in a trilogy aimed primarily at younger readers. As described in the novels, the gravitational geometry of this system almost works, although Antigeos and its major moon Philemon would be effectively a binary planet, and the density of worlds in the Antigeos system would have to be rather more extreme than the Earth-Luna system.

Straying more into fantasy, and returning to the origins of the antichthon concept as a location for the imaginary and different, is the very long Gor series of science fantasy adventures by John Norman (published 1966-1988, 2001-2022). These describe a counter-Earth which has populations which present a strong parallel to various Earth societies, ranging from ancient to modern cultures. These co-exist with alien species, allowing for a variety of adventures, while a running theme involves sexual practices, bondage and now-outdated stereotypical representations. While aspects of the series are problematic, the author, Norman, is a philosophy professor and consciously used his writing to explore aspects of modern society and philosophy in an unthreatening mirror to the Earth - as the ancient philosophers had done long before.

Twin Planets

While a counter-Earth is fascinating for its sheer unknowability and ability to reflect our own aspirations and ideas for our planet, counter-balanced twin planets have also appeared around other suns in science fiction.

Television series Space:1999 (which was, in fact, made by the same creative team as the film Doppelganger) describes the journeys of a colony of humans trapped on the Moon when it is ejected from the Solar System. In “The Last Enemy” (1976), the wanderers are initially excited by the possibility that there might be two habitable planets in the solar system they are approaching. However it soon becomes clear that the counter-poised planets Betha and Delta exist in a state of perpetual warfare, and the Bethans are determined to use the peripatetic moon as a missile launch platform. The solar system in this story is a very simple one, with only the two counterpoised worlds seen or mentioned - as a result, while this an unlikely configuration to form, it is not an impossible one. The cause for the war in this system (or how it started when the two planets can’t see or reach one another) is never explained in any detail, but a hint can be found in the fact that only female Bethans are seen, and only a single male Deltan. Here the balanced, equal but opposite planets are being used as emblematic of conflict between the two sexes, albeit in an oversimplistic manner.

Another study in opposites - in conflict but also in balance - can be found in the children’s science fiction-influenced cartoon Thundercats (1985-1989). Despite somewhat fantastical elements, the premise of this series has a small group of aliens, the eponymous Thundercats, fleeing their home planet Thundera and crash-landing on Earth. They are opposed by another group of aliens (in the form of an unlikely bunch of mutants) originating from the planet Plun-darr, which some sources suggest orbits the same sun as Thundera in a counterweight orbit. In this context the opposed forces the planets represent are not gender norms but the simpler concepts of good and evil, or perhaps courage versus selfishness.

Other fictional narratives with a similar theme of binary values or experiences are often shown in the related context of binary planets (which orbit a shared centre of mass, as is the case for Pluto and Charon, or arguably the Earth and Moon - although in the latter case the centre of mass is still within the Earth) - and I might discuss these at a later date.

Fine-tuned Formation

Counter-weight worlds feature in fiction primarily as a mirror of our own concerns and an opportunity to explore alternatives or consider the nature of opposing view points. The 2011 film Another Earth made full use of this conceptual role. While set entirely on contemporary Earth, the science-fictional elements considered how the discovery of another world would affect individuals and relationships. It presented a study of the choices made by one individual, and how those affected others, as well as how they might differ on a mirror planet on which the previously exact parallels are conveniently broken at the moment of discovery. This focus on mirrors, on choices and alternatives underlies most of SF of counter-worlds.

To the best of our current understanding, planets typically form from slightly over dense regions in the dust and gas disks that surround their new-born suns. The gravity of these regions begins to gather more material towards them, while interactions with the disks gradually clear the orbit of loose debris. It is probably common for several mid-sized proto-planets to form on close to the same orbit before merging to form a single dominant world. Indeed, the Moon is believed to have been formed by a collision between the proto-Earth and a Mars-sized planetoid sharing its orbit known as Theia. However unless the proto-worlds are precisely the same distance from the sun, and separated by a precise fraction of the orbit, they will not orbit in the same length of time and so one will always catch up with the other and collide with or eject it. Similarly, unless the balanced planets are alone in their solar system, the slight tug in each orbit from any neighbours will act like the regular gentle pushes of a parent on a playground swing, gradually sending the swing higher and higher - or the planet further and further away from its stable point. Again, the likely results are either collision with another planet or ejection from the solar system due to a close encounter. In other words, the formation of counterbalanced planets requires fine-tuning of the initial conditions and evolution of the planetary system.

As a result, a counterweight world, or antichthon, is fairly unlikely to exist in any genuinely habitable exoplanet system, let alone our own solar system. This probably won’t stop science fiction writers continuing to explore the idea: we now know that Earth is the only body in the solar system which humans can inhabit without extensive technological support. The idea that another habitable world could be lurking unseen, let alone one which might embody our uncertainty about human, technological and societal progress, is just too tempting to easily abandon. The only halfway-plausible location for such a world was in the counter-weight position, before probes, comets and planetary observations ruled it out once and for all.

As long as we continue to speculate about “what ifs”, however, and continue to see the world in terms of black and white, good and evil, or other binary divisors, we will likely continue to see fiction about twin worlds - and as long as one of those sides remains hidden or mysterious, the counterweight world will continue to exist, if only in the human collective imagination.