Planet X

The discovery of first Neptune and then Pluto, adding to the family of Solar System planets otherwise unchanged since antiquity, introduced the idea that there might be other planets lurking in the outer fringes of our Solar System. The discovery of Eris, Haumea, Makemake and others in the early 2000s and creation of the dwarf planet category in 2006 did nothing to dampen the interest in this idea. Here we take a look at the idea of a trans-plutonian planet, or Planet X, in science and science fiction [1].

 How Many Planets?

The discovery of Neptune in 1845 was the result of a concerted search, motivated by observed variations in the orbit of Uranus which required a large mass outside the latter's orbit to cause them. The subsequent discovery of Pluto, initially classed as a planet and later as a dwarf planet, in 1930 followed directly from the same method, with a search based on perturbations in the orbit of both Uranus and Neptune. However Pluto’s orbit was unusual compared to other planets - more highly elliptical and inclined to the Solar System’s disk-like plane. Explanations proposed for this orbital peculiarity have varied over the decades, from the idea that it was captured from the Oort Cloud (which lies on the very fringes of the Solar System), to the suggestion that it was a lost moon of Neptune, or the idea that it might in turn have been perturbed into this orbit by a more distant object still [2]. The search for such an outer planet has never been entirely abandoned ever since. Part of this search was the campaign by astronomer Mike Brown, which led to the discovery of several Pluto-scale objects in the outer Solar System (known as Kuiper Belt Objects or KBOs) and thus to the need for a redefinition of what constituted a planet or a decision on whether these distant objects qualified. Brown’s entertaining account How I killed Pluto and why it had it coming (non-fiction, 2010) is well worth a read, and spells out the key dilemma: either Pluto was not a planet or our Solar System might host dozens of objects with equal claim to planet status.

The resulting IAU decision requires a planet to fulfill three primary criteria: (i) to be orbiting a sun, rather than any other point outside its own radius, (ii) to be sufficiently large for gravity to have pulled its mass into a sphere and (iii) to have cleared its orbital zone of comparably sized objects (or perhaps captured them as moons) through gravitational interaction. None of the new trans-plutonian objects met this last criterion, but their very existence meant that neither had Pluto itself. The new category of dwarf planet, which need only satisfy the first two points, encompassed the new objects, Pluto, and also the largest of the Mars-Jupiter asteroid belt, Ceres (which had, indeed, initially been declared a planet when discovered, only to be demoted when its orbital companions like Vesta and Juno were discovered). This naturally met strenuous objection from some who were emotionally connected to Pluto and had always known it as a planet.

Partly because of these redefinitions, the number of planets in the solar system has always caused some debate.

For many astronomers of the nineteenth and very early twentieth century, the first planet was Vulcan (an illusory planet which was believed to orbit close to the Sun), by which counting the Earth was the fourth. With Vulcan largely disproved in the early twentieth century, the numbering of Mercury, Venus, Earth and Mars as 1-4 was fairly well established thereafter.

[Image: graphic showing the planets of the solar system, as known or hypothesised at different times. Uses icons from FlatIcon. Click to expand.]

However the idea that Ceres was a planet, or that both Ceres and Vesta were, or that either or both were the remnants of a now-destroyed planet orbiting in the current asteroid belt, persisted in popular opinion for longer, with that now-missing planet often referred to as planet 5, making Jupiter number 6 (or indeed anywhere up to number 20). The fact that tidal forces and perturbations from the giant Jupiter had prevented a planet from ever forming in that region was slow to be accepted in science fiction, but was widely recognised by the 1920s in astronomy, where Jupiter itself was firmly recognised as the Sun’s fifth planet.

With Jupiter in number 5, Saturn, Uranus and Neptune are the Solar System’s planets 6, 7 and 8 respectively. When a search was made for a planet beyond that, a common nickname was Planet X (as in unknown), but Pluto was eventually named and identified as planet 9.

After that the Planet X moniker was pleasingly appropriate both for “planet unknown” and for “planet 10”, and it is as the Tenth Planet that most science fiction of the twentieth century discussed the idea of a trans-plutonian planet. More recently, since Pluto’s redefinition, the “Planet X” name has seemed less apt. While the idea of another large planet in the outer solar system is occasionally revived, it is occasionally referred to as “Planet Nine”, “Planet Y” or by alternative names. Most recently, some evidence for a tenth planet has been seen in the orbital resonances of the Kuiper Belt Objects. On the other hand, the growing sensitivity of wide-field astronomical surveys, together with their evolution into the infrared (where even dark objects can be seen as long as their temperature exceeds the background) has continued to cut down the range of possible parameters of such a planet, rendering its presence ever more unlikely. The true answer, whether a large planet may lurk at very high distances, whether other dwarf planets could be causing the orbits seen, or whether there are still-to-be-determined explanations, remains unknown.

Planetary Romances

As with Vulcan (which I’ve written about before) and many of the other planets, the unknown, distant planet on the edge of the solar system was a prime target for the planetary romances - stories of discovery and adventure on worlds throughout the Solar System.

In We Guard the Black Planet! by Henry Kuttner (novella, Super Science Stories, Nov 1942), for example, a tenth planet beyond Pluto proves to be the origin of legends about flying humanoids on worlds across the Solar System. Shielded from sight by a ‘negasphere’, it is a water world whose islands hosts a species which has abandoned spaceflight. In earlier days, these gave rise to the myth of the Norse valkyries, amongst others, and the people of this world are still capable of interbreeding with humans. The story’s protagonist must preserve this world (and - inevitably - the woman he loves) by resisting unscrupulous human explorers intent on invasion and exploitation.

Another interesting example of the more thoughtful kind of planetary romances involving the outermost planet is provided by Thornton Ayre in Twilight of the Tenth World (short story, Planet Stories, Winter 1940). The story begins with mysterious spaceships hovering over the cities of Earth, which launch shells that begin a spreading disintegration process. At the same time, an odd wave of suicides by people just waking from sleep (together with speculations about a sleeping mind’s psychic potential [3]) give clues to a bigger picture: In fact, Earth is an experiment by the population of a tenth planet who diminished themselves to produce experimental humanity. As the protagonist is told:

“Near as we can figure out, Mr. Dugan, it’s about twenty million miles beyond the orbit of Pluto—a tenth planet. We’ve thought for a long time there might be one. We have never found anything. We can try again now we have this data. The planet must have a mighty low albedo since we’ve never sighted it in the 400-incher at Mount Wilson.”

 We see little of this world, and most of that involves protective domes suggesting less-than-optimal conditions. However, once the existence of the tenth planet, and that its under threat from the same alien aggressors, is established the story becomes a rather odd (and not always logical) race to recombine humanity with its parent race, in order to ensure the survival of one or both.

Planetary romances often paid very little attention to physically plausible conditions on their planets, or just used them as bouncing off points to explain why an Earth-like environment still exists through some contrivance of the writer.

The planetary romance tradition persisted in some cases well into the 1960s. In the marionette-animated television series Space Patrol (created by Roberta Leigh, 1963-4), for example, the crew of Galasphere 347 routinely wander around on planetary surfaces of the Solar System with minimal protection, going into suspended animation for some of the longer journeys. In the episode “The New Planet”, the Galasphere goes off-course while the crew are so frozen, and heads into deep space faster than any rescue craft can track. Woken by a chance encounter with a comet, the crew discover a planet beyond Pluto, which - admittedly to their surprise - is not frozen, but rather habitable and covered with giant vegetation… and a giant person who tells them that “There is great heat, and boiling springs, in the heart of this planet”, suggesting that the ecosystem is maintained by the planet’s internal radioactivity. The pseudo-explanation attempted by the writers, that sustained bombardment by cosmic radiation on the edge of the solar system might have altered molecular structure to explain the gigantism, remains unconvincing.

As in Space Patrol, the recognition that the distant world should be cold did not stop other fiction which treated it as Earthlike through some mechanism or other. A story which explores the habitability of such a world (albeit through rather a Clarke’s Law magic device) is Passage to Planet X by Henry Hasse (Planet Stories, Winter 1945). Here characters set out to find somewhere to develop their ideas and inventions without the intervention of the Bureau for the Investigation of New and Worthy Inventions, a division of the Tri-Planet Council. This organisation takes control of the intellectual property and development (or suppression) of any invention in human space - an offence to the free-thinking crew.

Finally reaching Planet X, beyond Pluto, they discover to their surprise that it is a high gravity world, on which they can barely move and which is frigid. The natives of this world, Perlac, are the descendants of a once-technological civilisation, who developed a mysterious device that emits heat and light and reduces gravity in its near-environs to near Earth-normal, thus rendering part of the planet habitable. In this region, the protagonists come across a small colony of humans stranded after their generation ship lost its way to Mars some century and a half earlier, and decide that this will be a new world for them:

"And with your permission, we'll want to bring back some new men. Not rogues and adventurers like me, but scientific men who can come here and work out their ideas without fear of that stupid Earth Bureau."

In all honesty, this story is of its time, with a fine disregard for the rights or interests of the indigenous life, unexplained giant robots, no real concern over making their new civilisation dependent on a magic alien device they don’t understand, and an interesting insight into post-war concern regarding government control of scientific research.

Cold Worlds

While many stories of this era treated such worlds (or parts of them) as Earth-like in every important respect, some writers strove for more accuracy. Many recognised the basic fact that a world so far from the Sun would receive very little heat or light (indeed the Sun is barely more than a bright star) and so would be very cold, unless it has other energy sources.

John W Campbell, first as a writer and then as a very influential editor, attempted to inject scientific plausibility into science fiction. His research team of Penton and Blake set out to explore the Solar System, and in The Tenth World (Thrilling Wonder Stories, December 1937), they moved on from Ganymede to the blackness beyond Pluto, following records they had previously uncovered on Mars. As we’re told:

“this planet’s about 15,000 miles in diameter, I believe. We’re headed now for the equatorial, the hot zone. It must be all of 5° above absolute zero there. Helium may be a gas, but everything else in the Universe is a solid at that temperature.”

This unnamed world’s surface has frozen oxygen sand, solid nitrogen cliffs and rivers of liquid hydrogen. Much to their surprise though, they find slow moving, boulder-like objects and bulky black cylinders that prove to be alive and attracted to any trace of heat - including Penton, Blake and their spaceship. The cylinders prove to be long-lived, intelligent and telepathic, but utterly unable to control their bodies, which threaten to destroy the visitors in search of their heat. As one explains it, they concentrated so hard on thinking, leaving their bodies to eat and operate on autopilot, that the neural channels atrophied, and the body now reacts only to instinctual processes. The escape of the explorers relies on a cunning bit of chemistry-based problem solving.

The Tenth World was accompanied in the same issue by a “The Story Behind the Story” article by Campbell explaining his reasoning:

“The Tenth World originated basically In an attempt to picture a form of life sufficiently resistant to live on a world at near absolute zero temperatures. I have not, in the story, suggested how life might have originated under such circumstances, since it is evident that only highly organized forms could exist. But this might be accounted for by assuming an original radio activity of the planet which had once maintained the temperature sufficiently to permit life to start, with a subsequent gradual cooling that drove the now highly-organized forms to build defenses against encroaching cold.”

Campbell goes on to explain the physical principles and experiments which he considered in his world-building. In true scientifiction form, “The Tenth World is built up on the amplification of these speculations.”

The incredible coldness of this hypothetical distant world is also a key plot element of Peter Elliot Hayes’ 6-part radio drama series for the BBC, Orbit One Zero, which was broadcast in 1961. This opens with a scientific investigation at a Scottish radio telescope which reveals a structured signal indicating intelligence. The beam is localised to beyond Pluto’s orbit, to understandable scepticism:

“But nobody sane believes that any other planet beyond Mars can possibly support any life, beyond a few fungi and mosses.”

“Too far from the Sun! No light. No heat. And beyond Pluto? Half a billion miles. There’s nothing out there. Just freezing darkness.” (Episode 1)

Unfortunately, the intelligence on the unnamed tenth planet is not just real but malevolent. A combination of the radio signal and pure coincidence leads the researching team to uncover buried, vibrating capsules around the world which begin to spread frigid gases in an attempt to xenoform Earth, threatening the whole of humanity. They only manage to overcome this by the skin of their teeth. 

By contrast, the tenth planet named Miranda which appears in Edmond Cooper’s The Tenth Planet (novel, 1973) is both cold and naturally uninhabitable, complete with the now-familiar liquid hydrogen rivers and oxygen and nitrogen rocks. However it nonetheless provides humanity’s third chance, after ecocide causes the collapse of Earth, and internecine war destroys the survivors on Mars. In Cooper’s story, an Earth-born captain, frozen in vacuum by an explosive planted by saboteurs aboard his spaceship and revived five millennia later, learns about the now-moribund civilisation which lives underground on Miranda and finds it contrasts sharply with his conception of a humanity which evolved to strive for new horizons.

Other names used for distant tenth planets include Cerberus (in Charles Harness’ The Paradox Men) and Planet Ultra (in television series Space: 1999). Isaac Asimov, writing in a factual article on the hypothetical tenth planet in The Magazine of Fantasy and Science Fiction (July 1960) proposed the name Charon, which was eventually used for Pluto’s largest moon in 1978.

However one other name has stood out amongst the speculation.

Possible Persephone

A commonly proposed name for the hypothesised tenth planet was Persephone, seen as a fitting counterpart of Pluto. This name, or its Roman form Proserpine, won a poll for the naming of the largest of the KBOs in 2005, although it was pointed out that Persephone had already been used to name an asteroid, which might render it unavailable. The new dwarf planet was ultimately named Eris, after the goddess of discord, for the arguments it caused. However Persephone had been used as a name for the tenth planet well before this.

Published in 1937, The Blue Spot by Jack Williamson (novel, Astounding Stories, January and February 1937) was a tale of scientific rivalry between two families, and of two young lovers straddling the divide [4]. With an interstellar nebula approaching Earth and threatening to block the Sun’s light, a young man is selected to be converted to structured light and sent to a newly discovered planet:

“But, in the year I was born, he discovered a new planet that he named Persephone. It is a tiny world, only two thousand miles in diameter, smaller than the Moon. Four times more remote than Pluto, it keeps a mean distance of sixteen billion miles from the Sun, so far that the solar radiation takes a whole day to reach it.

“That small globe is immeasurably the most ancient of the planets. Its volcanic energy and its store of radioactive elements must have been long since exhausted. At its tremendous distance the Sun is no more than a very bright star, unable to warm it appreciably. It should be frozen, utterly dead, but a few degrees above the absolute zero.”

In the narrative, a warm spot on this world shows signs of harnessed nuclear power that could be the salvation of Earth. But, on arrival, the protagonist learns that Persephone was captured from another star, during a close encounter which triggered the formation of the planets [5]. He also learns that another individual, themself encoded as light, opposes his attempt to find the energy source. The protagonist is ultimately saved (as is often the case in pulp SF stories) by the love of a young woman.

After this, Persephone was used as the name for an outer planet by Arthur C Clarke in early examples of his science fiction such as Rescue Party (Astounding, May 1946), Against the Fall of Night (Startling Stories, Nov 1948) or Earthlight (Thrilling Wonder Stories, Aug 1951) and he continued to use it later, as for example in Rendezvous with Rama (Galaxy, 1973) and the novel adaptations of several of the earlier stories. James Blish used the Roman form Proserpine for a gas giant beyond Pluto in the Sargasso of Lost Cities (novel, 1953; where it hosts a far-flung human outpost on one of its moons), and also in other stories such as Writing of the Rat (short story, Galaxy, July 1956). Larry Niven returned to the greek form in his stories, such as Protector (novel, 1973; in which Persephone is a distant gas giant in a highly inclined orbit), The Children of the State (novel, Galaxy, Nov 1976; in which it is flung into the Sun during a war), and The Borderland of Sol (novella, Analog, Jan 1975; which also downgrades Pluto to an escaped moon of Neptune and names three further planets: Caina, Antenora and Ptolomea.).

The name has lingered in the science-fiction consciousness. Poul Anderson mentioned Persephone as a tenth planet in passing in his short story exploring the other end of the Solar System, Vulcan’s Forge (Amazing Stories, Jan 1983). Lin Carter used it for his deliberately-retro pastiche Beyond the Worlds We Know (novel, serialised in Astro-Adventures, October and November 1988). A few years later, in 1992, Douglas Adams also used the name Persephone for a cold, distant tenth planet from the Sun in his Hitchhiker’s Guide to the Galaxy novel Mostly Harmless, although there the less-formal nickname ‘Rupert’ stuck to the distant world.

Proserpine is also the name suggested by Dean Wesley Smith and Kristine Kathryn Rusch in their trilogy of novels The Tenth Planet (1999), The Tenth Planet: Oblivion (2000) and The Tenth Planet: Final Assault (2000). These start in a manner reminiscent of Isaac Asimov’s science fiction classic Nightfall, with archaeologists uncovering a two thousand and six year periodicity in a strangely widespread soot layer. The culprit proves to be a rogue planet which has been captured into a 2006 year orbit of the Sun. During each approach to the inner solar system, its inhabitants (who know it as Malmuria) emerge from suspended animation and harvest resources from Earth to feed their population [6]. Unfortunately for the aliens, by their arrival in 2017, the population density and technological achievements of humanity had radically altered from previous trips and they face more resistance than they expected.

Post Pluto

We now live in a Solar System in which at least the nearer regions of the Kuiper Belt and its larger objects have been explored by telescope. It is a solar system that readers of Larry Niven’s scientifically informed The Borderland of Sol might have recognised: 

“There are unbelievable amounts of garbage out there beyond Neptune. Four known planets and endless chunks of ice and stone and nickel-iron."

Substituting ‘dwarf planet’ for planet, this is not far from reality. Pluto, Eris, Haumea and Makemake are widely recognised as dwarf planets, while Quaoar, Sedna, Orcus and Gonggong are strong candidates.

As surveys in the thermal infrared have become deeper and covered more area in recent years, the spaces left in which another still larger planet might hide have become smaller and smaller. This has led to speculation that the body causing perturbations in the Kuiper Belt and Oort Cloud (if it exists) might be very small and very dense - indeed, taking these properties to their limits, that it might be a tiny black hole formed in the early stages of the Universe. 

An author to take note of such theories is Steven Baxter, whose writing has always tended to explore the limits of cosmological theories of space and time. In Creation Node (novel, 2023) he describes a future in which humanity is divided between an Earth still recovering from the chaotic climate crisis and wary of resource usage, a moon struggling to assert its independence through a form of industrial expansionism, and a group known as Conservers who believe in minimal human impact on any environment. It is the latter who launch a solar sail vessel to investigate ‘Planet Nine’ - taking 35 years to travel 200 au from the Sun.

Once there they find what appears to be a black hole, with ten times the mass of Earth and yet less than a metre in diameter. However they soon discover evidence that it is more than a simple black hole, and indeed the artefact expands until it is three times the radius of Earth, giving an Earth-like surface gravity (and temperature) to a new ‘planet’ which is still referred to as Nine, even while humanity tries to probe its cosmic mysteries. The use of a trans-plutonian planet here is primarily for the sense of remoteness and distance, which prevents a lightspeed-limited humanity from reacting quickly to the discoveries, rather than for the sake of a distant planet itself.

Also noteworthy, more for its timing than anything else is Sedna, a short story by Andrew Frankham in the Doctor Who: Short Trips series, which appeared in The Solar System anthology in 2005. While the story itself is a fairly typical Doctor Who encounter with aliens, the anthology was ahead of its time in embracing the newly discovered KBOs - as an editor’s note explains, it went to press before the name of dwarf planet Sedna was even confirmed by the IAU.

Other than this, fiction which explores the newly-enriched family of large Kuiper belt objects, or the dwarf planets such as Eris, has been scarce [7]. Examples include Mike Adamson’s Sunrise on Eris (appearing in the 2023 short fiction anthology Rhapsody of the Spheres, ed. Juliana Rew), which is typical of the evocative descriptions of scientific surveyors contemplating the cold depths of space, exploring the cold dwarf planets and looking back at the warmth of the inner solar system. By contrast, G. David Nordley’s Haumea (appearing in the 2014 short fiction anthology Extreme Planets, eds. Conyers, Harris & Kernot) is a tale of human survival and improvisation where a small crew stranded on the dwarf planet Haumea must make the best use of its resources and those of previous robotic probes in order to attempt a return to safety. As Nordley noted when including the story in his own 2015 anthology Prelude to Stars:

“As with realistic science fiction in general, the background of this story is something that plausibly could happen, not necessarily something the author believes will happen.”

In this case, the survival of the crew as described is perhaps unlikely, but it is set against the best contemporary understanding of the nature and formation of the small world.

The increasing detail with which astronomers can determine the conditions of these objects has rendered stories treating them as Earth-like implausible to modern audiences, while stories which employ the advanced technologies needed to reach or exploit them tend to be focussed beyond the edges of our Solar System towards exoplanetary systems. The handful of recent stories which explore the Kuiper belt and Oort cloud tend to focus on the smaller objects, treating these as an outer counterpart of the asteroid belt (as it is) rather than on a single large world. However, as Baxter ably demonstrated, there is still scope for mysteries in the borderlands of Sol, and it still remains possible that a distant planet lurks somewhere in that outer darkness.

The discovery of the trans-neptunian dwarf planets did much to reframe the discussion of the outer solar system in science fiction. While the concept of a Persephone, a Planet X (or Y or Z), has yet to go away, it is far less compelling than once believed. The idea nonetheless has left its mark on science fiction. As in Edmond Cooper’s Tenth Planet, it is emblematic of the urge of humanity to explore ever outwards and to seek beyond the limits of our knowledge, and also to build imaginatively on the limits of contemporary science and take that one step further. And, who knows, perhaps one day we’ll go and take a look for ourselves.