Habitable Exomoons

In our Solar System, we know of one habitable planet (Earth) and a handful of potentially habitable worlds (e.g. Mars, Titan, Enceladus, Europa). Strikingly, several of these are not planets in their own right, but instead are moons orbiting gas giants. The prospect that moons outside our Solar System (exomoons) might offer some of the more habitable environments is an intriguing one in both science and science fiction.

Exomoons first started attracting serious attention during the planetary romance era best known from the pulp science fiction of the 1930s. These stories placed tales of high adventure, romance and exploration on habitable and apparently Earth-like worlds, often with only minimal acknowledgement of the astronomical constraints extant at the time. Solar System worlds such as Mars, Venus, Mercury, Vulcan, the gas giants and their larger moons (e.g. Titan, Ganymede [1]) were often the venue of such adventures.

Where stories were set outside the Solar System, the planets were typically fairly generic. However an interesting example of an exoplanetary system can be found in one of the most famous multi-media science fiction franchises. Flash Gordon was created as a comic book character in 1934, spread into a movie serial drama, and has since appeared in books, television series and feature length films. A space-travelling pilot, the majority of Gordon’s adventures involve confrontations with the evil emperor Ming the Merciless, who rules the planet Mongo, including a large number of rebellious populations and species. The nature of Mongo varies between different incarnations of the Flash Gordon universe. In the original comics, the peoples of Mongo occupied different nation states on the planet surface. By contrast, in several realisations of Flash Gordon, notably the 1980 big screen movie version and 2017 television version, a number of the rebel populations are associated with Mongo’s habitable moon system. These moons range from classic Earth-like planetary-romance fare in the early comics, to jagged lumps of rock supported by an ultradense Mongoan atmosphere in the film version. The isolation of these moons goes someway toward explaining the very different nature of each Mongoan subject race.

Habitable exomoons (i.e. those able to support life without substantial modification) nonetheless remained relatively rare in science fiction during the space age. The arrival of planetary probe missions to the gas giant planets, such as the Pioneers and Voyagers, in the 1970s and 80s may have been, at least in part, responsible for once again raising the profile of the miniature worlds orbiting them [2].

This is reflected in the appearance of two of the best known exomoons in the 1977 blockbuster film Star Wars and its sequels. The original Star Wars (later redubbed Star Wars IV: A New Hope; dir. Lucas) based its climactic scenes in and around Yavin 4, a jungle-swathed moon in orbit of a gas giant, Yavin Prime. This was the location of the hidden Rebel Base, and the target of the attack by the Empire’s first Death Star. As shown on screen, Yavin 4 has a gravity, atmosphere and environment very Earth-like in nature, and has hosted a substantial (although now extinct) civilisation.

First appearing in Star Wars VI: Return of the Jedi (film, 1983), the Forest Moon of Endor is the home of the small, furry Ewoks and was the construction location of the second Death Star. Again, this world is shown as Earth-like, thickly-forested and in orbit of a large gas giant, also called Endor. While Yavin Prime was shown as red, Endor appeared as a blue gas giant. In our Solar System, such colours are indicative of the composition and temperature of the atmospheres, suggesting that Endor might be slightly further from its primary star than Yavin.

Like most aspects of Star Wars, these exomoons have been explored extensively in the various spin-offs and media associated with the franchise. Yavin has recently become a major location in the prequel drama series Andor (TV and streaming, 2022) while Endor is best known from the classic animated series Ewoks (TV, 1985). Both have obvious day and night cycles and there is no evidence of one of the most likely properties of exomoons. Any small object orbiting a much larger one is likely to become tidally-locked, always turning one face to its primary (as is the case for all the major moons of Saturn and Jupiter). In the case of an exomoon, this would mean perpetually facing the gas giant, which will appear to remain fixed in the sky rather than rising or setting. The local equivalent of night would correspond to the sun being eclipsed by the giant, rather than the sun setting, and assumes the moon is not significantly illuminated by that planet.

Another, less well-known but more carefully considered fictional exomoon developed from a symposium organised by science fiction writer and editor Harlan Ellison in 1975 (as part of what must have been a truly amazing series of evening classes). Ellison tasked Hal Clement, Poul Anderson, Larry Niven and Frederik Pohl (all known for writing moderately hard, scientifically-motivated science fiction) with developing the specifications for the alien world Medea, with Clement designing its astrophysics, Anderson its physical conditions, and Niven and Pohl its ecosphere and societies. These five designers and six other prominent SF writers (Jack Williamson, Thomas M Disch, Frank Herbert, Kate Wilhelm, Theodore Sturgeon and Robert Silverberg) each wrote a story set in and motivated by the world, which were published individually in science fiction magazines, and collectively in the anthology Medea: Harlan’s World in 1985.

A brief synopsis of the world accompanied publication of the first story, Farside Station by Jack Williamson in Isaac Asimov’s Science Fiction Magazine in November/December 1978:

“Medea is actually a moon, locked in an orbit that keeps the same face of it toward the huge, red-hot gas planet Argo, which in turn orbits Castor C. Deformed by Argo’s massive gravity, it is a little egg-shaped, with the Ring Sea between the hot end and the cold end. Most of its heat comes from Argo, though the twin dwarf suns give most of its light. The first human settlements were on the tropic coasts of the Ring Sea, safely between the Hotside, where temperatures reach 80°C, and the Farside, where the maximum is -110°C and carbon dioxide freezes out of the air. The Medean day is about 76 hours, which the colonists have divided into their own more convenient three-day week.” 

By 1978, the star known as Castor was known to actually contain six components - three binaries. As the introduction above suggests, Castor C is a binary of two red dwarf stars, making Argo a circumbinary planet. The high temperature of the planet results primarily from heat released by its own collapse under gravity, which must have fallen not far short of triggering nuclear burning (i.e. on the borderline between a giant like Jupiter and a brown dwarf). By making the planet massive and circumbinary, the creators avoided having to make it tidally locked to its planet. The careful consideration given to worldbuilding for Medea (which included irradiation calculations and lengthy essays from the creators, development of a coherent biosphere, and recognition of factors such as tidal locking) make this a remarkably plausible and accurate setting for the eleven Medean stories. These vary in style and focus, varying from a concentration on the human settlers to a focus on the native life.

Medea was an unusual collaborative effort, deliberately designed to showcase a cooperative but scientifically-motivated creative method. However other authors have also deliberately created exomoons in order to explore their unusual properties.

In Nemesis (novel, 1989), Isaac Asimov set out to describe humans reaching a red dwarf. As he described in Asimov’s SF, March 1990, he described his thinking:

“Of course, you can’t very well have a habitable planet circling a red-dwarf star, but I wanted one. It would give me greater flexibility than simply to have the settlement go into orbit about the red dwarf. That meant I had to think up a set of conditions that (if you don’t question things too closely) would make it sound as though a habitable planet could exist. For that I had to invent a gas giant, with an Earth-sized satellite, and it would be the satellite that would be habitable.”

Current researchers question the assumption here that red dwarf planets are intrinsically uninhabitable, as I’ve discussed before. However the habitable zone of single red dwarfs (the region warm enough to permit liquid water) is so close that any planet is tidally locked to the star. However an exomoon itself tidally-locked to a tidally-locked planet would experience eclipsing and heating from the planet, and so has more opportunity to redistribute its heat from one side of the world to the other. As a result, Asimov’s system contains the M-dwarf Nemesis, the giant planet Megas and its moon, Erythro, which hosts the first exo-planetary system human settlement, as well as native alien intelligence.

 An inhabited exomoon was also used by John Barnes and Buzz Aldrin in their novel Encounter with Tiber (1996). This imagined an alien civilisation in the Alpha Centauri system, which is threatened by cometary bombardment. The story narrative follows an attempt by Nisuans to settle ancient Earth, and an attempt by modern humans to recover evidence of that attempt and to reach Nisu in return. 

The nature of Nisu (dubbed Tiber by humanity), and its relationship with its primary (the gas giant Sosahy), is explored in the opening chapters of the section on Nisuan history, with its explorers taken to a region of badlands situated directly under the giant, at the centre of the locked side of Nisu facing its planet. This is where both of the two subspecies of Nisuans are believed to have originated.

“The sunlight was bright - the sun had climbed most of the way up the sky and was drawing on toward Sosahy, which formed a great lighted bow, its centre bent outward toward the sun. Within the arms of the bow lay the dark bulk of he huge planet, the part not lit by the sun at that moment.”

…

“The noon eclipse in Palath was the only time you could see the stars, because Sosahy, hanging over us in the sky, was bright enough to turn the sky blue most of the time. But right during the daily eclipse, Sosahy blocked the sun, and we faced the part of it that was having night, and then the stars would pop out.”

(Encounter with Tiber, Part 2, chapter 2)

The sun in this case is Alpha Centauri A. Like Castor, Alpha Cen is a multiple system, with two bright components (A and B), and a fainter red dwarf tertiary (Proxima). It is likely that the relative instability of multi-component stars was a consideration for the authors when they designed the system. The orbits of small planets are much more easily disrupted than those of larger worlds. By making their world Nisu a tidally-locked exomoon of a much larger exoplanet, they were able to create a more plausible habitable environment than Nisu alone would have presented. This nonetheless leads to a complex maze of tidal and gravitational force that the explorer ship sent out from the world had to navigate. Nisu itself is a fairly well realised planet, tidally-locked but with a large ocean and a dense enough atmosphere to redistribute heat from one side of the world to the other.

The first real-world exoplanets to be found around alien suns were all massive objects known as hot jupiters. This was largely a selection effect, since close-in, jupiter-sized planets are far easier to locate than any other. However the confirmation that many gas giant planets do certainly exist around other stars seems to have triggered a resurgence of interest in the potential habitability of exomoons.

Habitable exomoons in science fiction of this period included Netu, an exomoon converted to a hellish prison colony by a ruthless Gua’uld overlord in television series Stargate SG-1 (episode “Jolinar’s Memories”, 1999), Ninfadel in Sheri S Tepper’s Raising the Stones (novel, 1990), the world Masada in Neal Asher’s Polity series (e.g. The Gabble, short story, Asimov’s, March 2006), Brimstone in Laura J Mixon’s At Tide’s Turning (short story, Asimov’s, April 2004), Hela in Alaistair Reynold’s Absolution Gap (novel, 2003), and Dawsheen in Robert Reed’s River of the Queen (short story, F&SF, Feb 2004), amongst others.

The last of these, River of the Queen, is interesting in the way it explores the consequences both of the very long lifespans of the protagonists (who are near-immortal humans from the Great Ship introduced in Reed’s novel Marrow, published in 2000), and of the nature of Dawsheen: 

“The largest moon of a massive gas giant, it was a blue body with tiny continents and tidal-chumed tectonics. The climate continued swinging in and out of the snowball state with the precision of a pendulum clock. Predictability was a blessing. Predictability allowed the ancient Dawsheen to adapt to their suffering.” 

As a result of the orbit of the moon, the gravitational influences of its star and planet, and varied irradiation, life on the planet is forced to preserve itself through regular deep freezes. As each winter approaches, the entire biosphere deposits its genetic material in a “queen” who will originate the next life cycle after the winter ends, and who is preserved together with as much knowledge as possible from the extant civilisation.

In 2005 the Star Trek universe also acquired a prominent exoplanet. The Star Trek Enterprise episode “The Aenar” took the ship to the homeworld of the Andorians (a species first seen in the original series in 1967) and the related Aenar. This was shown as a class-M (i.e. habitable) if frigid world in orbit of a ringed gas giant, and so as the first major species homeworld in the franchise to be an exomoon [3]. As with many aspects of the Star Trek universe, there are a range of conflicting information sources about the planet (variously called Andor or Andoria) and its star. Indeed, two of the Enterprise series writers have suggested in a DVD feature that they chose to represent a planet and its exomoon specifically to explain the two names. Enterprise also established this as what we know as the Procyon system (itself a binary), 11 light years from Earth.

The rediscovered potential of exomoons as habitable worlds was again spread to wider audiences by a blockbuster movie. Avatar (film, 2012; dir. Cameron) was groundbreaking in its special effects and visual impacts. The film was set on Pandora - a world with forests, oceans and a full biosphere that included the sentient Na’vi. Much of this biosphere was capable of neural interfacing with other organisms and collectively formed the world-spirit Eywa. However, as the opening scenes of the film established, Pandora was not a planet but rather an exomoon in orbit of the gas giant Polyphemus, itself in orbit of Alpha Centauri A. Polyphemus is shown as a bluish gas planet, with storm bands similar to those seen on Jupiter and Saturn in our own system. By contrast, Pandora is shown as extremely Earth-like, with a similar ratio of water to land and a similar amount of cloud coverage to our own planet, although slightly lower gravity and a smaller radius.

 As with Medea, considerable thought was given to the scientific plausibility of Pandora during the film’s development (and that of associated spin-offs such as computer games, sequels and the 2012 book The Science of Avatar by Stephen Baxter). The exomoon’s atmosphere composition, size, surface gravity, eclipsing by other moons and irradiation from both Alpha Cen B (the binary companion of its sun) and Polyphemus were considered. Tidal heating from Polyphemus was given credit for vulcanism on Pandora’s surface, and hence potentially for its rather toxic atmosphere. Night scenes on the exomoon showed the prominence of the gas giant in Pandora’s sky.

Interestingly though, there is no indication in the Avatar franchise that Pandora is tidally locked. This could be plausible because of its relatively large size. Although Polyphemus is canonically similar in mass to Saturn, Pandora is about 35 times more massive than Saturn’s largest moon Titan. In systems closer to equal mass, it takes longer for the satellite to lose its angular momentum and its rotation to become locked. It’s also likely that perturbations from the system’s other moons and Alpha Cen B would prevent a full tidal lock from becoming established.

The increased prominence of habitable exomoons in fiction has continued to parallel the interest in the topic by the astronomical community. Further exomoons appeared in recent Star Trek series including Star Trek Discovery (e.g. icy exomoon Geryon in TV episode “All is Possible”, 2021) and Strange New Worlds (e.g. "Those Old Scientists", 2023).

They have also been a feature of several recent novels, including the works of Becky Chambers. In The Long Way to a Small Angry Planet (2014) she describes Hashkath, the Mars-sized moon of the planet Theth, which is the homeworld of the Aandrisks, a reptilian sentient race. As an Aandrisk pilot, guiding the spaceship Wayfarer into her home system, explains:

“You guys lucked out, though. There’s nothing floating around your planet except the stuff you put up there yourselves. Your flyers could just orbit round and round for ever. Smooth sailing. But your moon’s got moons of its own, and it’s orbiting a ringed planet. That takes some very tricky manoeuvring, especially when you’re talking about a little metal can with flimsy sails. And that was before antigrav made it our way, so you’re just floating there, hoping you touch ground again. Being able to say that you were all the way out there and got yourself safely back home - that made you a hero.” (The Long Way to a Small, Angry Planet, Day 397).

Despite this complicated system, and Theth looming in the sky, Hashkath has a full ecosystem and a human-breathable atmosphere, as well as desert and more temperate climate zones. Like Pandora, there is no indication of tidal locking.

Chambers also visited an exomoon in her novella To Be Taught if Fortunate (2019). This follows a human crew exploring the biospheres of four distinct worlds orbiting a red dwarf star. The first of these, Aecor, is the moon of a gas giant planet in the outer solar system. Like the potentially-habitable solar system moons Europa and Enceladus, this is a frigid world with a liquid ocean under a thick cap of solid ice, and the explorers soon identify abundant life in that ocean, spending four years investigating it through camera traps and ice cores, before heading towards the terrestrial planets further into the system.

At least in part, this increased interest in habitable exomoons has been driven and encouraged by the engagement of the science fiction community with cutting edge research. While no exomoon has been unambiguously confirmed (and may not be until the planned Habitable Worlds Observatory is launched in the 2040s), there are several candidates. These result from a range of relatively high profile observational surveys, and have motivated a range of theoretical work and simulations exploring the potential properties and habitability of exomoon systems. Indeed, more than 90 peer-reviewed academic papers featuring the term exomoon in their title or abstract have been published since 2020 alone. Twenty years after the first detection, the number of confirmed exoplanets in the NASA Exoplanet Archive is approaching 6000, of which the majority are significantly larger than Earth.

The fraction of these which orbit in their habitable zones is difficult to calculate since this is an ever-changing number and somewhat dependent on how that zone is defined and calculated. A large number are believed to be hot Jupiters - too close to their primaries for any satellites to support life - but this is largely a selection function. Many more are what is known as sub-Jupiters and mini-Neptunes, comparable to our eighth planet in mass and radius, but in far warmer orbits. If even a small fraction of these have moon systems comparable to those of their namesake (which hosts a large but possibly captured moon, Triton), the potential for habitable worlds is significant. Theoretical and analytical models suggest that this should be the case. What is clear is that new space telescope missions such as TESS and PLATO and ground-based instruments such as HARPS, SPHERE and soon MICADO are becoming ever more sensitive to such planets (and possibly their moons), and that the numbers continue to rise rapidly.

Exomoons are likely to remain difficult to confirm for some time to come. They are unlikely to ever create the same excitement as the fabled (and as yet undiscovered) Earth 2.0 or even circumbinary planets (sometimes referred to as Tatooines after their Star Wars counterpart). However confirmation of the first potentially habitable exomoon (perhaps to be dubbed a Yavin or Pandora) is perhaps just as likely - and would be just as thrilling on both a scientific and an emotional level.

As our understanding of the miniature worlds of our own Solar System has grown, and then been complemented by the discovery of genuine exoplanets, science fiction has responded, reflected and encouraged the academic research. Discussions of tidal locking, irradiation, tidal heating, day-night cycles and potential orbital stability in the academic literature have been echoed in thought experiments played out in the pages of science fiction literature, and on screens large and small. If both research and fiction have taught us anything, it’s that the universe is likely full of more variety in environments than we can or do imagine. Whether life is to be found in those environments remains to be seen. But habitable exomoons are probably a pretty good place to look.