The energy source for virtually all life on Earth is the Sun. The Sun controls the weather, climate and seasons on our world. Its radiation maintains Earth’s surface temperature, and provides the energy that plants and algae use to photosynthesise, thus sitting at the base of the food chain for all the higher life forms [1]. Given its essential role in life on our world, perhaps its natural that a large body of science fiction explores the concept of how life might survive on worlds without their own sun - rogue planets.

 On a relatively superficial level, the identification of a world as a rogue or wandering planet has been used to add an air of mystery and differentness about it, casting it as Other from the worlds we know and can identify with. Examples of such a usage include descriptions of Mongo (the homeworld of Ming the Merciless in Flash Gordon), Cybertron (homeworld of the Autobots and Decepticons in Transformers) and the Founders’ homeworld in Star Trek: Deep Space Nine [2] as rogue planets. In these cases the consequences of their sunless status aren’t usually explored in detail, at least in the original source material, although they are often discussed in auxilliary or supporting texts.

A little more detail was provided in the Star Trek: Enterprise season 1 episode “Rogue Planet” (2002). Here the eponymous planet, Dakata, is a subject for scientific study by the NX-1 Enterprise, but proves to be used as a perpetually-night time hunting ground by an alien species. The bulk of the plot focuses on that hunt and the plight of the native species being hunted. Nonetheless, the planet is interesting. Its surface is richly vegetated, with a breathable atmosphere and a temperature comfortable for unprotected humans despite the lack of a sun. This circumstance is explained as being the product of extensive thermal venting and geothermal activity. On Earth, geothermal heat flow contributes about 80 mW of power per square metre at the surface, compared to the Solar irradiation of 1.4 kW per square metre. A third of the solar radiation is reflected, but even so this planet must have a surface heat flow almost twenty thousand times higher than that of Earth. While this might be possible if the planet is young and formed from a nebula highly enriched in radioactive elements, it’s hard to imagine that such a world would have a surface and ecosphere as stable as that shown in the episode.

In fact the lack of thermal energy from an external source is likely the biggest challenge to life on a Rogue Planet. An interesting exploration of the consequences of losing the Sun’s heat is Fritz Leiber’s 1951 short story “A Pail of Air”. Here a “dark star” has swept through the Solar System, pulling Earth from its accustomed orbit. While the characters believe that the dark star (presumably an old, cold neutron star or a black hole) has captured the Earth into its own influence, the net effect is equivalent to ejecting the Earth from the Solar System to turn it rogue. The Earth is no longer heated by the Sun. The atmosphere has condensed onto the surface, first as a layer of water ice and then as a layer of crystals containing the remaining gas. These must be melted over a heat source to provide a breathable atmosphere. Of course, this supposes that the crystals can be gathered in the right proportions of oxygen, carbon dioxide and inert gases such as nitrogen. However crystallised ices of the various atmospheric gases can indeed be found in the Outer Solar system, and this is an evocative tale of an Earth drawn past the snow line.

While this is an accidental displacement of Earth, a more deliberate decision to remove the Earth to a new orbit of Proxima Centauri is taken in Cixin Liu's novella "The Wandering Earth" (2000, filmed in Mandarin in 2019, published in English translation 2012). I'll come back to this story and the idea of deliberately-constructed world ships on another occasion, but I mention this here as it echoes many of the features of "A Pail of Air" - the calamitous geological and tidal disruption caused by a change in orbit of either the Earth or the Moon, the death of large sections of the population in such catastrophes, and the crystalisation of the atmosphere into an irregular layer as it freezes out due to the lack of solar heating.

In both “A Pail of Air” and "The Wandering Earth", the Earth is ejected while the Moon is left behind in the Solar System. In the television series Space:1999 (broadcast from 1975 to 1977), the reverse is true. The Moon is ejected from the Solar System by a thermonuclear explosion in a waste disposal ground. Redubbed “Alpha” it wanders between the stars, occupied only by the crew of a small moonbase who were carried along with it. In truth, the physics and astrophysics of Space:1999 are probably best left unexamined. It’s easy to show that any surface explosion large enough to accelerate the Moon to a Solar System escape velocity would also destroy it entirely. And the tendency of Alpha to stumble into and out of new solar systems on a weekly basis, and apparently to accelerate and decelerate erratically is best treated as a narrative conceit. Nevertheless, there are elements of the series which do demonstrate the challenges which would be faced by any population surviving ejection into interstellar space - amongst them the need for a power supply (in this case, rather ironically, a nuclear reactor) and the difficulty of establishing sustainable food and resource management - all problems also encountered in the context of generation starships [3].

Perhaps a more realistic, and certainly a more terrifying, picture of rogue planets is found in the 1933 novel When Worlds Collide by Philip Wylie and Edward Balmer. As is explained early on in the novel:

“It might be assumed, for purposes of explanation of the Bronson Bodies, that they were once planets like our Earth and Uranus, circling about some life-giving sun. A catastrophe tore them away, together with whatever other of her planets there might have been, and sent them into the darkness of interstellar space. These two - Bronson Alpha and Bronson Beta - either were associated originally, or else established a gravitational influence upon each other in the journey through space, and probably have travelled together through an incalculable time until they arrived in a region of the heavens which brought them at last under the attraction of the sun.”

In the movie version of the same story (1951, dir: Maté), the two planets are instead renamed as a sun Bellus and its planet Zyra. Either way the premise is the same: the larger body will swing past the Earth, causing massive devastation, and loop around the Sun before returning to collide with the Earth, destroying it utterly. The only hope of saving even a few representatives of humanity is to try to reach the smaller body - Bronson Beta or Zyra. The novel and film follow this effort at creating an ark, while revelling ghoulishly in the annihilation of virtually all humanity and scenes of destruction and desolation.

There are, however, some interesting aspects to this text, particularly to the original novel. It’s a relatively early manifestation of the distrust-of-scientists trope that became increasingly prevalent through the twentieth century - the novel begins with a conspiracy of silence amongst the world’s research astronomers, a community that has repeatedly proved incapable of keeping any results secret in recent years. When the news of the Bronson bodies and Earth’s fate finally does break, it’s accompanied by general disbelief and political demands that the scientists come up with a better prediction - a situation rather reminiscent of President Trump’s demand that his medical scientists come up with a quick cure for covid-19 and make more optimistic predictions at the start of the pandemic.

The book also contains an early recognition of both the uses and difficulties of using atomic power. Written a decade before the construction of the world’s first nuclear pile, it discusses the use of atomic rockets for interplanetary travel, and also the difficulty of finding a material capable of channeling and containing the necessary temperatures without melting. It’s a little unclear whether fission or fusion is being discussed, although the mention of beryllium might suggest the latter. Although magnetic fields are now used for holding the hottest plasmas away from the surfaces of containment vessels, materials science innovation remains a key challenge in the development of commercialisable fusion reactors.

Returning to the topic of the rogue planets themselves, a great deal of discussion understandably focuses on their potential habitability and its history. While some of this has religious overtones (both in terms of divine retribution on Earth and divine providence in providing Bronson Beta/Zyra), there are also some interesting scientific inferences in particular regarding the habitability of the new world and how it can be established. As one character notes:

“There are two worlds coming towards us - two worlds torn, millions of years ago, perhaps, from another star. For millions of years, probably, they’ve been wondering, utterly dark and utterly frozen, through space; and now they’ve found our sun; and they’re going to attach themselves to it - at our expense. “

The freezing out of both its oceans and the atmosphere - as seen in “A Pail of Air” and "The Wandering Earth" - is discussed, together with their reconstitution as Bronson Beta approaches the Sun and is captured into orbit. The need for any colonists to take with them oxygenators and base-of-foodchain photosynthesizers such as algae is discussed, as well as decomposers such as fungi and the bacterial load of pond water or other organic-rich materials. It’s recognised that surface life on the planet will not have survived the freezing process, although there’s a lengthy discussion of the then-established ability of spores (from fungi, moss, lichen etc) to survive extended freezing and the expectation is that some of this native life in will prove to have been viable even after millions of years at near-absolute zero. The once-existence of that life, and in a highly developed form, is confirmed by the observation of the ruins of a civilisation on Beta itself. 

Unfortunately - or perhaps not in light of the potential threat from alien bacteria and viruses - it’s highly unlikely that these expectations of a resurgent vegetation are realistic in the light of current knowledge. It is now possible for us to generate far lower temperature storage than was possible in the 1930s. While this does indeed preserve seeds and spores for longer than room temperature stores, it is far from providing indefinite storage, and also cannot prevent the genetic damage that would accumulate from millions of years of bombardment by cosmic radiation - none of which was understood when the novel was written. It is unlikely that even the robust protective endospores of extremophile bacteria (a threat against which human-built spacecraft are decontaminated before being sent to alien worlds) would survive such an extended deep freeze.

One matter which is left for speculation in When Worlds Collide is the process by which the two Bronson worlds have been set adrift - both a stellar explosion and gravitation disruption due to the passage of another nearby star (as in “A Pail of Air”) are discussed as possibilities, and both could certainly result a planet becoming unbound from its host star. However the majority of the rogue planets now believed to exist in the Milky Way are likely to have other origins. Such worlds are not easily detected, and have most often been found through a phenomenon known as gravitational lensing. A planet passing between us and a distant star will act as a magnifying glass, briefly magnifying that star and causing it to brighten. Such glinting stars need chance alignments, but the tens of objects which have been seen suggest that free-floating planet-mass objects must be relatively common in our universe. Some of the largest numbers of these objects have been found in association with young stellar clusters, hinting that planets are more likely to be set adrift early in the life of their host stars rather than later.

This is consistent with the modern picture in which planets are typically thrown out of solar systems in the early stages of their formation. As the protoplanetary disk, made of gas and dust, develops, small planet-sized objects form in its depths. Some of these will merge through a series of collisions until larger planets are formed. Some will form beyond the snow line where they can start to accrete the atmospheres of gas giants. And others will be thrown out of the solar system by close encounters with their neighbours, particularly if any giant planets migrate through the system due to drag from the residual disk or as a result of changes in the stellar evolution. If a smaller body comes into a resonance with two larger ones (for example an Earth-sized planet resonating with a sun and a gas giant), a process called the Kozai-Lidov mechanism results in the smaller planet being ejected, carrying away with it a large amount of angular momentum and allowing the two larger bodies to approach one another. Even before the first rogue planets were detected, the existence of “hot jupiters” (gas giants close to their sun) strongly suggested that they were common.

If rogue planets are really as common as current theory suggests, then the coming years should see ever larger numbers of such objects identified and studied. However these will always be challenging targets - with neither substantial internal heat nor the light of a sun to reflect, they will always be dark and seen primarily through their effects on other things around them. The challenge of maintaining life on such a world cannot be understated, but the fascination these planets have for writers of science fiction suggests the challenge is one the imagination, at least, can take on.