World ships - planets which move under the deliberate control of their inhabitants or others - are a staple of science fiction. At one end of the size/mass spectrum, they merge into the hollowed-out planetoid class of generation ships. At the other, they can involve efforts to move entire solar systems. But how plausible are representations of world ships in SF, and why are they so popular?

Cosmic Bowling Balls 

One of the earliest but least plausible reasons for shifting planets to appear in SF is for use as kinetic weapons. In the later books of E E ‘Doc’ Smith’s Lensman series, from Gray Lensman (serial 1939, novel 1951) onwards, planets are used as bowling balls to defeat the defences of other worlds [1]. The essential premise of all interstellar travel in the Lensman universe is that objects can be rendered “free” of their inertial mass. This means that no force is required to move them (because force = mass x acceleration) and no kinetic energy is needed (because work = force x distance). In this state, they can be moved instantaneously between points in space (because acceleration can be infinite). Importantly, when the objects are released and revert to being ‘inert’, they still have the kinetic energy, mass, and orbital velocities they started with (both around their hosts and around the Galactic centre), and planets can be picked and released from a chosen point such that these will put them in catastrophic collision with a selected target. This is akin to the old cartoon-animation trick of plucking a missile out of the air, turning it around, and releasing it to strike a different target.

Applying this process to planets is predictably devastating, and fits naturally into the scale of Smith’s vast cosmic-history space opera. However, setting aside for a moment the implausibility of achieving the “free” state and its associated faster-than-light travel, Smith does not address the change in gravitational potential in his planets as they’re shifted from within their home systems to elsewhere, or issues of conservation of angular (rather than linear) momentum, all of which require further breaches of different known laws of physics.

Even so, at least some measure of rational explanation is attempted, unlike the conceptually similar “playing pool with planets”, accomplished in comedy television series Red Dwarf’s episode “White Hole” (1991). Here a nuclear device is used to create a solar flare, which in turn knocks a planet out of orbit and triggers a series of collisions, in a scene played firmly tongue-in-cheek and paying little attention to the underlying physical principles.

Blink and You’ll Miss It

If a planet is expected to survive its movement, then perhaps the best option might be a near-instantaneous transfer, whether by teleportation or some kind of hyperspace passage, where a planet disappears from one point in our space-time and reappears in another.

The First Sirian Bank, an artificial intelligence formed naturally by a silicon planetary crust in Terry Pratchett’s early science fiction novel The Dark Side of the Sun (1976) effectively travels this way. The planet has matrix engines for interstellar flight, allowing it to move between primaries. This does appear to take some perceived duration, but during that interval, the planet is elsewhere and not subject to the physical laws of our Universe - hence habitability, gravitational effects and other changes are unlikely to be severely impacted until the body (in this case a legal Human, in the broad sense of recognised sentience) emerges back into a real-world orbit. The Dark Side of the Sun treats this relatively lightly, allowing the Bank to travel without significant consequences in terms of solar irradiation, surface temperature or gravitational disruption, although one has to ask over what range of temperatures and surface-quakes its silicon-based computing matrix is stable.

The Doctor Who story “The Pirate Planet” (1978, written by Douglas Adams), by contrast, features a world that can not only be moved but also preys on other worlds. Zanak is, in fact, a hollow shell which is materialised around resource-rich planets at the behest of its captain. These are then cannibalised for their mineral wealth, unbeknownst to the majority of Zanak’s population, and regardless of any native life. The planetary engines in this case are transmats (instantaneous matter transporters): the planet disappears in one place and immediately rematerialises under new skies with no obvious transition except the change in the star field. Even setting aside the current implausibility of matter transport, there are other concerns here. How did such a hollow planet form? Is it itself an artificial construct? And for there to be no substantive surface quakes, every planet consumed would have to be precisely the same mass and experience the same orbital forces as the one before… or these would have to be compensated for artificially using advanced technology. The script makes clear that the latter option applies in this case:

“..the whole system is so perfectly aligned by the most exquisite exercise in gravitational geometry that every system is balanced out within itself, which is why we can stand next to billions of tons of supercompressed matter and not even be aware of it. With each new planet I acquire, the forces are realigned but the system remains stable”

 “Then it’s the most brilliant piece of astrogravitational engineering I’ve ever seen!”

- The Captain and the Doctor, The Pirate Planet, episode 3

Presumably similar technologies deal with the angular momentum of the planets (around their own axes and their primaries) and for the huge amount of waste heat that would be generated by the mining/consumption of the prey worlds. Of course, for the surface conditions to remain stable, the energy flux from the primary would also have to be identical or compensated for (i.e. solar heating on the atmosphere, colour of the sky and sun etc), requiring entirely different advanced technology, and this point is not discussed. All in all, such a predatory planet would require an unlikely set of circumstances and a considerable range of Clarke’s Third Law (i.e. indistinguishable from magic) technologies.

Another text with a near-instantaneous planetary relocation, Greg Bear’s novel Moving Mars (1993), rather gives away the punchline of its plot in the title. A coming-of-age story, it follows the life of young Martian-born Casseia Majumdar from early days as a naive student rebel, through a developing political career, and on to a leadership role during a crisis between Mars and its parent civilisation on Earth, triggered by a scientific revolution. As political tensions morph into preemptive strikes, the decision is taken to relocate Mars and its population into a new solar system entirely. This is achieved by a technology that enables modification of the “descriptors” of matter - its properties including particle-type, quantum state… and spatial coordinates. 

Inevitably for a premise that requires a paradigm shift in physics, the mechanism here is unclear and implausible given our current understanding, but Bear does consider a few of the key issues - he discusses the possible impact of the changed gravitational tides on Mars’ interior, and also notes that the energy required is “stolen” from a field distributed through the entire Galaxy as a whole:

“...cheating on nature by pulling the “energy” required to shift Mars from the total energy of the larger system, the Galaxy. In fact, the energy would never be expended, not in any real sense; the universe would simply have its demanding book-keeping balanced, under the tables, while it wasn’t looking” - (1993 Legend Books edition, p. 434).

However he glosses over a number of other issues, notably the difficulty of correctly placing the planet into a new orbit that will be near-circular (to avoid extreme seasons) and stable (in a system with native planets and its own gravitational resonances) over long timescales, or how conservation of angular momentum is ensured overall.

 The Long Haul

A slower and likely more challenging path lies ahead of any planet which is unable to access hyperspace, quantum reality or some other form of faster-than-light get-out clause.

The alien race known as Pierson’s Puppeteers in Larry Niven’s Tales of Known Space series have installed planetary drives on the worlds they inhabit, in order to escape first the red giant expansion of their home star and then a wave of supernovae triggered by a starburst in the Galactic core. As Ringworld (1970) explains, this was achieved by the purchase of an inertialess drive (perhaps a nod to Smith’s earlier Lensman novels) from still more advanced and incomprehensible aliens - a conveniently Clarke’s Third Law explanation which avoids the need to discuss physics any further. While this enables them to travel faster-than-light when necessary, apparently they choose not to, rather travelling at sunlight speeds in an artificial ring configuration of five worlds. Four of these have orbital artificial suns, while the last is entirely urbanised and is a net producer of heat, with a population of over a trillion individuals. The puppeteer character in that novel actually claims “we found that a sun was a liability rather than an asset” (Sphere 1977 edn, pg 63) due to difficulties dealing with solar expansion phases and due to industrial overheating of their worlds. A group of late entries in this universe, Fleet of Worlds (2012, and its sequels, cowritten by Edward M Lerner), set some 400 years before the more famous Ringworld, explore this phase of Puppeteer history in more detail.

The 1980s BBC radio series Earthsearch, written by James Follett in 1981-2, also features a planet moved to avoid its sun’s expansion. While the titular spaceship was originally designed to find new habitable Earths, on return to its home system it finds its homeworld gone and the search changes focus. As gradually becomes apparent, the Earth has been moved by its occupants to avoid solar instability. Although the precise mechanism adopted (and how it satisfies the energy needs of a moving planet) is unclear, it’s interesting to note that Follett has considered at least some of the requirement of maintaining habitability; an array of sixteen fusion-powered orbital platforms called solaria are constructed to act as artificial suns ‘balanced so that the cycle of the seasons, of life and death itself, would remain undisturbed during the great voyage’ (Earthsearch II, ‘Solaria’, 1982). A late addition to this series, Earthsearch: Mindwarp (Follett, novel 1994; Big Finish audio production, 2001) clarified that at least some of the population was resident in underground cities before the project to move Earth got underway, which would help mitigate some of the problems associated with the move.

Earthsearch’s planet Earth is not the world as we know it - as rapidly becomes apparently to listeners. But several authors have indeed considered the possibility of moving our own world.

An altogether grittier and less optimistic view of the destructive energies required to move a planet, and the catastrophic effects of doing so on its environment and habitability, are articulated in the novella The Wandering Earth by Cixin Liu. Here huge “heavy-element fusion” torches are used first to stop the Earth’s axial rotation and then to push it into an elliptical orbit where it can pick up a gravity assist (i.e. extra tug and transfer of energy) from the Sun to escape the Solar System. The torches and orbital shifts destroy the atmosphere and all life on the surface, and so this process requires the entire population to move to underground cities. However, as Liu explains:

“The changes in velocity and trajectory generated by the Earth Engines disturbed the equilibrium of Earth’s iron-nickel core. The turbulence passed through the Gutenberg discontinuity and spread to the mantle. As geothermal energy escaped to the surface, volcanic eruptions ravaged every continent, which posed a lethal threat to humanity’s underground cities.”

Even after escape velocity is gained (after decades of power injection), the engines have to be sustained for five hundred years, and the story ends with the journey of Earth to the Alpha Centauri system expected to take another twenty-five centuries, for the entirety of which, humanity will be reliant on artificial environments while the surface remains frozen and uninhabitable. Like much of Liu’s writing, The Wandering Earth reads primarily as a meditation on the prices people are willing to pay for survival, the role of the individual in a focussed and driven society, and the complex ties of duty, honour and family in a time of adversity. The power source required for Earth’s motion is beyond our current (or very possibly also our future) capacity, since fusion experimentation is currently limited to light elements such as hydrogen and helium. Liu’s work is nonetheless interesting for its plausible representation of the potential impacts on Earth’s habitability of deliberately moving the planet with near-future human technologies and the sacrifices it would require, as well as the very long timescales required.

Less rigorously, but just as memorably, Doctor Who has also explored the possibility of moving Earth, albeit against the wishes of its inhabitants. In “The Dalek Invasion of Earth” (BBC television 1964, remade as the feature film Daleks - Invasion Earth: 2150AD in 1966), the eponymous aliens employ a slave labour force to dig a vast mine in Bedfordshire. Their goal is to crack open the Earth’s crust, expelling its magnetic core towards the Sun. The Daleks will then install a planetary motor to relocate the Earth to the Skaro system for future settlement. Here, at least, there is no illusion that habitability can be maintained on the surface. The advantage of being meglomaniacal militaristic aliens is that no one expects you to keep your slave labour force alive, or to have similar habitability requirements. More seriously, the mechanisms to be installed are never explained. Nor is the ejection of the core very plausible (at least while keeping the mantle and crust mostly intact). Fortunately, the Dalek plan is doomed to failure at the hands of the Doctor (or, in the film version, the hands of "Dr. Who", played by Peter Cushing).

You Can’t Take it with You

So why is the concept of shifting planets so popular in science fiction?

Such stories speak to a primal fear in humanity that the life-giving Sun may prove instead life-taking. Particularly in the 1960s and 1970s (when the solar neutrino problem suggested the Sun’s evolution had not been well understood) there was a fear that it would not remain stable, and that the only way of saving more than a remnant of humanity from destructive solar flares (or worse still, a nova) might be to leave the Solar System en masse. This idea spread well beyond the relatively small number of people who understood the issue, and informs a broad range of science fiction (including examples written after the problem was solved!). Even in the absence of flares, fiction has speculated about the need for the human race to escape supernovae, or simply the red-giant end states of natural solar evolution.

This is usually envisaged as occurring over a long time scale, through the gradual spread of extraterrestrial settlement and with small craft moving outwards in an expanding wave. Isaac Asimov’s popular science book Extraterrestrial Civilizations (1980), for example, envisages that many civilisations may go through a phase of living on “free-worlds”: medium-sized space settlements with populations in the tens of thousands (i.e. space colonies or inhabited asteroids rather than planets) that may drift between host stars as required. This premise is repeated in the same years’ A Choice of Catastrophes (also non-fiction) in which he states explicitly that moving the Earth out of our solar system is simply infeasible in terms of energy and momentum, even for a culture using fusion power. However texts such as The Wandering Earth and Moving Mars envisage relocation as being required at a technological stage before such widespread space travel can save more than a tiny fraction of the population, or on much too short a timescale to permit extra-solar emigration on small craft

In some ways, moving planets are a logical extension of the spaceship and generation ship concepts. Not only does the planet provide a sense of freedom, familiarity, historical grounding and comfort for the (often unwilling) travellers, but the size of a planetary geo- and eco-sphere providing some buffer against the potential failure modes of such ships (e.g. environmental inbalances, overpopulation, underpopulation, radiation exposure etc). This is particularly true if a planet’s sun, or an artificial substitute, can be carried with the planet - as is the case for the Puppeteers in Niven’s Known Space, or the titular Earth of Earthsearch. These worlds are expected to remain fully habitable during and after their movement. Of course, such advantages are only valid if the act of moving does not itself destroy habitability - as is the case in The Wandering Earth, and to some extent in Moving Mars.

An interesting mid-way house for such schemes has been proposed by Korycansky, Laughlin and Adams (2001). They suggest that it might be possible to move Earth to a wider, Mars-like orbit through a gradual process involving exchange of momentum with Jupiter through repeated encounters with planetoid intermediaries. While this would not enable humanity to escape the Sun’s eventual demise, it would substantially extend the habitable lifetime of the Earth as the Sun gradually brightens over the next few billion years. However such gradual migration (with timescales of gigayears) would itself be beyond our current technology, and yet still fall far short of the kind of world-engines assumed in science fiction narratives.

As a result, stories which involve routine (rather than emergency) planetary transport associate the feat with highly advanced civilisations, exploiting the entire energy output of their suns or other still more exotic sources. Indeed, researchers (mainly associated with the venerable British Interplanetary Society) have speculated that identifying such world-ships in time-domain astronomical surveys might be one avenue towards the search for extraterrestrial intelligence, acting as a technosignature. Badescu & Cathcart (2000), for example, defined categories of interstellar engines that advanced civilisations might build to move their entire solar systems, while in 2013 Duncan Forgan proposed looking for a specific class of such engines that would use a parabolic mirror megastructure and leave an identifiable trace in current exoplanet transit surveys. Given our current technologies, only weak upper limits can be placed on the prevalence of such systems, and even those would be extreme examples of their type (if one existed), on scales much larger than a single moving planet.

So is planet-moving ever likely to be feasible? In the unlikely event that humanity were ever able to capture the Sun’s entire energy output (for example using a Dyson sphere) and utilise it with 100% efficiency, becoming a Kardashev Type II civilisation, it could collect enough to overcome Earth’s gravitational potential with respect to the Sun in several months of energy accumulation. However the technology both to collect such energy and to convert it to kinetic energy (let alone do so in such a way that the planet doesn’t melt, explode or otherwise experience catastrophe) lie so far beyond any current paradigm that this seems firmly in the realm of science fiction. In the meantime, stories of piloted worlds continue to reflect humanity’s aspiration to reach the stars, its reluctance to leave behind the only home its known, and the deep fear that one day we may have no choice but to do so.