Centauri Dreams
Imagining and Planning Interstellar Exploration
Biological Evolution in Interstellar Human Migration
Centauri Dreams is happy to welcome Dr. Cameron M. Smith, a prehistorian at Portland State University’s Department of Anthropology in Portland, OR, with an essay that is the capstone of this week’s worldship theme. Dr. Smith began his career excavating million-year-old stone tools in Africa and today combines his archaeological interests with a consideration of human evolution and space colonization. He is applying this interest in his collaboration with the scientists at Icarus Interstellar’s Project Hyperion, a reference study for an interstellar craft capable of voyaging to a distant star. Recently Dr. Smith presented a paper at the NASA/DARPA ‘100 Year Starship Study’ conference in Houston, Texas. His recent popular science publications in this field include “Starship Humanity” (Scientific American 2013) and the book Emigrating Beyond Earth: Human Adaptation and Space Colonization (Springer-Praxis, 2013). We can look forward to a follow-up article to this one in coming weeks.
by Cameron M. Smith
1. Interstellar Migration: An Insurance Policy for the genus Homo
Planets orbiting distant stars are now being discovered at a rapid pace, with hundreds known and countless worlds implied. As an anthropologist, I take a wide and long-term look at human evolution, and this development is very exciting to me; those almost unimaginably-distant planets are where humanity is headed, in the longer or shorter term. Humanity has been characterized by spreading itself wide across the Earth, and after first colonizing Mars we will surely wish to go farther, just as, in Polynesian legend, the siblings Ru and Hina—having explored the whole Pacific—chose to build a special vessel for a trip to the moon. Although civilization as practiced so far is perhaps its own worst enemy, I am optimistic that humanity’s better side will generally prevail, and that our species will invest in space colonization as an insurance policy for our lineage. Figure 1 indicates the five most recent mass-extinction events in Earth history, and Figure 2 indicates that even in recent times, civilizations have repeatedly collapsed and disintegrated, with no guarantee of recovery [click on figures to enlarge as needed]. In the larger picture, over long time, space migration is the best means of surviving such disasters.
Recently humanity has spent just over a generation exploring the solar system just beyond our atmosphere, sometimes with robotic voyagers, and sometimes with our own bodies; for the past 23 continuous years human beings have already lived off of the surface of the Earth in various orbital stations; cosmonaut Sergei Krikalev has spent over 800 days in space, and his colleague Valeri Polyakov once remained continuously in orbit for well over a year. Despite some close calls, nobody has died in this nearly quarter-century of continuous space habitation, which has taught us some basics of space biology. Clearly, our species’ technical capacities are superb, and the essential technologies for long-term stays in space are being sketched out. We also have realistic destinations in view, and highly-focused, forward-thinking physicists and mathematicians working on how to reach those destinations with unique propulsion systems.
Will developing space technology be enough, however, to pave a way to space colonization? It will be part of the equation, but the equation is not complete. Space colonization will be about humans living off of Earth, but human biology off of Earth is only barely understood, and only in the cases of the individual physiology of adults, for short periods of the entire life course. Space colonization, however, will be about humans living out entire lifetimes—from fertilization to embryo development and so on through adulthood—and in populations (rather than just individuals) and populations over multiple generations, not short stints in Earth orbit. For these reasons, at least, we need an anthropology of human space colonization. Anthropology studies the biocultural evolution of our lineage and some of our closest relatives. In this article I would like to introduce some of the biological issues involved in space migration, specifically among multigenerational, interstellar voyages. In a second article I will introduce cultural evolution in such vessels.
2. Where to Begin?
Space colonization will be a continuation of human evolution. I don’t mean this in the shallow sense of a ‘March of Progress’, but in a deeper, more mature and more useful way. Humans are life forms, and thus we change through time, both biologically and culturally. When such change allows us to live in new places—for example, the Canadian Arctic (over 5,000 years ago), the islands of the Pacific (over 3,000 years ago), or Earth orbit—it is called adaptation. Space colonization, then, will be an attempt at adaptation, and if we want to succeed in this adaptive endeavor we should be informed with everything we know about evolution in general and adaptation in particular.
Generally speaking, while it is clear that humanity has adapted to many ecological niches over time, it is equally clear that our adaptation differs significantly from that of most other life forms. This is because humanity adapts more culturally and technologically than biologically; witness our essential biological similarity across the globe, despite a few regional variations in skin color, hair texture and so on. Rather than adapting largely by body, then, we adapt largely by mind, which generates complex behavior (such as adjusting our kinship rules and sex taboos to match our distribution across landscapes) and complex technologies (including everything from inventing sailing vessels and stellar navigation in the Pacific to walrus-hunting watercraft and harpoons in the Arctic) allowing our species to flourish in places never dreamed of by our earliest bipedal ancestors. Having said this, humanity continues to evolve biologically even today, and will continue beyond Earth, and that evolution must be considered.
While technology is a cultural invention, in this article I will leave the technologies of space colonization to the engineers, and focus on the biological issues involved in interstellar voyages. These have been sketched out before in disparate literature; here I would like to update the material and consider it specifically in the case of current draft studies of interstellar vessels.
3. A Thoughtscape for Planning Interstellar Voyages
Current plans for interstellar voyaging essentially envision a gargantuan starship hurtling on a one-way voyage from Earth towards a distant star system. Interstellar craft would have to be very large—perhaps kilometers on a side—to house populations in the thousands, as I will discuss below. The most distinctive characteristic of these colony ships is that they would essentially be closed systems, with no opportunity for bringing in new genes, maintenance or repair materials, or consumables, such as water or breathing gases. These ‘Space Arks’ would have to be regenerable and self-contained, a fascinating challenge not just in engineering but also concerning biology and culture.
Presuming that we could build such vessels, the main engineering challenge is going to be propulsion (an issue being tackled right now by the scientists at Icarus Interstellar). Our galaxy, the milky way, contains an average density of .004 stars per cubic light year, and the closest star to Earth, Proxima Centauri, is 4.2 light years away. At light speed that is 4.2 years of travel time, but today, attaining that speed (or, technically, something very close to it, as light speed is not thought to actually be attainable by anything other than individual photons or similar particles) is unrealistic. How fast can we go? The fastest humans have moved is about 24,790mph (just over .00003% the speed of light), on Apollo X in 1969. At that pace, Proxima Centauri is about 140,000 years away. Maybe we can do much better, though. In the last 150 years we have increased human travel speeds almost a thousandfold up from the locomotive’s 30mph (the Voyager spacecraft speed along at about 1,000 times the speed of a locomotive). If we can do something similar in the near future, travel times become significantly more manageable. A 100-fold increase in speed from Apollo X (bringing us to just under 3,000,000 miles per hour) would take us to Proxima Centauri in something under 1,400 years. This is getting manageable, but is still a long time; imagine launching the ship during the Dark Ages (just after the collapse of Rome, about 1,400 years ago) and having it arrive at its destination in the time of such oddities as desktop computers. If, however, we manage to make a 1,000-fold increase in speed (bringing us to about 30,000,000mph) in the next century or so, then the heavenly wonders of Proxima Centauri (actually we don’t know what it is like there, yet, but I am being optimistic) could be achieved in about 140 years (see Figure 3). This is a very manageable timescale, a distance only that from today’s world to that of 1872–less than five generations distant and essentially comprehensible.
This, then, is the thoughtscape I am exploring in this article; voyages on the order of under two centuries, carrying some thousands of people (a number discussed below) before reaching another solar system. What biological and cultural issues need to be addressed on such voyages? While the answer will be some time coming (I’m currently investigating it in research for my forthcoming textbook on the anthropology of space colonization), I can sketch out some below.
4. The Biology of Interstellar Voyaging
While evolutionary biology is currently undergoing significant revision in the light of new genomic data, the principles of evolution are largely intact. These include the four main processes (genetic mutation, selection, migration and drift) that change the properties of the gene pool over time. Each is discussed below.
4.1 The Biology of Interstellar Voyaging: Mutation
Mutation is ultimate source of new variations, such as coarser hair, or darker skin than one’s fellows. In common speech a mutation is thought of as being deleterious, but scientifically speaking mutations could be advantageous, disadvantageous or neutral. Some mutation is a result of radiation, and interstellar voyage designs will have to consider trapped particles, radiation energy near Earth and constrained within the Van Allen belt (and presumably about other planets with analogs of the Van Allen belt; cosmic rays derived from many sources outside this region, and solar radiation blasts out of suns. Radiation can degrade biological tissues and cell functions, and it can also be a powerful mutagen, altering DNA; recently it has been suggested that cosmic radiation could increase the incidence of brain disorders.
Certainly radiation is an issue to be addressed, but it does not seem to be a showstopper for human space colonization. First, many shielding schemes have been devised. In interstellar craft some kind of radiation shielding will be necessary, but because shielding is heavy, designers will probably try to get away with as little as possible. How thin is too thin? This is difficult to answer; aside from clear cases of lethal exposure to high doses of radiation, its long-term effects are mysterious. One 1995 study tracked the health of over 70,000 children of parents who were within a kilometer of the nuclear bombings at Hiroshima and Nagasaki, Japan, during World War II. Surprisingly, there was no statistically significant difference between the study population and populations of children of non-irradiated parents, in terms of malignant tumors in early age, differences in sex of offspring, chromosomal abnormalities and other mutations. On the other hand, recent studies have shown highly elevated mutation rates in people living near the Chernobyl disaster site. We have plenty to learn.
The second reason that radiation will not be a deal-breaker for interstellar voyaging is that mutagenesis itself has recently been found not to be largely the result of such ‘one-off’ ‘zaps’ from space, but rather more the result of the failure of DNA-repair mechanisms on the molecular level itself (over 300 such mechanisms and processes have been identified in the human genome alone). Thus, DNA repair therapy will probably be a large part of mitigating radiation issues in interstellar voyages.
In short, anywhere beyond Earth’s natural radiation shielding, under which we have evolved for millions of years, the mutagenic environment will be new and therefore a candidate for affecting our genome. Obviously we will use technology to mitigate such effects, including shielding and management of DNA repair, but we must remember that we do not know everything and that it will take time to adjust—both biologically and culturally–to new environments.
4.2 The Biology of Interstellar Voyaging: Selection
Natural selection occurs when life forms are prevented from reproducing—or simply have less offspring than others of their generation—due to their genetic characteristics. For example, a fly born without wings is unlikely to have offspring, or at least less offspring than its cohort (because of its reduced health and reduced ability to find mates) such that ‘wingless’ genes are likely to be ‘selected out’. Concomitant to most ‘selection against’ certain characteristics is selection ‘for’ alternatives. It’s often thought that humanity has halted natural selection with modern medicine and technology, but this is an illusion; many people continue to die before giving birth because of their genetic properties, particularly in populations without access to modern health care; and several recent studies have shown natural selection to be underway in modern populations. Even so, by the time of interstellar travel health technology will be much advanced…and yet selection will continue, for at least three reasons.
First, conditions off of Earth will differ from the conditions on Earth, where a relatively narrow environmental envelope of temperatures, atmospheric pressures and elemental compositions, nutrient supply and gravity have prevailed. The conditions in interstellar craft might well approximate these conditions, but much experimental data indicates that even small alterations in such variables as breathing gas composition and pressure can negatively effect gene expression (the switching on and off of genes on very complex schedules) and development of vertebrate embryos during the critical phases of early formation (e.g. gastrulation). We will of course try to control such variables, but it is unlikely that we will be able to anticipate everything. It seems certain that there will be a degree of increased infant mortality as the human genome adjusts to new conditions in interstellar colony craft, however carefully we design them. I am currently researching the genetic aspects of human development and the life course in environments slightly different from those on Earth.
Second, selection will likely play out in the cases of sweeps of novel diseases through interstellar craft populations. Again, we will be very careful, but it is impossible to anticipate all biological change, and in smallish populations (e.g. the 10,000+ that I suggest for interstellar craft; see below) inhabiting closed environments, sweeps of new disease, I believe, might well occur. Whether such sweeps structure the interstellar craft genome structurally is impossible to know, but we must be prepared for this possibility.
Finally, most interstellar voyage plans head not for other stars per se, but for their planets, where the resources and landscapes of alien worlds will allow humans to once again take to a planetary surface. In such a case, it is certain that new environmental conditions will be encountered. We will use technology to mitigate environmental selection, of course, but we must remember that we are only just appreciating the significance of epigenetics, the throwing of genetic switches by environmental factors. How will new planetary conditions shape the human genome? We simply can’t say, but we can be sure that it will occur. For example, even the Apollo lunar walkers commented on the lunar soil they inadvertently tracked into their lunar modules; you can bet that they breathed it in. They stayed on the moon only for days…but what would be the effect of such close contact of the human body with new chemistries of different worlds over the course of a lifetime? What genetic switches could be activated in such conditions? We don’t and can’t know; we can estimate and model, but we can’t be certain, and it is likely that selection will occur on our genome again in new planetary environments, at least until we control it with technology.
To succeed in migrating from Earth we are going to have to accept some risk. We do this on a daily basis; in the U.S. many of us take a daily commuting gamble, with nearly a hundred losing that bet—dying in car crashes—daily. If we can take that risk, surely we can adjust to the return of a degree of selection in our dream to colonize space and supply our genus with an insurance policy against extinction or even ‘just’ civilization collapse.
Ultimately, natural selection can be strongly mitigated with technology and is unlikely to strongly structure our genome in the 140-year ‘thoughtscape’ I am currently exploring. But natural selection should not be discounted in plans for interstellar colonization, particularly because it will ultimately involve generations of humanity adapting to new environments.
4.3 The Biology of Interstellar Voyaging: Migration and Drift
Migration is the flow of genes into and out of gene pools (populations) and it is a major factor on Earth, where humans have vast travel networks and today mate even across different hemispheres of the globe. However, in interstellar voyaging craft populations will be relatively fixed (rather than expanding, until new planets are reached), and migration will be between rather small sub-populations of the interstellar craft, an issue returned to below.
Drift, on the other hand, is the result of chance events in the history of a species; a good example, and one most suitable here, is the founder effect in which the genetic composition of a population is strongly conditioned by its founding members. This factor will be of critical importance in interstellar colony voyages because they will be closed genetic systems—as just mentioned—whose genetic structure will be largely established by the founding populations.
Regarding populations, we should take a minute to consider the size of interstellar colony groups. Should we send tens, thousands, millions of people? One way to tackle this issue is to consider our species’ MVP or ‘minimum viable population’, the figure required to avoid the deleterious effects of inbreeding. This figure has been much debated; anthropologist John Moore has suggested a figure of about 150, while others have suggested closer to 500. Such small populations, however, are highly vulnerable to single catastrophes, and my own calculations have suggested an MVP of about 3,000, multiplied by a ‘safety factor’ of 4 to 6 for an interstellar colonization population ‘reference figure’ of 12,000-18,000, which I consider significantly capable of surviving both biological disasters such as disease sweeps and a number of significant technological failures, over the low-centuries figure to reach, for example, Proxima Centauri.
From 12,000 to 18,000 people, then, as a ballpark figure for a founding population for interstellar migration; how do we pick them from the human population? Evolutionary ecologists measure s species’ health by its genetic diversity because a diverse gene pool allows for adaptation to new, unexpected conditions; thus our colonists should be biologically diverse, representing the human genome worldwide (which includes variations adapted to low and high altitudes, for example). However, an over-inclusive approach could endanger future populations if certain genetic maladies are allowed among the founding population. The screening process—determining that certain humans should not and could not participate in off-Earth colonies—seems to go against the very Enlightenment values of equality and freedom at the heart of Western Civilization, but if we are going to succeed in human space colonization we cannot ignore genetics. This is nothing new: over thousands of years human cultures have already devised many and elaborate kinship systems and sexual regulations that prevent the genetic disorders associated with close inbreeding; a survey of Yale’s Human Resources Area Files ethnographic database indicates that most cultures ban marrying or mating between parents and siblings of parents, siblings themselves, grandparents, and first cousins. Humanity has been looking after our genome for a very long time.
The main issue in genetic screening is the detection of genetic disorders that might send a biological ‘time bomb’ into future populations, particularly small and closed populations. But even this ‘simple’ issue includes moral and practical hurdles. In practical terms, this is evident in a depressing poster generated by the federal Genomic Science Program, a rather gruesome document pointing out the location–on each of our chromosomes–of many hundreds of genetically-controlled disorders, from cancers to deafness (we should remember that genes don’t exist simply to cause problems, and that in most cases they build healthy individuals!). Screening for interstellar suitability here seems simple enough: people carrying certain genes would have to remain Earthbound. The significant complication, though, is that while many genetic disorders are known to be simply correlated with certain genes—these are called Mendelian traits—modern genetics finds that more disorders are not so easily ‘pinned’ onto just one easily-spotted genetic marker. Indeed, in a recent paper Professor Aravinda Chakravarti of the Johns Hopkins School of Medicine noted that the textbook concept of simple Mendelian inheritance—’evolution basics’ that I teach in my own classes—seems to be melting away in light of his ‘genetic dissections’ of the real mechanisms of genetic disease, replaced by far more complex models. For example, many disorders are polygenic, the complex result of the interactions of many genes. And, single genes can be pleiotropic, affecting multiple characteristics of the individual organism! To further complicate matters, even though one might carry the gene or genes ‘for’ a certain disorder, environmental factors encountered during the course of life can determine whether or not those genes are activated in such a way as to ‘express’ the genetic disorder.
We must address such issues because, as geneticist David Altshuler of the Harvard Medical School recently noted in the New York Times, “Even if you know everything about genetics, prediction will remain probabilistic and not deterministic.”
And what if we could identify, say, a ‘gene’ for deafness? Just after thirty years of age, Beethoven became deaf; it this were due to a genetic disorder, should he have been ‘selected out’? Should Beethoven’s deafness have been ‘pre-emptively corrected’? He completed much fine music even after his deafness. And what about Stephen Hawking’s genetic disorder, amyotrophic lateral sclerosis? Would screening out someone carrying a certain likelihood of expressing that disorder be a good idea? We already make mate choices, some based on actual or perceived health and future of our partners, and even the fates of some of our embryos. For the health of off-Earth populations, we must be willing engage in these complex discussions to determine what levels of probability we are comfortable with in terms of deciding whether or not a given person can participate in space colonization. Philosophers of morality could be of great help in clarifying the issues in a secular way, and designing real-world solutions.
On the surface, we might think that a solution to these issues would be to encourage the breeding of a master ‘Space Race’, as in the science fiction film Gatacca. But this idea is counter to nature and all of population genetics. If all are identical, all are subject to the same evolutionary ‘sweep’–for example, a single devastating disease. This is why ecologists measure—as mentioned–the health of a species not by its sameness, but by its genetic diversity, which is a well of untapped variation that might provide for a changed future. Any ‘super-race’, then, would be genetically imperiled in the manner of the closely-inbred royal families of Europe, who, according to Dr. Alan Rushton, author of Royal Maladies: Inherited Diseases in the Royal Houses of Europe, have suffered statistically more than their share of genetic disorders.
All in all, from a genetic perspective we have plenty of both moral and genetic reasons to begin studying the genetics of space colonization today. And, critically, we will have to ensure the genetic health of our domesticates and symbionts as well: we will be taking many domesticated plant and animal species off of Earth, some as food, some as companions, some as providers of such things as fibers. A good way to proceed would be to set clear milestones for what we want to know before we can leave the Earth, and work towards meeting them; otherwise, the endless, question-generating process at the heart of science might keep us here too long–and it is only a matter of time before a civilization- or species-destroying event will occur again on Earth.
In the end, if we are going to migrate from the Earth we are going to have to grapple with the probabilistic world of the genome in order to make smart—and moral—choices about the genetic health of our descendants. Regarding the important issue of preserving genetic variability, this could be maintained by ensuing gene flow among sub-populations of the interstellar craft as well as such technological means as carrying along from Earth ‘novel’ genetic material in the form of stored sperm and egg, as well as artificial mutagenesis. All of these measures are under investigation.
5. The Biology of Interstellar Voyaging: Final Comments
Over the course of several generations, as on voyages to nearby stars with propulsion systems that are beginning to seem reasonable, interstellar voyaging is entirely possible from a genetic perspective, with two provisions. First, we will have to ensure the genetic health of the colonist population as it will be under strong founder effect. Gene therapies, carrying genes from Earth in the form of stored eggs and sperm, and even the artificial induction of mutations can all be used to mitigate such effects, but at some point it will cease to be desirable to keep ‘pushing’ a human Earth genome into interstellar space. This brings us to the second proviso, and that is that natural selection will in fact return as a significant concern in human evolution, particularly when the unknowns of new planetary environments are encountered (even if they are surveyed by reconnaissance vehicles first).
We should note that, in the currently-considered timelines and populations, according to what we know about human biology it is unlikely that humanity will undergo speciation in less than a few thousand years (Figure 1, lower right).
These lessons remind us that adaptation is a continual process of the adjustment of the genome to environmental conditions. In non-humans that evolve reactively, with no conscious effort, this equilibriating process is slow and uncentralized and results in many extinctions over time. In humanity, consciousness can be used to help proactively shape our evolution, but we must remember that the only way to stop evolution is by extinction. We should accept and learn from the fact that if things live, they evolve and adapt. We should plan our adaptation to space as students of evolution. We must internalize the truth that the nature of the universe is change, not fixity, and allow this truth to condition our plans for the human colonization of space.
In the next article I will address cultural evolution, and some issues in the coevolution of DNA and culture, in biocultural evolution.
Habitable Zone Planets: Upping the Numbers
Whether we’re planning to go to the stars on a worldship or with faster transportation, the choice of targets is still evolving, and will be for some time. Indeed, events are moving almost faster than I can keep up with them. It was in early February that Courtney Dressing and David Charbonneau (Harvard-Smithsonian Center for Astrophysics) presented results of their study of 3897 dwarf stars with temperatures cooler than 4000 K, revising their temperatures downward and reducing their size by 31 percent. The scientists culled the stars from the Kepler catalog, and their revisions had the effect of lowering the size of the 95 detected planets in their data.
They went on to deduce that about 15 percent of all red dwarf stars have an Earth-sized planet in the habitable zone. [PG note: The 15% figure is a revised estimate that I’ve just learned about from Ravi kumar Kopparapu. Dressing and Charbonneau call attention to this change at the end of their paper. See citation below].
That would make the nearest Earth-like planet in the habitable zone about 13 light years away (I’m still holding out for the much closer Proxima Centauri when it comes to M-dwarfs). But Ravi kumar Kopparapu (Penn State) has revisited Dressing and Charbonneau’s work because of a key fact: The latter used habitable zone limits based on a 1993 study by James Kasting which Kopparapu believes are not valid for stars with effective temperatures less than 3700 K. The scientist was in the news almost as recently as Dressing and Charbonneau with his study of habitable zones around main sequence stars, where he presented an improved climate model developed with Kasting and other colleagues that allowed him to move the habitable zone boundaries out a bit further from their stars than they had been before (see Habitable Zones: A Moving Target for more).
Rory Barnes (University of Washington) called the work of Kopparapu and colleagues ‘the new gold standard for the habitable zone,’ and in a paper just accepted by Astrophysical Journal Letters, Kopparapu now uses his habitable zone revisions to estimate the rate of occurrence of terrestrial-sized planets in the habitable zone of M-dwarfs. His new paper was based on the Harvard team’s data and used the same calculation method. But with the new habitable zone parameters worked in, the number of habitable planets is greater than previously thought. Four out of ten of the nearest small stars should have potentially habitable planets.
Image: The graphic shows optimistic and conservative habitable zone boundaries around cool, low mass stars. The numbers indicate the names of known Kepler planet candidates. Yellow color represents candidates with less than 1.4 times Earth-radius. Green color represents planet candidates between 1.4 and 2 Earth radius. Planets with “+” are not in the habitable zone. Credit: Penn State.
Kopparapu’s new work would place the average distance to the nearest habitable planet at around 7 light years. Given that there are eight stars within 10 light years of the Sun that fit this model, we could expect to find perhaps three Earth-sized planets in the habitable zones there.
M-dwarfs are becoming increasingly important in the search for terrestrial-class worlds. Because their orbits are close to the parent star, habitable worlds in such systems would transit often and produce a stronger transit signal than a similar planet around a G-class star like the Sun. That makes M-dwarfs good Kepler targets and also suggests that future space-based missions may find this class of star an extremely useful target. Given that M-dwarfs may comprise as much as 80 percent of all stars in the galaxy, it could turn out that most life-bearing planets orbit small red stars, assuming life can indeed develop around them.
The paper is Kopparapu, “A revised estimate of the occurrence rate of terrestrial planets in the habitable zones around kepler m-dwarfs,” accepted at Astrophysical Journal Letters (preprint). The Dressing and Charbonneau paper is “The Occurrence Rate of Small Planets Around Small Stars,” to be published in The Astrophysical Journal (draft version online).
Life Aboard the Worldship
Konstantin Tsiolkovsky is the first person I know of to talk about worldships and their ramifications, which he did in an essay originally published in 1928. “The Future of Earth and Mankind” was the rocket pioneer’s take on the need for enormous ships that could reach the stars in journeys taking thousands of years. The notion percolated quickly through science fiction, and by 1940 we have Don Wilcox’s “The Voyage that Lasted 600 Years,” which ran in Amazing Stories. Wilcox, who taught creative writing at Northwestern University, imagined a ship’s captain who, though kept in hibernation, wakes up every 100 years to check on his ship, watching the gradual degeneration of the successive generations of the crew.
It’s a bleak take on worldship travel that has often been echoed in later science fiction. But would a worldship actually be this horrific, a cruise from hell that lasted entire lifetimes? See Ken MacLeod’s Learning the World: A Novel of First Contact (2005) for the worldship as a place of unlimited opportunity, and consider reading some of the many worldship stories covering the entire range of possibility that SF has produced, from Lawrence Manning’s “The Living Galaxy” (Wonder Stories, 1934) all the way forward to Gregory Benford and Larry Niven’s Bowl of Heaven (2012), which depicts what might be described as a traveling Dyson Sphere, though one with unusual design parameters.
Population Density Enroute to the Stars
Stephen Ashworth’s papers on worldships, recently published in the Journal of the British Interplanetary Society contain his speculation that reasons of economic efficiency will produce colony vessels and then worldships with high population densities, supporting thousands of inhabitants per square kilometer. What to do with a human spirit in need of open spaces? One solution is virtual entertainment in which the crew — or colonists, or whatever we choose to call them — could tap into the experience of their choice, perhaps mixing Earth-like desert, jungle and ocean adventures with customized fantasy according to whim.
When Gerard O’Neill wrote about colony worlds in The High Frontier (1976), he imagined a different outcome. Huge communities like his Island Three would not necessarily need to be space-going versions of Manhattan, but might take advantage of the abundant resources of the Solar System to provide living room in a variety of settings:
…as colonists from various countries of Earth arrive to settle the many communities in space, there will be a great variety in the ways in which land area will be used. Some immigrants may choose to arrange their land area in small villages, with single-family homes, the villages being separated by forests. Others may prefer to build small, intimate towns of high population density, to enjoy for example the color and excitement and human interaction that is so much a feature of small villages in Italy. With many new communities to choose from, the emigrants from Earth will settle in those they like best.
Of course, O’Neill was not talking about worldships but what might be their predecessors, the kind of colony worlds that would define a truly system-wide economy. Even so, we can imagine scenarios where the inhabitants of such a colony, having produced several generations of space-adapted descendants, may decide that long journeys are not a problem if their entire world goes with them. Imagine, then, a truly sylvan setting like O’Neill’s:
I would have a preference, I think, for one rather appealing arrangement: to leave the valleys free for small villages, forests and parks, to have lakes in the valley ends, at the foot of the mountains, and to have small cities rising into the foothills from the lakeshores. Even at the high-population density that might characterize an early habitat, that arrangement would seem rather pleasant: a house in a small village where life could be relaxed and children could be raised with room to play; and just five or ten miles away, a small city, with a population somewhat smaller than San Francisco’s, to which one could go for theaters, museums, and concerts.
Image: The vast interior of an O’Neill cylinder presents a more spacious view of what a worldship might become. Credit: Rick Guidice/NASA.
Requirements for the Journey
The idea that we might one day build such artificial worlds, whether or not we translate them into star-voyaging worldships, seems fantastic. And for a bit of sobering up, Ashworth’s papers are just the ticket. In “The Emergence of the Worldship (II): A Development Scenario,” he looks at the requirements for sustaining human life on a vessel whose travel time might be measured in millennia. Space precludes running through all of these, but many are obvious:
- Closed cycle production and recycling of food, water and oxygen;
- A source of electrical power independent of Sun or planets and sustainable over the course of the journey;
- Thermal management through radiation of waste heat to space;
- Full control over the microbiological environment;
- Viability of all food and ornamental species in functional reproductive health at stable population sizes over an indefinite number of generations
and so on. The list is understandably extensive, and includes factors like ensuring social and psychological stability, the need for continuing maintenance of all systems, and the long-term stability of the gene pool. None of these are factors that could be resolved in short time frames, and Ashworth believes they may emerge through a series of colonies set up at progressively greater distances from the Earth. Eventually such colonies learn how to operate independent of planetary surfaces and a manned starship — the worldship — may emerge, seen here as the logical endpoint of a lengthy period of technological and social growth. From the paper:
Viewed as a government research programme such a demonstration seems implausible, due to both its complexity and the timescales required to achieve operational maturity. Viewed as a gradual evolution of humanity into space, however, aimed not at starflight as such but at space colonisation within our own Solar System, and motivated by the large energy and material resources available for expansion, it seems likely that after a timescale on the order of centuries the preconditions listed above may be fulfilled in the course of normal economic growth.
We’ve come a long way in our conception of worldships from Robert Heinlein’s “Universe” (Astounding, 1941) and its sequel the following month, “Common Sense.” Published in book form as Orphans of the Sky, the story follows a ship’s crew that has long forgotten the nature of the voyage and no longer realizes that its world is a ship. The result is social deterioration and a rigidly stratified society of the sort Brian Aldiss explored to much greater effect in his 1956 novel Non-Stop. One can only imagine how the inhabitants of a worldship structured more or less along O’Neill lines might fare over the course of a similar journey, and we can assume that science fiction isn’t yet through exploring the ramifications of this intriguing concept.
The citation for Stephen Ashworth’s paper is “The Emergence of the Worldship (II): A Development Scenario,” JBIS Vol. 65, No. 4,5 (2012), pp. 155-175.
Space Habitats and Nearby Resources
If humans go out into the Solar System and beyond drawing on the resources they find along the way, they don’t necessarily have to do it on worldships of the kind we talked about yesterday. But it’s a reasonable assumption that creating large space habitats would make engineering projects in deep space easier to implement, housing workers and providing a base for operations. Ken Roy presented ideas about habitats in the Kuiper Belt at Huntsville, including the possibility of a large colony being created inside objects like Pluto. If we choose to go that route, we’ll have the kind of space expertise to create artificial objects similar to worldships to help ourselves along.
Of course, we hardly need to limit ourselves to the Kuiper Belt for this kind of thinking. Whatever the design of the ships we use, we can also consider expansion into the vast cometary resources of the Oort Cloud and any other objects that may lurk there, including so-called rogue planets. For years we’ve kicked around the idea of a possible brown dwarf closer than Proxima Centauri, and it seemed the WISE mission was putting the idea to rest. But maybe not. Yesterday we got word of the closest star system found in a century, a brown dwarf binary 6.5 light years out.
Image: This diagram illustrates the locations of the star systems that are closest to the Sun. The year when each star was discovered to be a neighbor of the Sun is indicated. The binary system WISE J104915.57-531906 is the third nearest system to the Sun, and the closest one found in a century. Credit: Janella Williams, Penn State University.
The Closest Brown Dwarfs Yet
You have to go back to 1916 to find another discovery this close, and that was Barnard’s Star — E. E. Barnard was not the actual discoverer of the star, but he was the first to measure its proper motion, thus pegging it as a near neighbor. Proxima Centauri was itself discovered just the year before by Robert Innes at Union Observatory in Johannesburg. The new system, WISE J104915.57-531906, reminds us that brown dwarfs may become a potential source of raw materials that facilitates outward expansion. We also know that some brown dwarfs have planetary systems while others have disks that may indicate planet formation has begun.
The new brown dwarf binary comes from WISE data and leads to the speculation that there are other interesting small systems yet to be discovered within 10 light years or so of Earth. The discoverer, Kevin Luhman (Penn State) spotted the signs of rapid motion in the WISE images and then looked for the object in older sky surveys, finding it in the Digitized Sky Survey as well as the Two Micron All-Sky Survey and the Deep Near Infrared Survey. All of this led the astronomer to work out the distance of the binary by parallax, with follow-up from the Gemini South telescope.
This exciting new find gives us a sense of how much we may yet discover. Says Luhman: “There are billions of infrared points of light across the sky, and the mystery is which one — if any of them — could be a star that is very close to our solar system.”
Meanwhile, Closer to Home…
We are a long way from operating so far from Earth, but the presence of resources between the stars may re-shape our expectations about small, fast expeditions, and lead to the slower, staged approach implicit in worldships. But whether it leads to this outcome or not, Stephen Ashworth makes a case in his recent JBIS paper for exploiting resources here in our own system that could lead to what he calls an ‘astro-civilization,’ in which the construction of large space habitats and the ability to mine the needed resources leads inevitably to a culture more at home in space than on a planet.
John Lewis considered the potential of asteroid mining some time ago in Mining the Sky, a book now on the must-read list for admirers of the asteroid mining operations like Planetary Resources and Deep Space Industries that are now emerging. Lewis believed that freely orbiting space colonies built from raw materials extracted from the asteroids would allow for huge increases in the human population. He suggested an increase by a factor of 106, though Ashworth, working the numbers anew, comes up with a more conservative figure of 2 X 1015 humans, which works out to a factor of 286,000 over the current planetary population.
Whichever figure we go with, we are talking about vast increases in living space for our species. Note, too, that both Lewis and Ashworth are talking about a population model that applies to artificial habitats constructed in the inner Solar System, meaning out to and including the main belt asteroids. Neither writer is including the moons and Trojans of the gas giants, nor the abundant resources of the Kuiper Belt and the Oort Cloud, and of course neither works into his calculations the possibility of rogue planets without stars or potentially nearby brown dwarfs.
But a space-based civilization living in the inner Solar System could be the staging area for further expansion. Ashworth thinks the extensive development of the main belt asteroids and exploitation of resources closer in would produce an energy economy throughout the Solar System in the region of 2.5 X 106 zeta joules (ZJ) per year. A zeta joule is 1021 joules, the joule being a standard unit of energy equal to 0.2389 calories.
Ashworth’s figures show that a program consuming 1 percent of the Solar System GDP annually over a ten year period would demand 2.5 X 105 ZJ. At this level of energy production, the construction of a worldship designed for 107 passengers is 28 percent of the amount, meaning that a space-based civilization at this level would find it within its means to build such a vehicle.
Any culture building vast artificial structures in space is one capable of a long-term interstellar crossing, but it is clear that a worldship is not an object that can be completed and then left alone to function. Indeed, the humans aboard the vessel will be spending a great deal of time maintaining and improving their world. From Ashworth’s paper:
These trends involve an increasing reliance upon engineered control systems in order to maintain the correct temperatures, air composition, water purity, food supply, and biological and microbiological health. Space colonies are to be understood, not as small artificial self-regulating planets, but as large self-contained buildings requiring continual maintenance. An astro-civilisation must be constantly active in order to survive: repairing space colonies, scrapping superannuated ones, recycling their materials and constructing new ones, just as an urban civilisation on Earth today is constantly renewing its built infrastructure, or as the human body is constantly renewing the cells of which it is composed.
There too is the sense of purpose which some have speculated might be lost by the numerous generations on their way to another star. For constantly improving and upgrading conditions aboard a space habitat offers a challenge upon which the survival of all aboard depend. Learning to live off the resources that proliferate throughout the outer system and beyond — and the Oort Cloud may extend out a light year or more — the worldship inhabitants will be interested in where their next resource collection will be. Habitable planets in a destination star system will be of fascinating astrobiological interest, but asteroids and small moons may be where the action is.
I haven’t yet gotten to the reflections on Gerard O’Neill and the internal conditions on a worldship that I promised yesterday, but time draws short this morning. More on all this tomorrow. Repeating the Ashworth citation, it’s “The Emergence of the Worldship (I): The Shift from Planet-Based to Space-Based Civilisation,” in JBIS 65, No. 4-5 (2012), pp. 140-154. You might also want to check out the JBIS website, which is being revised and updated.
Toward a Space-Based Civilization
The assumptions we bring to interstellar flight shape the futures we can imagine. It’s useful, then, to question those assumptions at every turn, particularly the one that says the reason we will go to the stars is to find other planets like the Earth. The thought is natural enough, and it’s built into the exoplanet enterprise, for the one thing we get excited about more than any other is the prospect of finding small, rocky worlds at about Earth’s distance from a Sun-like star. This is what Kepler is all about. From an astrobiological perspective, this focus makes sense, as we want to know whether there is other life — particularly intelligent life — in the universe.
But interstellar expansion may not involve terrestrial-class worlds at all, though they would still remain the subject of intense study. Let’s assume for a moment that a future human civilization expands to the stars in worldships that take hundreds or even thousands of years to reach their destination. The occupants of these enormous vessels might travel in a tightly packed urban environment or perhaps in a much more ‘rural’ setting with Earth-like amenities. Many of them would live out their lives in transit, without the ability to be there at journey’s end. We can only speculate what kind of social structures might emerge around the ultimate mission imperative.
Moving Beyond a Planetary Surface
Humans who have grown up in a place that has effectively become their world are going to find its norms prevail, and the idea of living on a planetary surface may hold little interest. Isaac Asimov once wrote about what he called ‘planetary chauvinism,’ which falls back on something Eric M. Jones wrote back in the 1980s. Jones believed that people traveling to another star will be far more intent on mining asteroids and the moons of planets to help them build new habitats for their own expanding population. Stephen Ashworth, a familiar figure on Centauri Dreams, writes about what he calls ‘astro-civilizations,’ space-based cultures that focus on the material and energy resources of whatever system they are in rather than planets.
Ashworth’s twin essays appear in a 2012 issue of the Journal of the British Interplanetary Society (citation below) that grew out of a worldship symposium held in 2011 at BIS headquarters in London. The entire issue is a wonderful contribution to the growing body of research on worldships and their uses. Ashworth points out that a planetary civilization like our own thinks in terms of planetary resources and, when looking toward interstellar options, naturally assumes the primary goal will be to locate new ‘Earths.’ A corollary is the assumption of rapid transport that mirrors the kind of missions used to explore our own Solar System.
Image: A worldship kilometers in length as envisioned by space artist Adrian Mann.
An astro-civilization is built on different premises, and evolves naturally enough from the space efforts of its forebears. Let me quote Ashworth on this:
“A space-based or astro-civilisation…is based on technologies which are an extension of those required on planetary surfaces, most importantly the design of structures which provide artificial gravity by rotation, and the ability to mine and process raw materials in microgravity conditions. In fact a hierarchical progression of technology development can be traced, in which each new departure depends upon all the previous ones, which leads ultimately to an astro-civilisation.
The technology development Ashworth is talking about is a natural extension of planetary methods, moving through agriculture and industrialization into a focus on the recovery of materials that have not been concentrated on a planetary surface, and on human adaptation not only to lower levels of gravity but to life in pressurized structures beginning with outposts on the Moon, Mars and out into the system. Assume sufficient expertise with microgravity environments — and this will come in due course — and the human reliance upon 1 g, and for that matter upon planetary surfaces, begins to diminish. Power sources move away from fossil fuels and gravitate toward nuclear and solar power sources usable anywhere in the galaxy.
Agriculture likewise moves from industrialized methods on planetary surfaces to hydroponic agriculture in artificial environments. Ashworth sees this as a progression taking our adaptable species from the African Savannah to the land surface of the entire Earth and on to the planets, from which we begin, as we master the wide range of new habitats becoming available, to adapt to living in space itself. He sees a continuation in the increase of population densities that took us from nomadic life to villages to cities, finally being extended into a fully urbanized existence that will flourish inside large space colonies and, eventually, worldships.
An interstellar worldship is, after all, a simple extension from a colony world that remains in orbit around our own star. That colony world, within which people can sustain their lives over generations, is itself an outgrowth of earlier technologies like the Space Station, where residence is temporary but within which new skills for adapting to space are gradually learned. Where I might disagree with Ashworth is on a point he himself raises, that the kind of habitats Gerard O’Neill envisioned didn’t assume high population densities at all, but rather an abundance of energy and resources that would make life far more comfortable than on a planet.
Tomorrow I’ll want to take a look at O’Neill’s thoughts on how human society might conduct itself in space, and then return to Ashworth’s ideas on a natural progression to worldships. For now, though, let me give you the reference on Ashworth’s paper. It’s “The Emergence of the Worldship (I): The Shift from Planet-Based to Space-Based Civilisation,” in JBIS 65, No. 4-5 (2012), pp. 140-154. As you can see, the paper puts worldships in the far broader context of humanity’s future in space as we tap new sources of energy and materials. See Astronautical Evolution for more of Ashworth’s extensive contributions to the field.
Stranger Than Fiction
Just what does it take to make a habitable world? Keith Cooper is editor of Astronomy Now, the British monthly whose first editor was the fabled Patrick Moore. An accomplished writer on astronautics and astronomy as well as a Centauri Dreams regular, Keith has recently become editor of Principium, the newsletter of the Institute for Interstellar Studies, whose third issue has just appeared. In this essay, Keith looks at our changing views of habitable zones in light of recent work, and takes us to two famous science fictional worlds where extreme climates challenge life but do not preclude it. How such worlds emerge and how life might cope on them are questions as timely as the latest exoplanet findings.
by Keith Cooper
Literally overnight, two habitable planets – tau Ceti f and HD 85512b – were rendered barren and lifeless. What was the cause of this cataclysm? A nearby supernova? Asteroid impacts? On the contrary, it was something far more mundane.
A dozen light years away, scientists at Penn State University were re-analysing the extent to which habitable zones penetrate the space around stars; in other words, at what distance liquid water could potentially exist on a planetary surface assuming an Earth-like atmosphere. The basics for habitable zone theory had been worked out in part by, among others, Penn State’s James Kasting in decades previous. Building on his work, Ravi Kumar Kopparapu and Ramses Ramirez discovered that habitable zones are found further from their stars than had been envisaged (see Habitable Zones: A Moving Target for more).
The result was bad news for our two exoplanets. Suddenly, as the habitable zone shifted imperceptibly around them, they found themselves on the wrong side of the inner habitable zone boundary, too close to their respective stars. Consequently the Planetary Habitability Laboratory at the University of Puerto Rico, Arecibo, declared them uninhabitable. Too bad for any life-forms living there.
Despite only knowing the scarcest of details about these worlds – mass, radius, density, the amount of heating from their stars – these two worlds have been cast into the obsolescence in a manner that seems shockingly final. We know so little about these planets, how can we possibly say whether they are habitable or not, especially when the only standard we are holding them to is habitability for human beings?
Key Factors for Habitability
Determination of habitability is based on worlds not necessarily having exactly the same atmosphere as Earth, but at least having water and carbon dioxide, which are abundant and vital for life, Dr Abel Mendez of the Planetary Habitability Laboratory at the University of Puerto Rico, Arecibo, tells me. “The problem of the inner edge is that once you evaporate more water you get into a runaway greenhouse effect that will make the planet lose all its water,” he says.
There are other factors that play a part though. Just because a planet is inside a habitable zone doesn’t mean it is automatically habitable. The presence of an atmosphere, water, a global magnetic field, plate tectonics and a not too heavy impact rate are all factors. For those worlds close to the edges of the habitable zone, the margins are even narrower.
For example, habitability of planets on the edge could be largely dependent upon cloud cover, says Mendez, which can increase a planet’s albedo, or reflectivity, preventing heat from reaching the surface, but if there’s no way to see clouds on a planet many light years away, how can we just write off worlds like tau Ceti f and HD 85512b? Mendez admits nothing is for certain. “The intention of the habitable zone is to determine the limits [at which habitable planets can exist from their stars], but I will not call them hard limits yet due to uncertainties such as the effects of clouds.”
A constrained, limited view of habitability that says only Earth-like conditions will do limits the number of worlds we think would look friendly. And there’s nothing wrong with this approach – we know that a planet like Earth is suitable for life, so that is what we look for, whereas we don’t know yet whether life could exist on worlds like Europa or Titan, for example. It’s not that planetary scientists are ignoring other kinds of worlds, either. “Many groups are considering the more exotic possibilities, such as tidal habitable zones, habitable planets around white dwarfs, etc,” says Mendez. “The problem is that the habitability of such conditions are harder to observe or interpret than known biosignatures, and observational astronomers need to measure things, but we will get there.”
Science Fiction at the Boundaries
Until we do, however, we’re left to speculate with our imaginations and where is that not done best but in science fiction? So let’s take a look at a few imaginary worlds that are different to Earth but which could exist on the boundaries of the habitable zone and see how they stack up in comparison. Could reality really be as strange as fiction?
One common science fiction trope is the planet with the same climatic conditions over its entire globe, for example the desert planet, the ice planet, the jungle planet. In reality things are more complex – you can have what seems like all four seasons in one day on parts of the Earth. We don’t expect the same climate at the equator as at the poles. Meanwhile the change of seasons see cycles of weather, not just on our planet but on Mars, Saturn and Titan to name but three. What then do we make of our first two science fiction choices, the desert world Arrakis from Frank Herbert’s Dune, and the ice planet Hoth from The Empire Strikes Back?
Arrakis first. Dry as a bone, it has no surface water and no precipitation. What little atmospheric moisture there is is harvested by wind-traps and the water then ferried by canal to underground reservoirs in anticipation of using it as part of the terraforming of the planet. In the novels, however, the planet is mostly sand dunes, inhabited of course by the fearsome sandworms, except for at the pole where a large slab of bedrock ringed by mountains provides a more habitable zone.
Image: A sandworm rears up out of the desert of Arrakis on the March, 1965 cover of Analog. I can never resist the chance to display the artwork of the remarkable John Schoenherr. What memories…
So how did Arrakis end up like this? In Dune, Frank Herbert described the world as having salt flats, indicating that it once had lakes. The introduction of the non-indigenous sandworms, in their protoform as ‘sand trout’, saw them sequester all of the water. Arrakis, described as orbiting the star Canopus, changed from a fertile world to a desert planet.
We have our own desert planet in the Solar System in the form of Mars, skirting the outer edge of the habitable zone. While there are no sandworms on the red planet, liquid water dried up on the surface long ago and today only exists in frozen ice caps, sub-surface ice or possibly in aquifers deep underground. Indeed, what happened to Mars’ water, and the truth behind the climatic history of the planet, are still something of mystery, but we can hazard a best guess.
We know that Mars once had running water on its surface, in the form of rivers, lakes and even a northern sea. They existed billions of years ago. Today we see only their long-lasting consequences on the Martian terrain: river channels, floodplains, a surface chemistry forever altered by the presence of liquid water. The problem with Mars is that it is small, which results in a double whammy for the planet: its diminutive size means not only a smaller gravitational field but also a greater loss of heat from its core. As Mars’ interior began to cool, its molten core began to stiffen and the magnetic dynamo contained within began to stall. These two things conspired to allow the solar wind to strip Mars’ atmosphere, including its water vapour. (The European Space Agency’s Mars Express spacecraft has actually witnessed this stripping in action, watching Mars’ atmosphere lose oxygen at a rate ten times faster that Earth’s atmosphere; see Earth’s Magnetic Field Provides Vital Protection.
Move Mars closer in to the Sun and you could easily have a warmer Arrakis-type world. So desert worlds are feasible and you don’t require sandworms to create them either. But what about the other extreme, an ice planet like Hoth?
Life on the Outer Edge
Twice in Earth’s history – 2.5 billion years ago, and about 700 million years ago – our planet completely froze over [PG note: I must have had a typo here before; see the comments below re the 700 million year figure]. Even the oceans were covered with a thick layer of ice, right down to the equator. Dubbed ‘Snowball Earth’, what causes such events is uncertain, but a significant reduction in atmospheric carbon dioxide (possibly as a result of increased silicate weathering in the warm and wet tropics as continents gathered there) or methane (destroyed through oxidisation, as a result of an oxygen influx into the atmosphere from the first oxygen-exhaling life-forms) would do the trick. Both carbon dioxide and methane are potent greenhouse gases; without them the planet cooled and must have teetered close to the edge of an abyss from which it would never recover.
Of course, it did recover. The freezing of the planet brought the carbon-silicate cycle to a halt. Water vapour froze out of the atmosphere, which meant that precipitation ground to a halt. Ice covered the land so there could be no weathering and ice topped the oceans, preventing carbonates from reaching the sea floor. The way out of this predicament for the planet was that there was still an input into the carbon-silicate system, namely carbon dioxide belched out by volcanoes. Gradually the atmosphere accumulated carbon dioxide, with no rain to wash it out. Temperatures rose and the Earth thawed, but the point is that ice planets can very easily happen, particularly if a world lacks plate tectonics to provide that carbon dioxide input that acts as part of a thermal blanket for the world. If there were a ‘slushy’ belt around the equator, which doesn’t quite freeze over, then some life may be able to survive, although it’s hard to imagine what ecology could flourish on a planet like Hoth to permit a food chain with the monstrous yeti-like wampas at the top. Ironically, if methane was the primary greenhouse force in early Earth’s atmosphere, and was destroyed by oxygen, then the discovery of another snowball planet around another star could potentially be a biosignature indicating the presence of oxygen-exhaling life on that world.
Hoth was a world covered in ice. What about planets covered in water, such as Solaris in Stanislaw Lem’s novel of the same name (ignoring the fact that this fictional planet’s global ocean was actually a living entity)? According to the United States Geological Survey seventy percent of Earth’s surface is covered in water and simulations depicting planet formation suggest that planets could easily acquire much more water than Earth did; indeed, Earth is actually quite dry. Perhaps water is delivered to planets by comets and asteroids, or perhaps these water-worlds are born further out, beyond the ‘snow line’ where water-ice is prevalent, before migrating inwards to hotter climes where their ice melts. There’s even observational evidence for water-worlds – in February 2012 Hubble Space Telescope observations of the 6.5 Earth-mass world GJ 1214b, some forty light years distant, show that starlight passing through its atmosphere is being absorbed at the characteristic wavelength of water vapour, enough to contribute a large fraction of the planet’s mass.
All of these worlds – desert, ice and ocean planets – could potentially be habitable to a point; even in Earth’s own snowball periods, life persevered. However their occurrence was before the arrival of complex life and it is doubtful such life would have survived the onset of such a catastrophic change in climate. More to the point, Mars was once wet and warm with a thicker atmosphere, even if it was only for a short while, while still existing outside of the habitable zone. Now it is a barren. On the other hand Earth was once a frozen wilderness despite being in the habitable zone, but is now resplendent with life.
While habitable zones are a starting point, it is clear they are not necessarily the final word on habitability and locating planets within their limits does not guarantee that they are going to be Earth-like, nor does it automatically correspond that planets outside of the habitable zone will be inhospitable. Furthermore, astronomers also suspect that life could exist in such exotic locales as planets in ten hour-orbits around white dwarfs, on tidally locked worlds around red dwarfs, on exomoons orbiting gas giants and even on rogue planets that wander interstellar space, kept warm by their own innate radioactivity. Surely if any of these types of planet are discovered to be habitable it will prove that reality can be far stranger than fiction.
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