The following essay is being published in conjunction with the “(In)hospitable Space Conference” which explores the human dimensions of space exploration and is cosponsored between the English Department of Carnegie Mellon University, the Society for Social and Conceptual Issues in Astrobiology, and the European Astrobiology Institute.
By some accounts, humans face a stark choice. Humanity can remain on Earth, where inevitably some doomsday event will lead to our extinction. It may be a natural event, such as an asteroid impact or the death of the Sun, or it may be an event of our own making, such as thermonuclear war or malevolent AI – but sooner or later, our time on Earth will come to an end. Alternatively, we can reach for the stars. We can become a multiplanetary species, in which case, the end of life on Earth would not be the end of humanity. Our descendants would persist on other planets, perhaps populating other stars or even other galaxies. The human project could continue for billions or trillions of years
Given this portrayal, the appeal of multiplanetary existence is clear. But what is really involved in becoming multiplanetary, and should we, in fact, do it?
First, being truly multiplanetary involves more than establishing a lunar base or a research station on Mars. Those may be useful first steps, but to hedge against extinction an off-Earth settlement must be fully self-sufficient. It must be able be able to survive without any material or technical assistance from Earth. People must be able to reproduce; a healthy and genetically diverse population is required which can supply all its needs using local resources. An entire functional ecosystem of crops and oxygen-producing plants, pollinators, microbes to decompose waste and cycle nutrients, and a host of other ecological services will be needed. We are far from establishing anything like this level of self-sufficiency in any space environment.
Second, although the term “multiplanetary” points to settlements on other planets, in fact the range of space environments we might inhabit is broader. In our own solar system, some moons of Jupiter or Saturn, or some of the larger asteroids or Kuiper Belt objects may be targets for habitation. Additionally, we could construct large artificial habitats in space in which to live.
Finally, we need to think more clearly about the term “species”. On Earth presently, there is one species of human: Homo sapiens. However, in our evolutionary past we shared the planet with other species of Homo, notably the Neandertals (H. neanderthalensis), who displayed complex behaviors including tool use, cave art, body adornment and burial practices. If we establish self-sufficient human populations in space, it is likely that those populations will evolve to produce new species. As evolutionary biologist Scott Solomon suggests in his book Becoming Martian, the combination of small population size and strong selective pressures that would likely characterize a colony on Mars should spur rapid evolution. We may also supplement natural evolution by artificial selection or germ-line genetic engineering to enhance the adaptation of space colonists to their environments. What we really want to preserve is not a single “species” (Homo sapiens) but rather a lineage of descendants originating from our species. I will use the term “humanity” to encompass this idea.
We may think the notion of becoming multiplanetary is recent, but humans have long mused about the possibility of living in space. Based on advances in the science of his time, Anglican clergyman John Wilkins in 1638 wrote The Discovery of a World in the Moone arguing that the lunar surface could be a habitable environment. In 1869 the American author Edward Everett Hale wrote The Brick Moon, a novella relating the comical antics resulting in the launch of an artificial satellite carrying humans to live in orbital space. In the late 19th and early 20th century, the Russian rocket scientist Konstantin Tsiolkovsky promoted his vision of humanity spreading to the stars, declaring “Earth is the cradle of humanity, but one cannot live in a cradle forever.” American physicist Gerald K. O’Neill wrote The High Frontier, envisioning the construction of large artificial habitats in space where humans could spend their lives. Contemporary billionaires are investing in a multiplanetary future, with Jeff Bezos citing O’Neill’s vision as a motivating inspiration and Elon Musk aspiring to establish a Martian colony as a hedge against extinction.
The visions of multiplanetary existence promoted over the centuries are appealing. But to answer the question of “Should We Become a Multiplanetary Species?” takes us into the realm of ethics. I will examine the ethical issues from three different points of view.
First, let us adopt an anthropocentric point of view. What are the ethical considerations from the viewpoint of humanity? An obvious argument is that becoming multiplanetary mitigates the risk of humanity becoming extinct, and this is held to be inherently good. Longtermism is a philosophy that espouses this view. Longtermists argue that ethical decisions should be strongly influenced by benefits arising in the far future. Philosopher Toby Ord, in his book The Precipice, estimates that if humans remain solely on Earth we may expect 100 trillion (1014) future lives before human extinction. But if we inhabit the solar system or the galaxy, this may translate into a thousand trillion trillion (1027) or up to a trillion trillion trillion (1036) future lives. The vast amplification of human existence can be seen as good if those lives are happy, but what if life in space becomes brutish and miserable? Isn’t the most likely scenario one of both goodness and evil, joy and suffering? Longtermists view human extinction as a great evil, but is it necessarily so bad? If we adopt a person-regarding ethics (as advocated by philosopher Derek Parfit), then the absence of humans is not bad in itself. Perhaps eventual extinction is simply something we as a species need to accept, as we do our own individual mortality.
Still from an anthropocentric perspective, becoming multiplanetary will undoubtedly lead to technological spinoffs, and those could be beneficial to humanity (although some may also be detrimental). But, I wonder, are those technological spinoffs more advantageous than what would be achieved focusing on Earth-based projects?
Also, it might be argued that colonizing space fulfills something fundamental in the human spirit, that it is our destiny. Space is often referred to as the final frontier, however, transposing 18th and 19th century frontier attitudes from America’s westward expansion to 21st century endeavors would be a serious mistake. Becoming multiplanetary is a choice, not destiny.
Switching to the perspective of Earth and its planetary ecosystem, the Gaian point of view, there are other arguments for becoming multiplanetary. Expanding human presence in space could benefit the home planet. Migration of humans to space could reduce overpopulation on Earth and lessen demand for Earth’s resources. As O’Neill, Bezos and others argue, moving mining activity to asteroids or the Moon and placing polluting industries in space could be good for the terrestrial environment.
I think these arguments come with significant caveats. Reducing Earth’s population via migration to space would require massive numbers of launches to make a dent in the over 8 billion current inhabitants, and any reduction could easily evaporate as more births occur on Earth. Out-migration would only be an effective population control strategy if coupled with limits on fertility, which does not sound like a desirable future. The case for space leading to environmental benefits by displacing mining and other industries is stronger. A robust space economy could, in principle, reduce demand for mineral resources mined on Earth and reduce pollutant emissions. However, this is not guaranteed. It is not clear, for instance, whether metals mined from asteroids would actually reduce Earth-based mining, or just be added to the economy increasing total use. Also, space-based activities do have direct impacts on Earth’s environment, such as pollutant emissions from launches. Finally, all the proposed benefits are premised on development of a space economy and some human presence in space, but not necessarily permanent habitation. It seems that space mining and industry could rely on a combination of robotic and temporary human workers, so the Gaian point of view does not provide a strong argument for true multiplanetarity.
Some argue for humans becoming a multiplanetary species based on a cosmic point of view. Perhaps life, especially human life, or some quality associated with human life, such as intelligence or moral agency is inherently good. Then, the argument goes, a universe with more life (or intelligent beings or moral agents) spread more widely in the galaxy (or beyond) is better than a universe with life (or intelligence or moral agency) restricted to one planet. To distinguish this from the anthropocentric point of view, the claim is that the spread of humans in space is not just good for humans, but good in an absolute sense.
Chemist and astrobiologist Michael Mautner espouses a life-centered ethics in which actions that secure life are morally good, and self-extinction is the ultimate evil. The purpose of life is self-propagation, so he views it as imperative for us to spread life to the wider universe. He has focused on directed panspermia, the intentional seeding of other planets with organisms that can grow and evolve there. However, he remarks that, as intelligent creatures ourselves, we have reason to prefer the spread of intelligent species, and so he would endorse the goal of becoming multiplanetary.
The philosopher Hans Jonas in his book The Imperative of Responsibility sought to develop a future-oriented ethics, suited to technological capabilities of humanity to impact the distant future. He does not explicitly consider the possibility of becoming multiplanetary but emphasizes that we must take actions to avert human extinction. For him, human existence is imperative because humans carry the possibility of instantiating responsibility in the world, and preservation of that moral possibility is a cosmic duty.
Ethicist Brian Patrick Green similarly argues that humans are the only species known to be capable of complex morality, and that if humanity went extinct, morality would cease to exist. Following Hans Jonas, he sees this as an objectively bad outcome. Thus, he argues that rapid space settlement is necessary to avert the possibility of morality being extinguished.
All these arguments from a cosmic point of view assume that what is of value (life, intelligence, moral agency, etc.) is rare or nonexistent in the universe beyond Earth. That may be true, or it may not. Many astrobiologists would argue that conditions for at least primitive life exist on many planets throughout the galaxy. Intelligent life may be rarer, and so far, our search for it has not yielded positive results. However, there may be reasons for intelligent life to resist detection, such as the “dark forest” hypothesis explored in Liu Cixin’s sci-fi trilogy The Three-Body Problem where alien civilizations are assumed to be predominantly hostile. In any case, until we know more about the actual distribution of life, intelligence and moral agency in the universe, it is premature to conclude whether the extinction of humans would substantially alter the total.
Although these arguments are stated as being from a cosmic point of view, the conclusions seem awfully anthropocentric in nature. The universe is a better place when filled with intelligent, moral beings – thus, declares the intelligent, moral human. Are these values really objective, or are we preferentially valuing qualities we possess because they are important to us? I am not convinced that we have achieved the God’s-eye view required to decide what is good from a cosmic perspective.
Should we become a multiplanetary species? In the end, I favor a middle path. Becoming multiplanetary is morally permissible, but not obligatory. If done right, it could bring significant benefits to humans and extend the existence of humanity. But there are many ways things could go wrong, increasing human suffering and spreading environmental destruction beyond Earth. For now, I would emphasize scientific exploration involving robotic and human-crewed missions, but not permanent settlement. On the time scale of the likely persistence of humanity, a delay of decades or even centuries is trivial. Let’s understand the worlds we encounter before we inhabit and transform them.
