Suppose we woke up one morning to learn that scientists had made contact with an extraterrestrial civilization.
What would that change? On the one hand, we would still be bounded by our terrestrial situation: a species exhilarated by technological change, but buffeted by tribalism and struggling for resources on an increasingly bruised planet. On the other hand, we would know that we were not necessarily the pinnacle of creation, that we had kinship among the stars, and that we were not truly alone.
The question of whether or not we are alone in the universe is one of the biggest and most profound that science can address. Discovering remote examples of life would affect our self-image, and continue our sequential displacement from a central position in the scheme of creation that started with the Copernican revolution.
Astronomers are sanguine that life beyond Earth will be detected. Among my colleagues, pulses are quickening because we think it may happen within the next decade. What is the source of this optimism?
The universe is highly habitable. The major biogenic elements—carbon, nitrogen, oxygen, sulfur, and phosphorus—are routinely created within stars and then ejected into interstellar space, where they become part of the next generation of stars and planets. Although heavy elements are thousands of times less abundant than the primary constituents of the universe, hydrogen and helium, when a star forms these heavy elements concentrate and cool at the periphery of the nebula. Models suggest that biochemistry could have begun within a billion years of the Big Bang. This means natural selection, or something like it, could have been operating for three times longer than the age of the Earth. As the universe ages, and more carbon is produced, it gets more hospitable for life.
A second reason scientists are bullish about the prospects of life elsewhere is the fact that simple cells formed from primitive molecular ingredients in the first 10 percent of the history of this planet. Biology grips the Earth like a fever. Microbes have radiated into every conceivable ecological niche, including many that required adaptation to extremes of temperature, aridity, toxicity, pH, and pressure. Life doesn’t necessarily need a star and it might not even need a planet.
In the past few years the burgeoning haul of exoplanets has let astronomers project the number of habitable worlds in the Milky Way. The Kepler satellite has begun to find Earth-like planets in the habitable zones of their stars. We anticipate roughly a hundred million habitable worlds in the galaxy, and ten times more habitable “spots,” or moons of giant planets far from their stars where local energy is provided by tidal or geological heating. This assumes a conservative definition of habitability based on availability of liquid water.
The principal of mediocrity implies that the full census of habitable worlds among 100 billion galaxies in the observable universe is ten billion billion. It beggars belief that all these Petri dishes are sterile. Yet logically we have to accept the possibility that biology is exceptionally rare.
In our back yard, there’s solid evidence that Mars was habitable and hosted standing bodies of water several billion years ago. Even today there is likely to be water in aquifers that erupts episodically to the surface, and so there may be extant life in those locations. Europa and a number of moons in the outer Solar System have interior water, and Titan could host an alternative mode of biochemistry based on ethane and methane.
Taking the next step—proving biology not just habitability—will be difficult. Current and planned missions to Mars can only scratch its surface and life-finding missions to the outer Solar System will cost billions of dollars and are decades away. Astronomers are perfecting the techniques needed to detect biomarkers: the chemical imprints of life in an exoplanet atmosphere. This requires the feeble reflected light of the exoplanet to be separated from the billion times stronger light of its star and dispersed into a spectrum. The archetypal biomarker is oxygen since it was created on Earth by microbes and in the absence of life it would disappear in less than ten million years. This experiment can probably be carried out within the next decade.
What if it succeeds? The discovery would generate front page headlines and reverberate around the scientific community for some time. It would allow us to conclude that the events that took place on our planet weren’t a fluke or unique. Yet knowing that we live in a universe sprinkled with microbial life is unlikely to alleviate our loneliness.
In considering true cosmic companionship we enter a landscape littered with untested assumptions and anthropocentric thinking.
Frank Drake, eponymous creator of the equation that estimates the number of cosmic pen pals, modestly described it as a container for ignorance. We have no idea how often life starts given a habitable environment, no idea how often biology develops something we would recognize as intelligence, no idea how often advanced organisms harness technology, and no idea of the longevity of creatures with these advanced capabilities. The tools used to communicate with putative aliens—radio telescopes and powerful lasers—are likely to be fleeting technologies, and they can’t be wielded by the intelligent creatures who share our planet but who lack opposable thumbs, like orcas and cephalopods. We look outward, but really we’re looking in a mirror.
Undaunted, SETI researchers (who admit that the Search for Extraterrestrial Intelligence is more accurately the Search for Extraterrestrial Technology) haven’t folded up their tents, even after six decades of the “Great Silence.” They note that within a decade or so we’ll be able to detect analogs of our radio transmitters and pulsed lasers operating on planets around the nearest hundred million stars.
If the silence persists, they contend, it really will mean we’re operationally alone, that the transition from microbial to intelligent, technological life is incredibly rare. Or will it?
Absence of evidence is not evidence of absence. There are many reasons why advanced civilizations might exist without us being aware of them. To communicate requires motivation and means. To be comprehended requires commonality of culture and language. This is a high bar; after all, we can’t talk to animals that share 99% of our DNA. What is the likelihood that we could communicate with aliens of unknown function and form? Humans are still primitive, huddling as we do on the knife edge between the heat of our star and the absolute cold of deep space. We’ve barely ventured beyond our planet. Aliens might be to us as we are to bacteria.
We’re trapped in the box of terrestrial biology and the human condition. It might pay to jump out of the box. Some researchers have argued that we should move beyond strategies based on communication and “contact” and look for aliens by their artifacts. Freeman Dyson speculated that advanced alien civilizations might harness the total energy of their star by building a sphere around it, which would radiate distinctively at infrared wavelengths. They might become post-biological, suggesting that we should look for astro-engineering on an interstellar or even a galactic scale. Conventional SETI might be doomed to fail because it’s unduly timid. Astronomers have begun to act on these suggestions by data-mining in current large surveys.
In anticipation of the outcome, we’re not immune from a century of popular culture, dating back to Percival Lowell and H.G. Wells. Science fiction has primed us to expect a zoo of aliens out there. They’re roaming the galaxy, colonizing worlds, and subjugating their peoples. Yet they have music and arts and senses of humor too. They’re strange, yet strangely like us.
Even as we enjoy the tropes and the exotic imagery of science fiction, we can recognize the implied morality tales. Aliens might be allies but they also might be enemies. They can help us but they can also destroy us. They are metaphors for salvation and retribution. In movies like Steven Spielberg’s “ET” and “Close Encounters of the Third Kind,” the Christian iconography is literal. These alien myths are mesmerizing, but surely hollow.
This Big Question has an answer. Our best bet is to follow our science and be patient, leading to another question: can we handle the answer?
- If it turns out that life elsewhere is extremely rare, what would that imply about the history of the Earth and our place in the universe?
- Would the discovery of microbial life be less meaningful or profound than the discovery of intelligent life, and if so, why?
- Is it possible for us to talk about the nature of advanced forms of life beyond Earth without falling prey to anthropocentric thinking?
- Is the “great silence” meaningful? How many reasons could there be for us not to have any evidence of intelligent civilizations even if they actually exist?
- How strange could biology elsewhere be, and might it be unrecognizable? Related to that, how likely is post-biological evolution, where life takes mechanical or computational forms?
- If advanced forms of intelligence and civilization exist elsewhere, are we prepared for that information and how would it affect human culture?
The question “Are We Alone?” generated diverse responses and feedback, showing that in the absence of data, speculation can run in many directions. There was a tension among readers between the perspective that “aliens” will be somewhat like us, and have something to teach us, and the view of others who suspected that alien life would be unrecognizable.
Science cannot assign probabilities to these very different scenarios, and of course in a universe littered with biology, both may be true. The presence of a universal chemical toolkit for life does not prescribe the function and form of any advanced organisms that might evolve. In evolutionary biology, the arguments over the primary of convergence or contingency have continued for decades, unresolved. We are surrounded by a cosmic experiment where hundreds of millions of Earth-like planets with all the ingredients for biology have been basking in the energy from their stars, in some cases for longer than age of the Solar System. Even if microbial life exists on some or many of those planets, we have no way of estimating how many of those sites, if any, host complex life where information processing takes place in an organ like a brain as well as in the core genetic material. Since it’s possible to have intelligence without technology, as orcas and octopuses and elephants on this planet demonstrate, SETI’s “Great Silence” cannot be used to argue that intelligent life elsewhere does not exist.
I didn’t mention it in the essay but there is a “timing argument” made that since we have only just gained the capacity to travel and communicate in space, any civilization we discover or encounter will likely be substantially more advanced. There are several implications. We have been anatomically modern for nearly 40,000 years, but modern astronomy and space travel are less than a century old. We search for alien life with technologies that are rapidly evolving, and this frames what we can detect. Perhaps in a century we will be effortlessly generating gravity waves and consider this a superior strategy for SETI. Also, if an alien civilization had visited the Earth at most times in the past forty million years and looked for the brainiest creatures, they would have found them in the oceans, the ancestors of toothed whales and dolphins. Our ascendance in terms of brain capacity is fairly recent. All of these timescales are dwarfed by the time available for biological evolution beyond the Earth. A similar, but weaker, argument can be made that there have been 11 billion years for biological evolution to play out on Earth-like planets so we are also unlikely to be the first civilization to reach this level of technology. One reader alluded to this with the comment that we might be the creations of a more advanced, God-like, civilization.
Another thread of the discussion alluded not to life elsewhere, but to life on Earth, and some skepticism was expressed that the remarkable complexity of a biological organism could emerge naturally from primitive chemical components. This perspective leads into arguments based on design and teleology. Since the essay was outward-looking in its discussion of biology, a few additional comments are useful.
The original argument from design, formulated by William Paley nearly two hundred years ago, was undermined by Darwin’s theory of natural selection and it has been discredited by modern molecular biology, which provides a detailed explanation for the evolution of complexity in organisms. However, there is still a large gap in our understanding: the initial formation of genetic material and cells about four billion years ago. Since evidence of biogenesis has been mostly erased by our geologically active planet, this is historical science. But painstaking lab work has been able to show how many of the major components of a cell—a permeable enclosure, RNA fragments, and surfaces for growing complexity—can arise naturally in water. The research is still in its early days so it is premature to leap to a design premise when natural processes acting over large spans of time are likely to provide a satisfactory explanation.
The results of the discussion and the wide open conceptual landscape of astrobiology suggest several new Big Questions:
1. Can we prove that we are not the creations of a superior alien civilization or a God-like entity?
2. Might some forms of advanced intelligence be unrecognizable?
3. Given the lack of direct, physical evidence, will we be able to prove that biology arose naturally on the Earth four billion years ago?