Physicist Enrico Fermi famously asked the question "Where are they?" to express his surprise over the absence of any signs for the existence of other intelligent civilizations in the Milky Way Galaxy. Although many potential resolutions to this so-called “Fermi paradox” have been suggested over the years, there is still no consensus on which one, if any, is correct. The question of whether we are alone in the Milky Way (or in the universe at large) remains, however, one of the most intriguing questions in science in general, and in astronomy in particular.
Given the enormous uncertainties involved with the emergence, evolution, and survivability of any extrasolar life (if it exists), we shall attempt to briefly identify the most generic, remotely-detectable signatures of alien life (both simple and intelligent), and to examine the expected effectiveness of various search strategies. This topic has become particularly timely, because observations (primarily with the Kepler space telescope) have shown that the Milky Way contains no fewer than a billion Earth-size planets orbiting Sun-like (or smaller) stars in the “Goldilocks” region that allows for liquid water to exist on the planet’s surface (the so-called habitable zone). Furthermore, the search for extraterrestrial intelligent life has recently received a significant boost in the form of “Breakthrough Listen”—a $100-million decade-long project aimed at searching for non-natural transmissions in the electromagnetic bandwidth from 100 megahertz to 50 gigahertz.
Simple life appeared on Earth almost as soon as the planet cooled sufficiently to support water-based organisms. To be detectable from a distance, however, life has to evolve to the point where it dominates the planetary surface chemistry and has significantly changed the atmosphere, creating chemical “biosignatures” that can in principle be detected remotely. For instance, Earth itself would probably not have been detected as a life-bearing planet during the first two billion years of its existence. Concerning the evolution of intelligent life, the main open questions include:
—What are the geochemical constraints on the evolution of complex life?
—What are the timescales that those constraints dictate?
—Are there evolutionary "filters" or bottlenecks that make it extremely hard to make the transition to intelligence? On Earth, for example, it took about three billion years for the most basic multi-cellular life forms to appear. It took four and a half billion years (and a series of contingencies such as plate tectonics and asteroid impacts) to reach even the most rudimentary capability of interstellar communication (That is, via radio reception and transmission). These considerations demonstrate that it is important to first establish whether planetary systems that are older than the solar system are common in the Milky Way.
The current age of the solar system is about half that of the our galaxy's disk and also half of the sun’s predicted lifetime. We therefore expect that roughly one half of the stars in our galactic disk are older than the sun. A recent study that examined planet formation history concluded that the solar system formed close to the median epoch for giant planet formation, and that about 80 percent of currently existing Earth-like planets may already had been formed (pdf) at the time of Earth’s formation. This gives us great leverage for probing extrasolar intelligent life.
Which detectable biosignature may be considered the most reliable for the existence of simple life (on a sufficiently old, rocky planet, in the habitable zone)? Even though no single biosignature would be absolutely compelling, an atmosphere that is very rich in oxygen (say 20 percent or more) would probably be the most promising target initially. Wheras non-biological processes (such as the splitting of carbon dioxide by intense ultraviolet radiation) can produce oxygen in a planetary atmosphere, only under rare circumstances would these create such high levels of enrichment. Only in combination with other potential biosignatures, however, such as methane, would the credibility of a life-based origin for the oxygen be significantly strengthened.
Consequently, an excellent first step in the quest for signatures of simple extrasolar life in the relatively near future would be to: search for oxygen, but try to back it up with other biosignatures. This can (in principle) be achieved with large, ground-based arrays of relatively low-cost flux collector telescopes (such as a next-generation European Extremely Large Telescope; with a collecting area the size of a few football fields), if these are equipped with very-high-dispersion spectrographs. The oxygen lines from the exoplanet’s spectrum would be slightly Doppler-shifted relative to oxygen in Earth’s atmosphere, making it relatively straightforward (although definitely not easy) to detect them. The more difficult detection of methane in the infrared would have to follow.
What would be the requirements from a space mission? We would want to be able to at least place a meaningful constraint on the rarity of extrasolar life, if such a mission happens to not detect any biosignatures. Simulations show that in the case of non-detection, to be able to make a statement such as: “remotely detectable life occurs in less than about 10 percent of Earth-like planets around sun-like stars,” would require the ability to image and characterize the atmospheres of at least three-dozen or so exoEarths. Such a yield, in turn, would necessitate a space telescope aperture exceeding about 8.5 meters in diameter. The proposed Habitable-Exoplanet Imaging Mission (HabEx), under discussion for the next decadal survey, would have to be designed at the upper limit of its currently conceived aperture to meet this particular requirement. The more ambitious nine- to 12-meter Large UV/Optical/IR (LUVOIR) type space telescope such as the proposed High Definition Space Telescope would be a natural mission candidate for the 2030s.
One would ideally like to go beyond biosignatures and seek the clearest sign of an alien technological civilization. This could be the unambiguous detection of an intelligent, non-natural signal, most notably via radio transmission, the aim of the SETI (Search for Extraterrestrial Intelligence) program. Yet there is a distinct possibility that radio communication might be considered archaic to an advanced life form. Its use might have been short-lived in most civilizations, and hence rare over large volumes of the universe. What might then be a generic signature? Energy consumption is a hallmark of an advanced civilization that appears to be virtually impossible to conceal.
The two most plausible, long-term energy sources available to an advanced technology are through commanding stellar luminosity with a construction known as a “Dyson sphere”, possibly including harvesting the starlight from many stars, not just one, or even from an entire galaxy; the other is by controlled fusion of hydrogen into heavier nuclei. In both cases, waste heat would be an inevitable outcome, producing a detectable mid-infrared (MIR) signature. Other potential signatures of advanced civilizations that have also been suggested, such as various forms of atmospheric industrial pollution, or short-lived radioactive products, are necessarily transitory. (Basically those aliens either clean up their act or destroy themselves). Infrared emission, on the other hand, seems almost unavoidable. A recent large survey by the Wide-field Infrared Survey Explorer (WISE) satellite did identify five red spiral galaxies whose combination of high MIR and low near-ultraviolet luminosities are inconsistent with simple expectations from high rates of star formation. A conventional explanation for these observations, such as the presence of large amounts of internal dust, has not been ruled out, however. Such peculiar objects deserve follow-up observations before we explore whether they might represent the signatures of galaxy-dominating species.
More pessimistically, biologically-based intelligence may constitute only a very brief phase in the evolution of complexity, followed by what futurists have dubbed the “singularity”—the dominance of artificial, inorganic intelligence. If this is indeed the case, most advanced species are likely not to be found on a planet's surface (where gravity is helpful for the emergence of biological life, but is otherwise a liability). But they probably must still be near a fuel supply, namely a star, because of energy considerations. Even if such intelligent machines were to transmit a signal, it would probably be unrecognizable and non-decodable to our relatively primitive organic brains.
This could perhaps explain the Fermi paradox. If this scenario holds true, our chances of detecting simple life via biosignatures may be far greater than those of discovering intelligent ET’s. Still, the ultimate goal of detecting the signature of an advanced intelligence, whether biological or nonbiological, remains the most intriguing option. All power to proposed projects for the 2020s such as Japan's Space Infrared Telescope for Cosmology and Astrophysics (SPICA) and NASA’s Far Infrared Surveyor.
The key point is that for the first time in human history, we are only two or three decades away from being able to actually answer the “Are we alone?” question. Because the answer may affect nothing less than our last claim for being special in the cosmos, its importance cannot be overemphasized. In any case, echoing what Giuseppe Cocconi and Philip Morrison said at the end of their seminal 1960 Nature article on searching for extraterrestrials (pdf), we shall never know unless we search! (Scientific American is part of Nature Publishing Group.)
This article is based on research conducted at the Institut d’Astrophysique de Paris.
Winston Churchill, British prime minister and one of history’s most influential statesmen, was undoubtedly a man with weighty questions on his mind. How best to save the British Empire? he must have mused. What will the postwar world look like? he surely wondered. But the legendary leader also focused his prodigious mind on less pragmatic questions. For instance: Is there life on other planets?
In fact, in 1939, Churchill penned a lengthy essay on this very topic, which was never published. Besides displaying a strong grasp of contemporary astrophysics and a scientific mind, he came to a breathtaking conclusion: We are probably not alone in the universe. The long-lost piece of Churchilliana has just floated up to the surface again, thanks to an article written by astrophysicist Mario Livio in this week's edition of the journal Nature analyzing Churchill's work.
“With hundreds of thousands of nebulae, each containing thousands of millions of suns, the odds are enormous that there must be immense numbers which possess planets whose circumstances would not render life impossible,” Churchill concluded in his essay. He wrote these words on the eve of World War II—more than half a century before exoplanets were discovered.
Until last year, Churchill's thoughts on the problem of alien life had been all but lost to history. The reason: His 11-page typed draft was never published. Sometime in the late 1950s, Churchill revised the essay while visiting the seaside villa of publisher Emery Reves, but the text still didn't see the light of day. It appears to have languished in the Reves house until Emery's wife Wendy gave it to the U.S. National Churchill Museum during the 1980s.
Last year, the museum’s new director, Timothy Riley, unearthed the essay in the museum's archives. When astrophysicist Mario Livio happened to visit the museum, Riley "thrust [the] typewritten essay" into his hands, Livio writes in Nature. Riley was eager to hear the perspective of an astrophysicist. And Livio, for his part, was floored. “Imagine my thrill that I may be the first scientist to examine this essay,” he writes in Nature.
Churchill did his homework, Livio reports. Though he probably didn't pore over peer-reviewed scientific literature, the statesman seems to have read enough, and spoke with enough top scientists—including the physicist Frederick Lindemann, his friend and later his official scientific adviser—to have had a strong grasp of the major theories and ideas of his time. But that wasn't what left the deepest impression on Livio.
“To me the most impressive part of the essay—other than the fact that he was interested in it at all, which is pretty remarkable—is really the way that he thinks,” Livio says. “He approached the problem just as a scientist today would. To answer his question 'Are we alone in the universe?' he started by defining life. Then he said, 'OK, what does life require? What are the necessary conditions for life to exist?'”
Churchill identified liquid water, for example, as a primary requirement. While he acknowledged the possibility that forms of life could exist dependent on some other liquid, he concluded that “nothing in our present knowledge entitles us to make such an assumption.”
"This is exactly what we still do today: Try to find life by following the water,” Livio says. “But next, Churchill asked 'What does it take for liquid water to be there?' And so he identified this thing that today we call the habitable zone.”
By breaking down the challenge into its component parts, Churchill ended up delving into the factors necessary to create what is now known as the “Goldilocks zone” around a star: that elusive region in which a life-sustaining planet could theoretically exist. In our own solar system, he concluded, only Mars and Venus could possibly harbor life outside of Earth. The other planets don't have the right temperatures, Churchill noted, while the Moon and asteroids lack sufficient gravity to trap gasses and sustain atmospheres.
Turning his gaze beyond our own solar system raised even more possibilities for life, at least in Churchill's mind. “The sun is merely one star in our galaxy, which contains several thousand millions of others,” he wrote. Planetary formation would be rather rare around those stars, he admitted, drawing on a then-popular theory of noted physicist and astronomer James Jeans. But what if that theory turned out to be incorrect? (In fact, it has now been disproven.)
“That's what I find really fascinating,” Livio notes. “The healthy skepticism that he displayed is remarkable.”
Churchill suggested that different planetary formation theories may mean that many such planets may exist which “will be the right size to keep on their surface water and possibly an atmosphere of some sort.” Of that group, some may also be “at the proper distance from their parent sun to maintain a suitable temperature.”
The statesman even expected that some day, “possibly even in the not very distant future,” visitors might see for themselves whether there is life on the moon, or even Mars.
But what was Winston Churchill doing penning a lengthy essay on the probability of alien life in the first place? After all, it was the eve of a war that would decide the fate of the free world, and Churchill was about to become Prime Minister of the United Kingdom.
Such an undertaking was actually quite typical for Churchill, notes Andrew Nahum, Keeper Emeritus at the Science Museum, London, because it reflects both his scientific curiosity and his recurring need to write for money. It was skill with the pen that often supported Churchill and his family's lavish lifestyle (recall that he won the 1953 Nobel Prize for Literature, with a monetary award of 175,293 Swedish Kroner worth about $275,000 today).
“One recent biography is entitled No More Champagne: Churchill And His Money,” Nahum says. “That was a phrase he put into a note to his wife about austerity measures. But he didn't know much about austerity. He liked luxury so he wrote like crazy, both books and articles that his agent circulated widely.”
That’s not to say that Churchill was simply slinging copy about aliens for a paycheck. “He was profoundly interested in the sciences and he read very widely,” notes Nahum, who curated the 2015 Science Museum exhibition “Churchill's Scientists.” Nahum relates the tale of how as Chancellor of the Exchequer, Churchill was once sent a book on quantum physics, and later admitted that it had occupied him for the better part of a day that should have been spent balancing the British budget.
He not only read scientific content voraciously, but wrote on the topic as well. In a 1924 issue of Nash's Pall Mall Magazine, Churchill anticipated the power of atomic weapons. “Might not a bomb no bigger than an orange be found to possess secret power to destroy a whole block of buildings nay, to blast a township at a stroke?” he warned. In 1932, he anticipated the rise of test-tube meat in the magazine Popular Mechanics: “Fifty years hence, we shall escape the absurdity of growing a whole chicken in order to eat the breast or the wing, by growing these parts separately in a suitable medium,” he wrote.
In 1939 he authored three essays, tackling not just extraterrestrial life but the evolution of life on Earth and the popular biology of the human body. Two were published during 1942 by the Sunday Dispatch, Nahum discovered when reading Churchill's papers at the University of Cambridge. It remains a mystery why his thoughts on alien life went unpublished.
In the rediscovered essay, Churchill admits that, because of the great distances between us and other planet-harboring stars, we may never know if his hunch that life is scattered among the vastness of the cosmos is correct. Yet even without proof, Churchill seems to have convinced himself that such a possibility was likely—perhaps by swapping his scientific mind for one more finely attuned to the human condition during the troubled 20th century.
“I, for one, am not so immensely impressed by the success we are making of our civilization here that I am prepared to think we are the only spot in this immense universe which contains living, thinking creatures,” he wrote, “or that we are the highest type of mental and physical development which has ever appeared in the vast compass of space and time.”
Seventy-five years after Churchill's bold speculations, there's still no proof that life exists on other worlds. But, as was often the case, his analysis of our own still seems prescient.
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