Extraterrestrial: The First Sign of Intelligent Life Beyond Earth

by Avi Loeb

If evidence of extraterrestrial life appeared in our solar system, would we notice? If we are expecting the bang of gravity-defying ships on the horizon, do we risk missing the subtle sound of other arrivals? What if, for instance, that evidence was inert or defunct technology—the equivalent, perhaps, of a billion-year-old civilization’s trash?

Most of the evidence this book wrestles with was collected over eleven days, starting on October 19, 2017. That was the length of time we had to observe the first known interstellar visitor. Analysis of this data in combination with additional observations establishes our inferences about this peculiar object. Eleven days doesn’t sound like much, and there isn’t a scientist who doesn’t wish we had managed to collect more evidence, but the data we have is substantial and from it we can infer many things, all of which I detail in the pages of this book. But one inference is agreed to by everyone who has studied the data: this visitor, when compared to every other object that astronomers have ever studied, was exotic. And the hypotheses offered up to account for all of the object’s observed peculiarities are likewise exotic. I submit that the simplest explanation for these peculiarities is that the object was created by an intelligent civilization not of this Earth.

We could theorize a hypothesis starting from assumptions about ‘Oumuamua’s transit or from assumptions about its origins. If its peculiar shape and reflective properties had been the sum total of ‘Oumuamua’s distinctiveness, either theory might have been satisfactory. In that case, I would have remained curious but moved on. But I could not restrain myself from joining in this detective story for one simple reason. It concerned ‘Oumuamua’s most arresting anomaly. As I have mentioned, when ‘Oumuamua sped part of the way around the Sun, its trajectory deviated from what was expected based on the Sun’s gravitational force alone. There was no obvious explanation for why. This, for me, was the most eyebrow-raising bit of data we accumulated over the roughly two weeks we were able to observe ‘Oumuamua. This anomaly about ‘Oumuamua, along with the other pieces of information that scientists assembled, would soon lead me to form a hypothesis about the object that put me at odds with most of the scientific establishment.

Briefly ignore what the object is composed of and let’s consider more carefully its shape. No one at a breakfast table would ever confuse a cigar for a pancake. They are dramatically different. So are we really left to choose between these two outlier shapes when we envision ‘Oumuamua tumbling through space? Yet another scientist, an astrophysicist at McMaster University, went back to the evidence to see if he could provide an answer. He evaluated all the brightness models the data allowed and concluded the likelihood of ‘Oumuamua being cigar-shaped was small and the likelihood of ‘Oumuamua being disk-shaped was about 91 percent. You should keep this percentage in mind when you see the umpteenth artist’s rendering of ‘Oumuamua as a cigar-shaped rock. You should also keep it in mind when reading any explanation for a naturally occurring oblong object, such as the low probability process of melting and tidal stretching along a rare trajectory that passes very close to a star, the value of which is mooted when it comes to ‘Oumuamua, given this analysis. Is there a simpler way to achieve the required surface-to-volume ratio for a pancake-shaped object? Yes, there is. You could build a thin, sturdy piece of equipment capable of deviating due to the effects of solar-radiation pressure to exactly such specifications.

It was in this familiar setting that I typed out my plans for humanity’s first interstellar probe using lightsail technology. Two weeks later, I visited Milner’s home in Palo Alto and announced that we had a plan that fulfilled the mandates he had set out. Within our lifetimes, it was technologically feasible to send a craft to Proxima Centauri. Yuri was pleased and excited, as was Pete. After several months of extensive discussions, they decided to publicly announce the Starshot Initiative at the observatory atop One World Trade Center in New York City on April 12, 2016. This was Yuri’s Night, a commemoration of the launch of the first human into space, Yuri Gagarin, on April 12, 1961. There, on the stage with Yuri Milner, I was joined by Freeman Dyson and Stephen Hawking. The historic vision presented by the panel was recorded by TV crews and broadcast all over the world. The following morning, my wife brought our car in for an oil change, and the mechanic, who was used to seeing me perform this chore, asked where I was. Ofrit mentioned that I could not do it because of the announcement, and he replied: “This project is amazing; I read all the news reports about it.” The vision of visiting another star within our lifetimes captivated the public’s imagination in a way reminiscent of the Apollo 11 moon landing. Just seventeen months after that presentation, ‘Oumuamua was discovered by the telescopes of Pan-STARRS.

Enrico Fermi was one of the giants of twentieth-century physics. Among his accomplishments is the development of the first nuclear reactor and, as he was instrumental in the Manhattan Project and the production of the first nuclear bomb, he can claim some credit for the prompt ending of hostilities with Japan at the conclusion of World War II. Toward the end of his storied career, during a lunch with his colleagues, Fermi raised a simple, provocative question: How do we explain the paradox that, given the vastness of the universe, the probability of extraterrestrial life seems high, yet there is no certain evidence for anything but terrestrial life? If life is common in the universe, he asked, “Where is everybody?” Over the years, many answers have been formulated. One is especially arresting and especially pertinent to the unfolding mystery of ‘Oumuamua and its implications for us. In 1998, the economist Robin Hanson published an essay titled “The Great Filter—Are We Almost Past It?” Perhaps the answer to Fermi’s paradox was, Hanson argued, that throughout the universe a civilization’s own technological advancement overwhelmingly predicts its destruction. The very moment when a civilization reaches our stage of technological advancement—the window where it can signal its existence to the rest of the universe and begin to send craft to other stars—is also the moment when its technological maturity becomes sufficient for its own destruction, whether through climate change o...

Of this I am certain: The tenuous threads connecting humanity’s Earth-bound civilization as it exists today, and the promise of humanity’s possible interstellar civilization as it might exist tomorrow, will not be upheld by exercising conservative caution. In the words of Rebbe Nachman of Breslov, “The whole world is nothing but a very narrow bridge, and the key is not to be fearful at all.”

In 1939, as the world was collapsing around him, Churchill penned an article titled “Are We Alone in Space?” He never published it; the confluence of events that would bring him to the apex of his political influence would also sweep this essay aside and bury it for decades. A war was fought and won before Churchill, once more out of political fashion in the United Kingdom, revisited his article. In the 1950s, he gave it the more accurate title “Are We Alone in the Universe?” But it still sat unpublished at the time of Churchill’s death, and it entered the U.S. National Churchill Museum archives unknown and uncommented upon until its discovery in 2016. It is a shame that Churchill’s unusual essay was never published, for it contains ideas that were well ahead of their time and a sense of perspective that was sorely needed then, as it is now. Churchill evinced the modesty of a generalist as he wondered how unique the Sun and our planetary system might be, writing, “I am not sufficiently conceited to think that my sun is the only one with a family of planets.” He was also astute. Decades before the discovery of exoplanets, Churchill concluded it was reasonable to believe that a large number of planets existed “at the proper distance from their parent sun to maintain a suitable temperature,” had both water and atmosphere, and so could potentially support life. Indeed, given the expanse of space and the number of suns in it, he wrote, “The odds are enormous that there must be ...

But Struve’s paper was ignored, as was his proposal to search for close-in Jupiters. The scholars that sat on the time-allocation committees for major telescopes asserted that it was commonly understood why Jupiter lies as far from the Sun as it does, and they saw no reason to waste telescope time in searching for exo-Jupiters that were much closer to their host star. Their prejudice slowed down scientific progress by decades.

The abundance of exoplanets upon which we could fix our observational equipment reminds me of a common Jewish tradition during Passover Seder: the hiding of a piece of matzoh, called the afikomen. The task for the children of the household is to find it, and whoever is successful receives a reward. What I learned as a child—and what I am mindful of now as an adult in the nascent field of astro-archaeology—is that the question “Where to look?” trumps the question “What exactly are we looking for?” And my sisters and I also quickly learned that the best places to start looking were the places where the afikomen had been hidden in the past. Today, this same strategy is guiding the search for extraterrestrial life. Most of our telescopes and observational instruments are seeking evidence of life on rocky planets with features—most crucially liquid water—that are consistent with the one place we know life exists: Earth. But is this all we can do? Is there anywhere else we might look, even if we restrict ourselves to the orbits of alien stars?

On September 14, 2020, scientists on Earth announced the first report of a possible biosignature in the atmosphere of another planet. This new potential evidence of extraterrestrial life had not been discovered near some far-off star. Rather, much like ‘Oumuamua, it had been found right next to Earth, in our own solar system. A team led by Jane Greaves from Cardiff University in the UK had tentatively discovered a chemical compound called phosphine (PH₃) in the clouds of our neighboring planet, Venus. Searching for its spectral fingerprint in absorption of light at millimeter wavelengths, they had detected signs of the gas at an elevation of about 35 miles above the planet’s surface. The surface of Venus is currently too hot for liquid water to exist there, and so its rocky terrain is, as far as we know, inhospitable to life. But at such a height, the temperature and pressure resemble the conditions in Earth’s lower atmosphere—raising the distinct possibility that microbes could live inside liquid droplets suspended in the atmosphere of Venus. On Earth, phosphine is a product of life. And as of this writing, no alternative chemical pathways have been identified for producing phosphine at the detected levels in the Venusian atmosphere. This potential discovery galvanized the astronomy community much as did the sighting of ‘Oumuamua almost exactly three years earlier. As then, a flurry of calculations was inspired in my research group by the initial announcement. For instanc...