On a recent afternoon in February, as the midday doldrums began to set in, I did something I rarely do: I logged into Twitter. There, amid the tide of political mudslinging, non sequiturs, and navel gazing, I came across a surprisingly tense science scrap.  

A new book titled Extraterrestrial: The First Sign of Intelligent Life by Harvard astronomy professor Avi Loeb had just been published. It expands on an article that Loeb published back in 2017 with colleague Shmuel Bialy in The Astrophysical Journal Letters. The main subject is a strange, cigar-shaped object that entered our solar system in 2017 before careening at an estimated 200,000 miles per hour back into space. Scientists from the University of Hawai'i who first detected the object dubbed it ‘Oumuamua, a Hawaiian word that translates to “scout,” or “messenger from afar.” 

Most scientists speculated that ‘Oumuamua was simply an oblong asteroid ejected from a distant star system and brought into our midst by the sun’s gravitational pull. Loeb, however, had other ideas. He suggested that ‘Oumuamua’s strange shape and highly elliptical orbital trajectory—as well as the fact that it accelerated ever so slightly after passing around the sun—meant that it could not have been a comet or an asteroid. 

“If radiation pressure is the accelerating force,” Loeb wrote in the 2017 article, “then ‘Oumuamua represents a new class of thin interstellar material, either produced naturally, through a yet unknown process in the ISM or in proto-planetary disks, or of an artificial origin.” What he meant by “artificial origin” would become clear a few paragraphs later: “[O]ne possibility is that ‘Oumuamua is a lightsail, floating in interstellar space as a [piece of] debris from … advanced technological equipment.” A “more exotic scenario,” he continued, is that ‘Oumuamua may be a fully operational probe sent intentionally to Earth.”

These are not the sorts of conclusions you read in scientific papers every day—and the astronomical community took note. “Assertions that ‘Oumuamua may be artificial are not justified when the wide body of current knowledge about solar system minor bodies and planetary formation is considered,” concluded a paper published in Nature by researchers from the International Space Science Institute. The language is measured, but the conclusion is unequivocal: Loeb is wrong.  

The publication of Extraterrestrial has rekindled the hostility touched off after the publication of Loeb and Bialy’s article. Much of the angst, amplified by social media, has taken on a personal tone—namely a sense that Loeb, who occupies one of astronomy’s most prestigious posts, is intentionally stirring controversy to monopolize media attention that should rightfully be focused on serious research. Astrophysicist and science writer Ethan Siegel tweeted, “Harvard astronomer Avi Loeb has been telling everyone and their mother that 'Oumuamua was likely aliens. It wasn't.” (In a follow-up tweet, Siegel accused Loeb of “ruining science.”) Like ‘Oumuamua itself, the jibes seemed to accelerate, as each commenter attempted to outdo the previous one. 

The search for life in the cosmos is a foundational principle and motivating force of much of astronomical research. But it is a field of study riven by two hard-to-reconcile ideas: Based on the universe’s tremendous size and age—along with our understanding of how life began from basic building blocks here on Earth—it seems safe to assume that life exists, or has existed, beyond the confines of our planet. But the universe’s great size and age also mean that finding evidence of that life will be an extraordinarily difficult, if not impossible, task.  

For these reasons, the search for extraterrestrial life has become a kind of third rail of scientific discourse—even taboo among many researchers. And yet, some are willing to take the risk. How then do these scientists carry out their work—reconciling the idea that life is very likely to be out there but that it will be very, very hard to find?      

Extraordinary claims, as the old cliché goes, require extraordinary proof. To date, many of the most compelling cases of extraterrestrial life have been debunked, explained away by natural phenomena. Take the case of Tabby’s Star, in the constellation Cygnus. Discovered in 2015 by citizen planet hunters, the star exhibited a strange periodic dimming and brightening. This odd behavior, according to Jason Wright, a researcher from Pennsylvania State University, might be evidence of “Dyson spheres,” swarms of massive solar arrays built in orbit around the star to harvest solar energy. Follow-up observations, however, revealed that the star’s flickering is likely attributable to a far simpler explanation—dust. 

Some researchers who study the possibility of life beyond the surface of Earth prefer to do their speculating in terms of probabilities. Last year, for example, a group of scientists from the University of Nottingham used a calculation known as the Drake Equation—which takes into consideration a number of factors, including star formation rates, the numbers of stars with planets capable of harboring life, and the average lifespan of an intelligent civilization—to estimate that there should be 36 civilizations in the Milky Way capable of communicating with us at this very moment. This, of course, says nothing about where or how far away the planets are upon which these alien societies might exist.

Instead of trying to come up with a theoretical number of how many intelligent civilizations might be out there, others have set out to determine the probability of intelligent life emerging from the raw material of the cosmos. David Kipping, an astronomer at Columbia University, used a sophisticated statistical method called “Bayesian analysis” in an effort to determine the likelihood of life spontaneously emerging (a process called abiogenesis) on an Earth-like planet elsewhere in the Milky Way. By examining a host of factors, Kipping came to the less eye-popping but no less profound conclusion that life beyond Earth is probably common but that intelligence is rare. The trouble, of course, is that the power of statistics resides in large sample sizes. And as far as confirmed life-bearing planets go, we have a pitifully small sample size of one.

Which brings me back to Loeb and his unconventional ideas about ‘Oumuamua. In an interview with Nautilus, Loeb responded to his critics that the idea of the existence of extraterrestrial life is no more speculative than, say, the idea of dark matter—something that mathematical models suggest exists even though we have never observed it directly.  

It seems that the search for extraterrestrial life requires a balancing act of scientific rigor and creative thinking of the highest order. These are not antithetical but reinforcing concepts. How is one to deduce the average lifespan of an intelligent civilization without first imagining an alien city? Do these distant metroplexes have rush-hour traffic and drive-through coffee shops? Are real estate values through the stratosphere? 

It is this tension between creativity and rigor that drives the whole enterprise forward. Scientists like Loeb should not fear to pose explanations for unknown phenomena that defy conventional wisdom; but they should also not be outraged when those ideas are eventually refuted or replaced by more practical explanations. 

It’s nothing personal—it’s just science.