On October 19, 2017, a Canadian astronomer named Robert Weryk was reviewing images captured by a telescope known as Pan-STARRS1 when he noticed something strange. The telescope is situated atop Haleakalā, a ten-thousand-foot volcanic peak on the island of Maui, and it scans the sky each night, recording the results with the world’s highest-definition camera. It’s designed to hunt for “near-Earth objects,” which are mostly asteroids whose paths bring them into our planet’s astronomical neighborhood and which travel at an average velocity of some forty thousand miles an hour. The dot of light that caught Weryk’s attention was moving more than four times that speed, at almost two hundred thousand miles per hour.

Weryk alerted colleagues, who began tracking the dot from other observatories. The more they looked, the more puzzling its behavior seemed. The object was small, with an area roughly that of a city block. As it tumbled through space, its brightness varied so much—by a factor of ten—that it had to have a very odd shape. Either it was long and skinny, like a cosmic cigar, or flat and round, like a celestial pizza. Instead of swinging around the sun on an elliptical path, it was zipping away more or less in a straight line. The bright dot, astronomers concluded, was something never before seen. It was an “interstellar object”—a visitor from far beyond the solar system that was just passing through. In the dry nomenclature of the International Astronomical Union, it became known as 1I/2017 U1. More evocatively, it was dubbed ‘Oumuamua (pronounced “oh-mooah-mooah”), from the Hawaiian, meaning, roughly, “scout.”

Even interstellar objects have to obey the law of gravity, but ‘Oumuamua raced along as if propelled by an extra force. Comets get an added kick thanks to the gases they throw off, which form their signature tails. ‘Oumuamua, though, didn’t have a tail. Nor did the telescopes trained on it find evidence of any of the by-products normally associated with outgassing, like water vapor or dust.

“This is definitely an unusual object,” a video produced by NASA observed. “And, unfortunately, no more new observations of ‘Oumuamua are possible because it’s already too dim and far away.”

As astronomers pored over the data, they excluded one theory after another. ‘Oumuamua’s weird motion couldn’t be accounted for by a collision with another object, or by interactions with the solar wind, or by a phenomenon that’s known, after a nineteenth-century Polish engineer, as the Yarkovsky effect. One group of researchers decided that the best explanation was that 1I/2017 U1 was a “miniature comet” whose tail had gone undetected because of its “unusual chemical composition.” Another group argued that ‘Oumuamua was composed mostly of frozen hydrogen. This hypothesis—a variation on the mini-comet idea—had the advantage of explaining the object’s peculiar shape. By the time it reached our solar system, it had mostly melted away, like an ice cube on the sidewalk.

By far the most spectacular account of 1I/2017 U1 came from Avi Loeb, a Harvard astrophysicist. ‘Oumuamua didn’t behave as an interstellar object would be expected to, Loeb argued, because it wasn’t one. It was the handiwork of an alien civilization.

In an equation-dense paper that appeared in The Astrophysical Journal Letters a year after Weryk’s discovery, Loeb and a Harvard postdoc named Shmuel Bialy proposed that ‘Oumuamua’s “non-gravitational acceleration” was most economically explained by assuming that the object was manufactured. It might be the alien equivalent of an abandoned car, “floating in interstellar space” as “debris.” Or it might be “a fully operational probe” that had been dispatched to our solar system to reconnoitre. The second possibility, Loeb and Bialy suggested, was the more likely, since if the object was just a piece of alien junk, drifting through the galaxy, the odds of our having come across it would be absurdly low. “In contemplating the possibility of an artificial origin, we should keep in mind what Sherlock Holmes said: ‘when you have excluded the impossible, whatever remains, however improbable, must be the truth,’ ” Loeb wrote in a blog post for Scientific American.

Not surprisingly, Loeb and Bialy’s theory received a lot of attention. The story raced around the world almost at the speed of ‘Oumuamua. TV crews crowded into Loeb’s office, at the Harvard-Smithsonian Center for Astrophysics, and showed up at his house. Film companies vied to make a movie of his life. Also not surprisingly, much of the attention was unflattering.

“No, ‘Oumuamua is not an alien spaceship, and the authors of the paper insult honest scientific inquiry to even suggest it,” Paul M. Sutter, an astrophysicist at Ohio State University, wrote.

“Can we talk about how annoying it is that Avi Loeb promotes speculative theories about alien origins of ‘Oumuamua, forcing [the] rest of us to do the scientific gruntwork of walking back these rumors?” Benjamin Weiner, an astronomer at the University of Arizona, tweeted.

Far from being deterred, Loeb doubled down. Together with Thiem Hoang, a researcher at the Korea Astronomy and Space Science Institute, he blasted the frozen-hydrogen theory. In another equation-packed paper, the pair argued that it was fantastical to imagine solid hydrogen floating around outer space. And, if a frozen chunk did manage to take shape, there was no way for a block the size of ‘Oumuamua to survive an interstellar journey. “Assuming that H₂ objects could somehow form,” Hoang and Loeb wrote, “sublimation by collisional heating” would vaporize them before they had the chance to, in a manner of speaking, take off.

Loeb has now dispensed with the scientific notation and written “Extraterrestrial: The First Sign of Intelligent Life Beyond Earth” (Houghton Mifflin Harcourt). In it, he recounts the oft-told story of how Galileo was charged with heresy for asserting that Earth circled the sun. At his trial in Rome, in 1633, Galileo recanted and then, legend has it, muttered, sotto voce, “Eppur si muove” (“And yet it moves”). Loeb acknowledges that the quote is probably apocryphal; still, he maintains, it’s relevant. The astronomical establishment may wish to silence him, but it can’t explain why ‘Oumuamua strayed from the expected path. “And yet it deviated,” he observes.

In “Extraterrestrial,” Loeb lays out his reasoning as follows. The only way to make sense of ‘Oumuamua’s strange acceleration, without resorting to some sort of undetectable outgassing, is to assume that the object was propelled by solar radiation—essentially, photons bouncing off its surface. And the only way the object could be propelled by solar radiation is if it were extremely thin—no thicker than a millimetre—with a very low density and a comparatively large surface area. Such an object would function as a sail—one powered by light, rather than by wind. The natural world doesn’t produce sails; people do. Thus, Loeb writes, “ ‘Oumuamua must have been designed, built, and launched by an extraterrestrial intelligence.”

The first planet to be found circling a sunlike star was spotted in 1995 by a pair of Swiss astronomers, Michel Mayor and Didier Queloz. Its host star, 51 Pegasi, was in the constellation Pegasus, and so the planet was formally dubbed 51 Pegasi b. By a different naming convention, it became known as Dimidium.

Dimidium was the ‘Oumuamua of its day—a fantastic discovery that made headlines around the world. (For their work, Mayor and Queloz were eventually awarded a Nobel Prize.) The planet turned out to be very large, with a mass about a hundred and fifty times that of Earth. It was whipping around its star once every four days, which meant that it had to be relatively close to it and was probably very hot, with a surface temperature of as much as eighteen hundred degrees. Astronomers hadn’t thought such a large body could be found so close to its parent star and had to invent a whole new category to contain it; it became known as a “hot Jupiter.”

Mayor and Queloz had detected Dimidium by measuring its gravitational tug on 51 Pegasi. In 2009, NASA launched the Kepler space telescope, which was designed to search for exoplanets using a different method. When a planet passes in front of its star, it reduces the star’s brightness very slightly. (During the last transit of Venus, in 2012, viewers on Earth could watch a small black dot creep across the sun.) Kepler measured variations in the brightness of more than a hundred and fifty thousand stars in the vicinity of the constellations Cygnus and Lyra. By 2015, it had revealed the existence of a thousand exoplanets. By the time it stopped operating, in 2018, it had revealed sixteen hundred more.

NASA’s ultimate goal for the telescope was to work out a figure known as eta-Earth, or η⊕. This is the average number of rocky, roughly Earth-size planets that can be found orbiting an average sunlike star at a distance that might, conceivably, render them habitable. After spending two years analyzing the data from Kepler, researchers recently concluded that η⊕ has a value somewhere between .37 and .6. Since there are at least four billion sunlike stars in the Milky Way, this means that somewhere between 1.5 billion and 2.4 billion planets in our galaxy could, in theory, harbor life. No one knows what fraction of potentially habitable planets are, in fact, inhabited, but, even if the proportion is trivial, we’re still talking about millions—perhaps tens of millions—of planets in the galaxy that might be teeming with living things. At a public event a few years ago, Ellen Stofan, who at the time was NASA’s chief scientist and is now the director of the National Air and Space Museum, said that she believed “definitive evidence” of “life beyond earth” would be found sometime in the next two decades.

“It’s definitely not an ‘if,’ it’s a ‘when,’ ” Jeffrey Newmark, a NASA astrophysicist, said at the same gathering.

What will life on other planets look like, when—not if—it’s found? Arik Kershenbaum, a researcher at the University of Cambridge, takes up this question in “The Zoologist’s Guide to the Galaxy: What Animals on Earth Reveal About Aliens—and Ourselves” (Penguin Press). “It’s a popular belief that alien life is too alien to imagine,” he writes. “I don’t agree.”

Kershenbaum argues that the key to understanding cosmic zoology is natural selection. This, he maintains, is the “inevitable mechanism” by which life develops, and therefore it’s “not just restricted to the planet Earth” or even to carbon-based organisms. However alien biochemistry functions, “natural selection will be behind it.”

From this premise, Kershenbaum says, it follows that life on other planets will have evolved, if not along the same lines as life on this planet, then at least along lines that are generally recognizable. On Earth, for instance, where the atmosphere is mostly made of nitrogen and oxygen, feathers are a useful feature. On a planet where clouds are made of ammonia, feathers probably wouldn’t emerge, “but we should not be surprised to find the same functions (i.e. flight) that we observe here.” Similarly, Kershenbaum writes, alien organisms are apt to evolve some form of land-based locomotion—“Life on alien planets is very likely to have legs”—as well as some form of reproduction analogous to sex and some way of exchanging information: “Aliens in the dark will click like bats and dolphins, and aliens in the clear skies will flash their colours at each other.”

Assuming that there is, in fact, alien life out there, most of it seems likely to be microscopic. “We are not talking about little green men” is how Stofan put it when she said we were soon going to find it. “We are talking about little microbes.” But Kershenbaum, who studies animal communication, jumps straight to complex organisms, which propels him pretty quickly into Loebian territory.

On Earth, many animals possess what we would broadly refer to as “intelligence.” Kershenbaum argues that, given the advantages that this quality confers, natural selection all across the galaxy will favor its emergence, in which case there should be loads of life-forms out there that are as smart as we are, and some that are a whole lot smarter. This, in his view, opens up quite a can of interstellar worms. Are we going to accord aliens “human rights”? Will they accord us whatever rights, if any, they grant their little green (or silver or blue) brethren? Such questions, Kershenbaum acknowledges, are difficult to answer in advance, “without any evidence of what kind of legal system or system of ethics the aliens themselves might have.”

As disconcerting as encountering intelligent aliens would be, the fact that we haven’t yet heard from any is, arguably, even more so. Why this is the case is a question that’s become known as the Fermi paradox.

One day in 1950, while lunching at Los Alamos National Laboratory, the physicist Enrico Fermi turned to some colleagues and asked, “Where are they?” (At least, this is how one version of the story goes; according to another version, he asked, “But where is everybody?”) This was decades before Pan-STARRS1 and the Kepler mission. Still, Fermi reckoned that Earth was a fairly typical planet revolving around a fairly typical star. There ought, he reasoned, to be civilizations out there far older and more advanced than our own, some of which should have already mastered interstellar travel. Yet, strangely enough, no one had shown up.

Much human intelligence has since been devoted to grappling with Fermi’s question. In the nineteen-sixties, an astronomer named Frank Drake came up with the eponymous Drake equation, which offers a way to estimate—or, if you prefer, guesstimate—how many alien cultures exist with which we might hope to communicate. Key terms in the equation include: how many potentially habitable planets are out there, what fraction of life-hosting planets will develop sophisticated technology, and how long technologically sophisticated civilizations endure. As the list of potentially habitable planets has grown, the “Where are they?” mystery has only deepened. At a workshop on the subject held in Paris in 2019, a French researcher named Jean-Pierre Rospars proposed that aliens haven’t reached out to us because they’re keeping Earth under a “galactic quarantine.” They realize, he said, that “it would be culturally disruptive for us to learn about them.”

Loeb proposes that Fermi may be the answer to his own paradox. Humanity has been capable of communicating with other planets, via radio wave, for only the past hundred years or so. Seventy-five years ago, Fermi and his colleagues on the Manhattan Project invented the atomic bomb, and a few years after that Edward Teller, one of Fermi’s companions at the lunch table at Los Alamos, came up with the design for a hydrogen bomb. Thus, not long after humanity became capable of signalling to other planets, it also became capable of wiping itself out. Since the invention of nuclear weapons, we’ve continued to come up with new ways to do ourselves in; these include unchecked climate change and manufactured microbes.

“It is quite conceivable that if we are not careful, our civilization’s next few centuries will be its last,” Loeb warns. Alien civilizations “with the technological prowess to explore the universe” are, he infers, similarly “vulnerable to annihilation by self-inflicted wounds.” Perhaps the reason no one has shown up is that there’s no one left to make the trip. This would mean that ‘Oumuamua was the cosmic equivalent of a potsherd—the product of a culture now dead.

A message an earthling might take from this (admittedly highly speculative) train of thought is: be wary of new technologies. Loeb, for his part, draws the opposite conclusion. He thinks humanity ought to be working to produce precisely the kind of photon-powered vessel that he imagines ‘Oumuamua to be. To this end, he’s an adviser on a project called the Breakthrough Starshot Initiative, whose stated aim is to “demonstrate proof of concept for ultra-fast light-driven nanocrafts.” In the longer term, the group hopes to “lay the foundations” for a launch to Alpha Centauri, the star system closest to Earth, which is about twenty-five trillion miles away. (The initiative has funding from Yuri Milner, a Russian-Israeli billionaire, and counts among its board members Mark Zuckerberg.)

Loeb also looks forward to the day when we’ll be able to “produce synthetic life in our laboratories.” From there, he imagines “Gutenberg DNA printers” that could be “distributed to make copies of the human genome out of raw materials on the surface of other planets.” By seeding the galaxy with our genetic material, we could, he suggests, hedge our bets against annihilation. We could also run a great evolutionary experiment, one that might lead to outcomes far more wondrous than seen so far. “There is no reason to expect that terrestrial life, which emerged under random circumstances on Earth, was optimal,” Loeb writes.

When I was a kid, one of my favorite books was “Chariots of the Gods?,” by Erich von Däniken. The premise of the book, which was spun off into the TV documentary “In Search of Ancient Astronauts,” narrated by Rod Serling, was that Fermi’s question had long ago been answered. “They” had already been here. Von Däniken, a Swiss hotel manager turned author who for some reason in the documentary was described as a German professor, argued that aliens had landed on Earth sometime in the misty past. Traces of their visits were recorded in legends and also in artifacts like the Nazca Lines, in southern Peru. Why had people created these oversized images if not to signal to beings in the air?

I figured that von Däniken would be interested in the first official interstellar object, and so I got in touch with him. Now eighty-five, he lives near Interlaken, not far from a theme park he designed, which was originally called Mystery Park and then later, after a series of financial mishaps, rebranded as Jungfrau Park. The park boasts seven pavilions, one shaped like a pyramid, another like an Aztec temple.

Von Däniken told me that he had, indeed, been following the controversy over ‘Oumuamua. He tended to side with Loeb, who, he thought, was very brave.

“He needs courage and obviously he had courage,” he said. “No scientist wants to be ridiculed, and whenever they deal with U.F.O.s or extraterrestrials, they are ridiculed by the media.” But, he predicted, “the situation will change.”

It’s often said that “extraordinary claims require extraordinary evidence.” The phrase was popularized by the astronomer Carl Sagan, who probably did as much as any scientist has done to promote the search for extraterrestrial life. By what’s sometimes referred to as the “Sagan standard,” Loeb’s claim clearly falls short; the best evidence he marshals for his theory that ‘Oumuamua is an alien craft is that the alternative theories are unconvincing. Loeb, though, explicitly rejects the Sagan standard—“It is not obvious to me why extraordinary claims require extraordinary evidence,” he observes—and flips its logic on its head: “Extraordinary conservatism keeps us extraordinarily ignorant.” So long as there’s a chance that 1I/2017 U1 is an alien probe, we’d be fools not to pursue the idea. “If we acknowledge that ‘Oumuamua is plausibly of extraterrestrial-technology origin,” he writes, “whole new vistas of exploration for evidence and discovery open before us.”

In publishing his theory, Loeb has certainly risked (and suffered) ridicule. It seems a good deal more likely that “Extraterrestrial” will be ranked with von Däniken’s work than with Galileo’s. Still, as Serling notes toward the end of “In Search of Ancient Astronauts,” it’s thrilling to imagine the possibilities: “Look up into the sky some clear, starlit night and allow yourself the freedom to wonder.” ♦