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.