Oumuamua — Hawaiian for “scout” — was first noticed by a telescope on the island of Maui on Oct. 19, 2017, when it was already on its way out of the solar system, having passed closest to the sun a month before. It had come from outside the solar system, from the direction of the star Vega.
Nobody ever got a picture of the object, but from how its brightness varied as it apparently tumbled, astrophysicists were able to deduce that it was about a quarter-mile across and at least five to 10 times longer than it was wide. An artist’s interpretation of reddish, cigar-shaped rock was widely reproduced. Based on some surprising herky-jerky motions as it departed our realm, astronomers concluded that Oumuamua was a weird comet. Such objects often get accelerated by jets of evaporating gases on their surface, although in this case no evaporating gases were detected.
But Loeb argues that it is no more preposterous to suppose that Oumuamua was a lightsail, a thin material that gets its propulsive boost from sunlight or starlight, either launched in our direction or anchored like a buoy in space, where we ran into it on our planet’s travel around the galaxy. In which case the age-old question — are we alone in the universe? — has been answered.
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.”
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.