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If you don’t read the science magazines, you may not be aware of the asteroid, or comet, or object that entered our solar system, passed around the Sun some months before October of 2017, and just as quickly went somewhere else. The object was spotted by the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS) at Haleakala Observatory in Hawaii and was almost immediately identified as originating outside of our system—but only as it was already receding. Once so identified, however, it was given the designation 1I/2017 U1 and named ‘Oumuamua (pronounced “oh-moo-ah-moo-ah”), or “Scout” in the Hawaiian language.
Most astronomers consider it to be some kind of asteroid or comet, and the artist’s conception that was widely published (see nearby) shows a grayish or reddish oblong rock, clearly of natural origin. But let me be quick to point out that no telescope ever resolved the object’s image so clearly. All our telescopes could see—because ‘Oumuamua was already beyond Earth’s orbit when it was detected, and it was surprisingly small on an astronomical scale to begin with—was a faint point of light that varied in brightness over a regular eight-hour period. It was something really tiny and, by the time we saw it, pretty far away.
There the matter might have rested—a rock from beyond our solar system, an asteroid that had escaped from some other star system—if Avi Loeb had not taken up the issue. Loeb is an astrophysicist, an alumnus of the Institute for Advanced Study at Princeton, currently the Frank B. Baird, Jr., Professor of Science at Harvard University, formerly the long-serving chair of Harvard’s Department of Astronomy, and author of eight books of popular science and about 800 papers of serious scientific inquiry. He recently published his analysis of ‘Oumuamua, along with his lifelong involvement with the question of alien intelligence, in Extraterrestrial: The First Sign of Intelligent Life Beyond Earth (Houghton Mifflin Harcourt, 2021).
Being fascinated by the subject, I of course bought the book and devoured it right away. So consider this my book report on the subject. And accept that I find Loeb’s analysis convincing, even though most astronomers and cosmologists disagree and insist that ‘Oumuamua is still just a rock or other natural object.1 Remember, I’m a natural contrarian.
The first issue is ‘Oumuamua’s brightness. The observations suggest that its longest dimension is just about one hundred meters, or three hundred feet, the length of a football field. I imagine such a tiny object would not normally be visible at interplanetary distances, the distance at which we first detected it, unless it was really bright. The nature of the light we could see suggested it was reflected sunlight, not any artificial light the object might be emitting. At that distance, the reflective capacity—called “albedo” by astronomers—had to be much greater than that of a rock or even the ice of a comet, which is usually so contaminated that we call them “dirty snowballs.” ‘Oumuamua reflects sunlight like polished metal.
The second issue is the shape. From the variation in brightness, the observations suggest that the object was slowly tumbling. The amount of reflected light varying over time implies that if ‘Oumuamua’s longest dimension is three hundred feet, then it’s shortest is a little more than a tenth to a fifth of that, or about thirty to fifty feet. The artist’s conception draws this as a cigar shape, and I think of it as about the size and dimensions of one of our nuclear submarines. But Loeb presents an alternative shape as more likely: a disk or a pancake. Here I am interpreting the reasoning as discussed in the book: if the oblong or cigar shape was the object’s true nature, then we would have to be viewing it practically end-on—that is, with the axis of spin at right angles to our line of sight—for the variation to be this complete. If we were viewing it from the side—with the axis aligned with our viewpoint—then the variation would not be as great. But a tumbling disk could display that degree variation from a variety of angles to our line of sight.
The third issue is the path that the object took in its travels. It deviated on its course around the Sun and accelerated slightly on its exit from the system. An asteroid, a solid rock, doesn’t do this but instead follows the course that its starting speed and the Sun’s gravity give it. A comet often deviates and accelerates slightly because sunlight heats the ice, causing outgassing that functions like tiny rocket motors, pushing the comet randomly this way and that and, on its outward trip, with the Sun at its backside, perhaps accelerating it. But ‘Oumuamua did not have a coma of dust and water vapor surrounding it or the long tail pointing away from the Sun, both features typical of a comet. Astronomers studied the object at various wavelengths—for example, in the infrared, where carbon dioxide from a comet’s emissions would show up clearly—and still they found nothing.
The scientists who dispute Loeb’s interpretation of ‘Oumuamua as a technological artifact suggest that it might have been composed entirely of frozen hydrogen, because the outgassing of hydrogen would be invisible to us. Such an object is possible, but it’s hard to imagine how, at the relatively slow speed it was traveling, such it would survive the long trip through interstellar space, where even starlight would eventually heat it up enough to melt it.
Scientists have also suggested that the object was pushed around and accelerated on its passage through our system by the Sun’s light itself. This idea is supported by the observation that ‘Oumuamua’s acceleration faded with the inverse square law as it went further and further away.2 But for the object to respond like that to mere sunlight, it would have to weigh almost nothing. The rock depicted in the picture would have to be less dense than the air on Earth, and it’s hard to see how such an object would hold together while it tumbles through space.
Here Loeb brings into play some of his personal experience. He recently participated in a privately funded project to conceive of and then design a probe that could be sent to a nearby star and return signals within a human lifetime. The probes that we have already sent out of the solar system, the Pioneer and Voyager programs, and more recently the New Horizons flyby of Pluto, all depended on chemical rockets to launch them and set their initial course, then used gravity assists from the outer planets to speed them on their way. They will take centuries, if not millennia, to reach any stars in their paths.
The Breakthrough Starshot program that Loeb participated in envisioned instead a small electronics package, a “Starchip,” attached to a lightsail. This vehicle could be put into space near Earth and then propelled by a laser fired from the planet’s surface and focused on the sail. A sustained laser blast could accelerate it to a high fraction of the speed of light. The sail would be very thin and light: think of the metal coating on a Mylar party balloon. It could take the package to the nearby star Proxima b in about twenty years. As the Starchip passed through the Proxima b system it could record images of the Earthlike planet that we know orbits this star. The chip could then send these images back to Earth in the 4.2 years that light (and any radio signals) from Proxima b takes to reach us. The beauty of the program is that the main capital and operating costs, including fuel, are in the Earth-bound laser, while the individual probes would cost almost nothing by comparison. So the program could send out hundreds or thousands of Starchips to different nearby star systems.
With this background, plus his lifelong interest in the search for extraterrestrial intelligence to begin with, Loeb was primed to see ‘Oumuamua as some form of lightsail: one hundred meters wide and perhaps no more than a millimeter in thickness, fully expanded, very reflective, and tumbling slowly. It might have been sent into our solar system as part of an alien Starshot program. However, from the mechanics of its parabolic orbit and its presumed entry speed, Loeb and other scientists think ‘Oumuamua must have been moving at the average speed of most of the stars rotating in the galactic plane—and then our Sun, which is moving a little faster than average, scooped the object into its gravity well and redirected it to who-knows-where. So, in that interpretation, ‘Oumuamua might have been an interstellar navigation buoy or repeater station, instead of an aimed probe.
As Loeb describes the situation, most astronomers consider ‘Oumuamua to be a natural object, and they cling to interpretations of its orbital deviation that involve either a hydrogen iceberg or some kind of super-lightweight mass that still has the internal strength to tumble and not fall apart. He believes these scientists resist the evidence of ‘Oumuamua’s artificial and possibly technological nature because the search for extraterrestrial intelligence (SETI) has leaves a bad taste with true scientists. The notion of alien intelligence brings to mind too much science fiction full of little green men, bug-eyed monsters, and evil space invaders, as well as too many years of aiming radio telescopes at various stars and listening for messages that never come.3
I have always believed that, in a universe filled with billions of galaxies and trillions of stars like our Sun, and now with growing evidence that many of these stars have Earthlike planets in their habitable zones, it would be the extreme of hubris to think that ours is the only planet to develop and support life, or that human beings are the only intelligent, tool-building and -using, and soon to be spacefaring species in all of that vastness.
I find Avi Loeb’s reasoning to be persuasive. We have just detected the handiwork of intelligent aliens that passed unannounced through our system. Maybe it was a lightsail or an interstellar beacon disturbed by the Sun’s gravity, as Loeb suggests. Or it could also have been a cargo cover, a blown hatch, or debris from a larger ship that suffered some terrible accident. All of that would be unprovable speculation. But what I no longer think is that ‘Oumuamua was an extrasolar asteroid or comet—not even one made out of pure hydrogen ice.
1. Much of Loeb’s book is autobiographical, demonstrating his solid scientific background. It also gives a detailed history of the science of astronomy in relation to comets and asteroids and various professional inquiries and disputes about the search for extraterrestrial intelligence, which makes for fascinating reading. But I’ll try to focus here on the issue of ‘Oumuamua itself.
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2. The inverse square law says that the amount of radiation from any point source that broadcasts in all directions decreases proportionally with the square of the distance from it. So, if the strength of a light is, say, 1,000 lumens at a distance of one mile from the source, then it is just 250 lumens at two miles (one-quarter being the inverse square of two), and only 110 lumens at three miles (one-ninth being the inverse square of three). You can test this by measuring the amount of light from a lamp as you walk away, from standing next to the bulb to standing across the room.
3. But, as Loeb points out several times in the book, many physicists devote their careers to studying the extra dimensions—beyond the three that we know of, plus time—needed to support string theory, or the nature of the multiple universes that support probability theory and the fate of Schrödinger’s cat. And we spend hundreds of millions of dollars on particle accelerators and experiments to prove the supersymmetry underlying quantum mechanics. These are just beautiful ideas without, so far, any hard evidence to back them up. And here a piece of alien technology—although the evidence is debatable and requires some thought and analysis—has just floated through our solar system.