An Astrophysicist’s Detective Story: On That Giant Space Object That Passed Through the Solar System

Avi Loeb Considers the Greatest Scientific Anomaly of Our Age

Science is like a detective story. For astrophysicists, this truism comes with a twist. No other field of scientific sleuthing confronts such a diversity of scales and concepts. Our chronological scope of inquiry starts before the Big Bang and stretches out to the end of time, even as we recognize that the very notions of time and space are relative. Our research descends to quarks and electrons, the smallest known particles; it reaches out to the edge of the universe; and it concerns—directly or indirectly—everything in between.

And so much of our detective work remains incomplete. We still don’t understand the nature of the main constituents of the universe, and so out of ignorance we label them dark matter (which contributes five times more to the cosmic-mass budget than the ordinary matter we are made of) and dark energy (which dominates both dark and ordinary matter and that causes, at least at present, the peculiar cosmic acceleration). We also do not understand what triggered the cosmic expansion or what happens inside black holes—two areas of study in which I have been deeply involved since switching to astrophysics all those years ago.

There is so much we do not know that I often wonder whether another civilization, one that had the benefit of pursuing science for a billion years, would even consider us intelligent. The possibility that they might extend us that courtesy, I suspect, will not be determined by what we know but by how we know it—namely, our fealty to the scientific method. It will be in our open-minded pursuit of data that confirms or disproves hypotheses that humanity’s claim to any universal intelligence will stand or fall.

Very often, what sets an astrophysicist’s detective story in motion is the discovery of an anomaly in experimental or observational data, a piece of evidence that does not follow our expectations and that cannot be explained by what we know. In such situations, it is common practice to propose a variety of alternative explanations and then rule them out one by one based on new evidence until the correct interpretation is found. This was the case, for instance, with Fritz Zwicky’s discovery of dark matter in the early 1930s; it was based on the observation that the motion of galaxies in clusters required more matter than was visible to our telescopes. His proposal was ignored until the 1970s, when additional data on the motion of stars in galaxies and the expansion rate of the universe provided conclusive evidence for it.

This winnowing process can divide, even fracture, whole fields of scholarship, pitting explanations and their advocates against one another until—sometimes—one side presents demonstrative proof.

There is so much we, astrophysicists, do not know that I often wonder whether another civilization, one that had the benefit of pursuing science for a billion years, would even consider us intelligent.

This has been the case in the debate over ‘Oumuamua, a debate that, for want of demonstrative proof, is ongoing. In fact, it is worth admitting up front that the likelihood of scientists ever obtaining demonstrative proof is very remote. Catching up to and photographing ‘Oumuamua is impossible. The data we have is all we will ever have, leaving us the task of hypothesizing explanations that fully account for the evidence. This is, of course, a thoroughly scientific undertaking. No one gets to invent new evidence, no one gets to ignore evidence that is at odds with a hypothesis, and no one gets to—as in the old cartoon of a scientist working through a complex equation—insert “and then a miracle happens.” Perhaps the most dangerous, most worrisome choice, however, would be declaring of ‘Oumuamua, Nothing to see here, time to move along, we’ve learned what we can and we’d best just go back to our old preoccupations. Unfortunately, as of this writing, that seems to be what many scientists have decided to do.

The scientific debate over ‘Oumuamua was relatively calm at the outset. I attribute this to the fact that early on, we were unaware of the object’s most tantalizing anomalies. At first, this detective story seemed like an open-and-shut case: the likeliest explanation for ‘Oumuamua—that it was an interstellar comet or asteroid— was also the simplest, most familiar one.

But as the fall of 2017 progressed, I, along with a significant portion of the international scientific community, found myself puzzling over the data. I—again, along with a significant portion of the international scientific community—couldn’t make the evidence neatly fit the hypothesis that ‘Oumuamua was an interstellar comet or asteroid. As all of us struggled to make the evidence fit that hypothesis, I began to formulate alternate hypotheses to explain ‘Oumuamua’s multiplying peculiarities.


Whatever else we conclude about ‘Oumuamua, most astrophysicists would agree that it was, and remains, an anomaly unto itself.

For starters, prior to ‘Oumuamua’s discovery, no confirmed interstellar object had ever been observed in our own solar system. That alone made ‘Oumuamua historic, and it was enough to draw many astronomers’ attention, which led to the gathering of more data, which was interpreted and found to reveal further anomalies, which drew more astronomers’ attention, and so on.

Whatever else we conclude about ‘Oumuamua, most astrophysicists would agree that it was, and remains, an anomaly unto itself.

With the revelation of these anomalies, the real detective work began. The more we learned about ‘Oumuamua, the clearer it became that this object was every bit as mysterious as the media reported.

As soon as the observatory in Hawaii announced its discovery, and even as ‘Oumuamua was fleeing toward the outer solar system, astronomers around the world trained a variety of telescopes on it. The scientific community was, to put it mildly, curious. It was as if someone had come to your house for dinner and only when she was out the door and heading down the dark street did you become aware of all her strange qualities. We scientists had questions about our interstellar visitor and confronted a rapidly closing window of time to gather information, which we did by revisiting the data about our dinner guest that we had already collected and by observing her receding figure as she disappeared into the night.

One pressing question was: What did ‘Oumuamua look like? We did not, and do not, have a crisp photograph of the object to rely on. But we do have data from all those telescopes that were dedicated for about eleven days to collecting whatever they could. And once we had our telescopes trained on ‘Oumuamua, we looked for one bit of information in particular: how ‘Oumuamua reflected sunlight.

Our Sun acts like a lamppost that illuminates not only all the planets orbiting it but every object that comes close enough to and is big enough to be seen from Earth. To understand this, you must first appreciate that in almost all scenarios, any two objects will rotate relative to each other when they pass. With that in mind, imagine a perfect sphere hurtling past the Sun as it makes its way through our solar system. The sunlight reflecting off its surface is unvarying, because the area of the tumbling sphere that faces the Sun is unvarying. Anything other than a sphere, however, will reflect the Sun’s light by varying amounts as the object rotates. A football, for example, will reflect more light when one of its long sides faces the Sun and less light when, as it tumbles, its narrow sides face the Sun.

One pressing question was: What did ‘Oumuamua look like?

For astrophysicists, an object’s changing brightness provides invaluable clues to its shape. In the case of ‘Oumuamua, the object’s brightness varied tenfold every eight hours, which we deduced to be the amount of time that it took to complete one full rotation. This dramatic variability in its brightness told us that ‘Oumuamua’s shape was extreme, or at least five to ten times longer than it was wide.

To these dimensions, we added further evidence about ‘Oumuamua’s size. The object, we could say with certainty, was relatively small. Its trajectory near the Sun meant that ‘Oumuamua should have had a very hot surface temperature, something that would have been visible to the infrared camera of the Spitzer Space Telescope, which NASA launched back in 2003. However, Spitzer’s camera was unable to detect any heat coming off ‘Oumuamua. This encouraged us to surmise that ‘Oumuamua must have been small and thus hard for the telescope to detect. We estimated its length at about a hundred yards, or around the size of a football field, and its width at less than ten yards. Keep in mind that even a razor-thin object often appears to possess some width at a random orientation in the sky, so ‘Oumuamua’s actual width could well be smaller.

Let’s assume that the larger of these dimensions is accurate and that the object measured a few hundred yards by a few tens of yards. This would make ‘Oumuamua’s geometry more extreme by at least a few times in aspect ratio—or its width to its height—than the most extreme asteroids or comets that we have ever seen.

Imagine setting down this book and taking a walk somewhere. You encounter other people. Perhaps they are strangers to you, and no doubt they all look different, but by their proportions, they are immediately recognizable as human. Among such passersby, ‘Oumuamua would be a person whose waist appears to be smaller than his or her wrist. Seeing such a person would cause you to question either your vision or your understanding of people. This was essentially the dilemma that astronomers faced as they began to interpret the early data about ‘Oumuamua.


As with any good detective story, the evidence that emerged about ‘Oumuamua in the year after its discovery allowed us to abandon certain theories and winnow out hypotheses that did not fit the facts. Its brightness as it rotated gave us vital clues about what ‘Oumuamua couldn’t look like and what it might look like. In the latter category, the object’s relatively small but extreme dimensions—with a length at least five to ten times greater than its width—allowed only two possible shapes. Our interstellar visitor was either elongated, like a cigar, or flat, like a pancake.

In the case of ‘Oumuamua, the object’s brightness varied tenfold every eight hours, which we deduced to be the amount of time that it took to complete one full rotation.

Either way, ‘Oumuamua was a rarity. If it was elongated, we had never seen any naturally occurring space object that size and that elongated; if it was flat, we had never seen any naturally occurring space object that size and that flat. Consider, for context, that all asteroids previously seen in the solar system had length-to-width ratios of, at most, three. ‘Oumuamua’s, as I have just noted, was somewhere between five and ten.

And there was more.

In addition to being small and oddly shaped, ‘Oumuamua was strangely luminous. Despite its diminutive size, as it passed the Sun and reflected the Sun’s light, ‘Oumuamua proved to be relatively bright, at least ten times more reflective than typical solar system asteroids or comets. If, as seems possible, ‘Oumuamua was a few times smaller than the upper limit of a few hundred yards that scientists presumed it to be, its reflectivity would approach unprecedented values—levels of brightness similar to a shiny metal.


When the discovery of ‘Oumuamua was first reported, all of these peculiarities were arresting. Together, they presented a puzzle to astronomers. Together, they demanded a hypothesis that could explain why a naturally occurring object—and at this point, no one was arguing that ‘Oumuamua was anything but—would have these statistically rare characteristics.

Perhaps, scientists reasoned, the object’s strange features were caused by its exposure to cosmic radiation over the hundreds of thousands of years it had likely traveled in interstellar space before reaching our solar system. Ionizing radiation, in theory, could have significantly eroded an interstellar rock, although why such a process would have produced ‘Oumuamua’s shape isn’t clear.

In addition to being small and oddly shaped, ‘Oumuamua was strangely luminous.

Or perhaps the reasons for its strangeness lay in ‘Oumuamua’s origin. Perhaps it had been violently expelled through a gravitational slingshot by a planet in a manner that explained some of its features. If a suitably sized object gets within a suitable distance of a planet, part of that planet could be pulled free and thrown, as by a slingshot, into interstellar space. Conversely, perhaps it was gently pulled free from a layer of icy objects orbiting the outer reaches of a solar system, something similar to our system’s Oort cloud.

We could theorize a hypothesis starting from assumptions about ‘Oumuamua’s transit or from assumptions about its origins. If its peculiar shape and reflective properties had been the sum total of ‘Oumuamua’s distinctiveness, either theory might have been satisfactory. In that case, I would have remained curious but moved on.

But I could not restrain myself from joining in this detective story for one simple reason. It concerned ‘Oumuamua’s most arresting anomaly.

As I have mentioned, when ‘Oumuamua sped part of the way around the Sun, its trajectory deviated from what was expected based on the Sun’s gravitational force alone. There was no obvious explanation for why.

This, for me, was the most eyebrow-raising bit of data we accumulated over the roughly two weeks we were able to observe ‘Oumuamua. This anomaly about ‘Oumuamua, along with the other pieces of information that scientists assembled, would soon lead me to form a hypothesis about the object that put me at odds with most of the scientific establishment.


Extraterrestrial: The First Sign of Intelligent Life Beyond Earth by Avi Loeb

Excerpted from “Anomalies” from Extraterrestrial: The First Sign of Intelligent Life Beyond Earth by Avi Loeb. Copyright © 2021 by Avi Loeb. Used by permission of Houghton Mifflin Harcourt. All rights reserved.

Avi Loeb
Avi Loeb
Avi Loeb is the former chair of the astronomy department at Harvard University (2011-2020), founding director of Harvard’s Black Hole Initiative and director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics. He also chairs the Board on Physics and Astronomy of the National Academies and the advisory board for the Breakthrough Starshot project, and is a member of the President’s Council of Advisors on Science and Technology.

More Story
Joan Didion: Why I Write Of course I stole the title for this talk from George Orwell. One reason I stole it was that I like the sound of the words:...