Extraterrestrial

by Avi Loeb

  It was never my intent to become what the science journalist Michelle Starr labeled me: “Harvard astrophysics enfant terrible.” My attitude toward anomalies remains what it has been since my first day of grade school—quizzical and questioning; I pause long enough to wonder what might follow if I pursue one course of action over another. When Starr asked Matthew Knight, an astronomer at the University of Maryland and one of the scientists on the ISSI ‘Oumuamua Team, to sum up the team’s findings, he declared, “We have never seen anything like ‘Oumuamua in our solar system. It’s really a mystery still,” and then added, “but our preference is to stick with analogues we know.”

  Fair enough. But what happens when we start from the mystery end of the trench rather than the familiar-analogues end? What questions arise and what new avenues for pursuing answers suggest themselves when we entertain possibilities that are counter to our governing presumptions but align with the data we have?

  This is not an idle question; the data we have forces us to entertain exceptionally rare explanations for it. Several other mainstream astronomers, not members of the above groupthink, analyzed ‘Oumuamua’s data carefully and found that only highly exotic explanations could account for the object’s anomalous behavior. To explain all the known facts, they were forced to imagine ‘Oumuamua was a fluffy object composed of material a hundred times more rarefied than air or that it was a comet composed of solid hydrogen ice.

  Scientists had to offer up “never seen before” options to explain Oumuamua’s proven peculiarities. Of all the many asteroids and comets that we have cataloged, none have had such peculiarities. If these scientific-mainstream explanations for ‘Oumuamua were deemed valid enough for thoughtful consideration, the hypothesis that it was extraterrestrial technology, similarly a “never seen before” possibility, deserves no less.

  The questions that the lightsail hypothesis raises, moreover, are intriguing. If we suppose that ‘Oumuamua is an exceedingly rare comet composed of frozen pure hydrogen, most of our questions dead-end. The same is true if we imagine it as a fluffy cloud of dust with sufficient internal integrity to hold together but still lightweight enough to be pushed by sunlight. In both instances, we can marvel, but that’s about all we can do. Statistical rarities belong on the shelves of a curiosity cabinet; they shouldn’t give rise to new branches of scientific inquiry. But if we acknowledge that ‘Oumuamua is plausibly of extraterrestrial-technology origin and approach that hypothesis with scientific curiosity, whole new vistas of exploration for evidence and discovery open before us.

  After the media had gotten over the initial shock that the chair of Harvard’s astronomy department and his postdoc were postulating that ‘Oumuamua was a relic of extraterrestrial technology, I was accused of seeing lightsails wherever I looked. After all, my involvement in the Starshot Initiative had been announced just two years earlier and our stated goal was to send electronic chips to the nearest star by harnessing the power of lightsail technology.

  The interviewer for the German newspaper Der Spiegel put it with admirable bluntness: “According to a proverb, whoever has only a hammer will see nothing but nails.”

  I replied that, yes, like everyone’s, my imagination was guided by what I knew, and, yes, like everyone’s, my ideas were influenced by what I was working on. But I should have added that the problem with the proverb was that it focused attention on the hammer rather than on the person wielding it. Not only do skilled carpenters most definitely not see nails everywhere, but they are trained to differentiate among those they do observe.

  6

  Seashells and Buoys

  One of my favorite activities is walking along the beach searching for seashells worth collecting. It is a pastime I indulge in while I’m on vacation, when I get to pick a beloved stretch of sand to stroll and study and I have the free time to do it. My daughters often join me as we examine what has been swept ashore. Over the years we have amassed a nice collection of delicately attached bivalves, rounded cowries, and curled triton and murex shells. A few of our shells are pristine, but far more are worn down and partially disintegrated, their tiny pieces now part of the white sand we’ve walked on.

  Sometimes while searching for shells, we find a piece of sea glass—a shard from a discarded bottle that, after years of being tossed and tumbled in the ocean, has been made smooth. Under these conditions, even industrial products can be things of beauty.

  Occasionally on our shell-hunting expeditions, we find other, less beautiful man-made objects—a plastic bottle, say, or an old grocery bag. But such discoveries are relatively rare, and that rarity is easily explained: we try to vacation near places where we are less likely to encounter trash.

  If our family wished, we could travel to beaches where encountering trash is a certainty. Sadly, our planet has a growing number of them. For instance, Hawaii’s Kamilo Beach; once beautiful, it is now known as “Plastic Beach” due to the extent of garbage that accumulates there. Its condition is sad but not all that surprising, given that the Great Pacific Garbage Patch—by some estimations, the largest of the world’s five offshore “plastic-accumulation zones”—sits between California and Hawaii. And the existence of five such patches is also not surprising, given that humanity puts about eight million metric tons of plastic in the oceans each year.

  The more there is of something, the more likely it is you will encounter it. This truism applies equally to seashells and plastic bottles—and to the two potential explanations for ‘Oumuamua I have described so far. Either it is a naturally occurring seashell or it is a piece of manufactured material, junk or otherwise.

  Viewing both possibilities through a lens made of beach glass, we can see why identifying the right one is so important—and what implications it will have for both science and our own civilization.

  …

  Let’s suppose that, rather than a plastic bottle, ‘Oumuamua was a seashell. An exotic seashell, certainly, but still a naturally occurring seashell.

  This line of reasoning has attracted the vast majority of scientists who have considered ‘Oumuamua’s anomalies. It comes undone almost immediately, however, when we ask how many interstellar seashells must there be for our solar system to have randomly come across one.

  No one is surprised to encounter an intact seashell while walking along the beach. The sea creatures that produce shells are vast in number, and even given the size of the world’s oceans, there are more than enough of them to make it commonplace to find shells worth collecting. Indeed, if we wished, we could estimate the likelihood of encountering not just a seashell but a specific kind of seashell along a given stretch of beach. Knowing a little about the number of quahog clams in the waters around Cape Cod, for example, would allow us to predict how often we could expect to discover one on the beaches around Provincetown. The same would go for a conch shell on a Florida beach.

  If ‘Oumuamua is a naturally occurring asteroid or comet, we can pose the following question: How many interstellar rocks need to be in the universe for human beings to regularly encounter them in our solar system? If interstellar space has a large population of asteroids and comets, just like the familiar family of those bound to the Sun, it wouldn’t be surprising for us to see them. After all, as I’ve noted, the more there is of a thing, the more likely you are to encounter it. If interstellar space has a small population of such rocks, however, it would be more surprising to encounter them in our solar system.

  Interstellar space is, of course, many magnitudes vaster than the Earth’s oceans. This means that for us to regularly encounter them in our solar system, there would have to be a very, very large population of such interstellar objects floating around out there. Such rocks are known to be the building blocks of planetary systems around stars.

  Actually, very, very doesn’t come close to doing this inferred population justice. To account for a population of rocks as large as the discovery of ‘Oumuamua implies is out there requires that each star in the Milky Way eject
1015 such objects from the rocky material around it during its lifetime. To get a sense of the size of that number—a quadrillion—grab a piece of paper and write down a 1 followed by fifteen 0s. It’s not quite the size of the number of habitable planets in the observable universe (1021), but still, it represents a lot of objects coming from each and every star in our galaxy. Planetary systems around stars are the regions where large solid objects are likely to form.

  Our own Sun has not been nearly so profligate with its planetary building blocks. In 2009, nearly a decade before ‘Oumuamua’s discovery, I published a paper with Amaya Moro-Martin and Ed Turner in which we used a dynamical history of our solar system to predict a population of random interstellar objects; it is smaller by two to eight orders of magnitude than the amount needed to explain the discovery of ‘Oumuamua. In other words, the number of predicted interstellar objects our projection came up with was at least one hundred times lower than that needed for the hypothesis that ‘Oumuamua was a random interstellar rock. This by itself does not rule out ‘Oumuamua being a familiar rock, but it does make its discovery in our solar system surprising on statistical grounds.

  Put another way, the idea that ‘Oumuamua was a naturally occurring rock implies that the population of random interstellar objects is far greater than what we expect and what our own solar system predicts. So either a great many other stars out there are very different from the one nurturing us, or there is something else going on.

  …

  In 2018, a small group of scientists revisited the question of the abundance of ‘Oumuamua-like rocks in interstellar space. While studying the ability of Pan-STARRS to detect objects similar to ‘Oumuamua, they reached some general conclusions. Among them was the broadly agreed-upon insight that “many aspects of ‘Oumuamua are both intriguing and troubling.” But they also found that the number per unit volume of interstellar material necessary to have ‘Oumuamua be a random rock requires “mass ejection rates” that far exceed expectations, up to a quadrillion (1015) ‘Oumuamua-size objects per star, yielding roughly one object for every interstellar volume whose circumference is carved by the orbit of the Earth around the Sun. In two follow-up papers, my former collaborator Amaya Moro-Martin showed that the natural abundance of ‘Oumuamua-like objects on random orbits falls short of the required value by several orders of magnitude, even if every planetary system ejected all of the expected solid material in it.

  These conclusions complicate this echo of our 2009 finding in interesting ways. One complication has to do with the origins of interstellar material, which falls into two broad categories: dry rocky material (which has little to no ice) and icy rocky material.

  Dry rocks are created primarily during planet formation. This happens as a result of dust particles sticking together and growing in size to planetesimals, which eventually combine to make planets. But the first of the above studies concluded that the number density needed for the random-rock hypothesis to explain a naturally occurring ‘Oumuamua “cannot arise from the ejection of inner solar system material during planet formation.” Not enough material is ejected during planet formation to get us to the necessary density.

  To get to the needed density of naturally occurring objects, these scientists had to posit an additional source of interstellar objects like ‘Oumuamua. And for that, they turned to material ejected from stars’ Oort clouds—the shells of icy objects at these systems’ outermost edges. When a star reaches the end of its lifetime, its gravitational hold on its Oort cloud bodies weakens, and they are released. But even if all dying stars contribute their Oort cloud debris to interstellar space, Amaya Moro-Martin found in her second paper, they do not provide enough material to achieve the needed density.

  The challenge that the “natural origin” explanation for ‘Oumuamua confronts is the need for a sufficient amount of interstellar material. The rough analogy of seashells helps; you need a great many seashells in the sea to make discovering an intact one on a beach probable. The same applies to a naturally occurring ‘Oumuamua arriving in our solar system. For it to be a randomly encountered object, we need a lot of such objects in the universe, and to get at that density, we need objects released from both planet formation and Oort clouds.

  Of course, we have already established that ‘Oumuamua wasn’t icy. (No outgassing, no ice.) So a naturally occurring ‘Oumuamua is very unlikely to have come from an Oort cloud.

  In short, if ‘Oumuamua was a natural object, it had to have been generated by planet formation. It also has to belong to an unknown class of objects generated by planet formation whose size, shape, and composition make them deviate from a path shaped solely by our Sun’s gravity without any visible outgassing.

  At the time of this writing, we know of no other object that fits the second set of criteria. But we know of at least one that fits the first.

  …

  Not long after the discovery of ‘Oumuamua, we encountered our second interstellar object. By the time you read this book, we may well have found others.

  This second interstellar object is named 2I/Borisov, after Gennadiy Borisov, a Russian engineer and amateur astronomer who on August 30, 2019, using a sixty-five-centimeter telescope of his own construction, identified the object in the skies above Crimea. And it was Borisov who first ascertained that its trajectory was hyperbolic. Just as had been true for ‘Oumuamua, 2I/Borisov was moving too fast to be gravitationally bound to the Sun. And so, just like ‘Oumuamua, 2I/Borisov had come from outside our solar system and was on a path that would send it through and beyond our solar system.

  But otherwise, 2I/Borisov was unremarkable. It was an interstellar comet, without question, and for this reason it was distinctive; any interstellar object is a rarity. But its distinctiveness ended there. Its coma and outgassing resembled our solar system’s comets’ in all characteristics; 2I/Borisov was icy and decidedly not exotic.

  The point is that the discovery of 2I/Borisov did not help us move toward a naturalistic explanation for the exotic ‘Oumuamua. If anything, it did the opposite, by underscoring how special ‘Oumuamua truly was. When I met my wife and realized how special she was, I married her. The many people I have encountered since then do not take away from her unique qualities; they only add to my sense of wonder at how rare she is.

  ‘Oumuamua and 2I/Borisov were both interstellar interlopers in our solar system, but other than that, they were decidedly different from each other, for among 2I/Borisov’s list of ordinary features, it was from an unexceptional origin in space-time.

  ‘Oumuamua wasn’t. Indeed, its origin in velocity-position space is another of its marked peculiarities—and another piece of evidence supporting an unusual origin. It is also another clue that can help us to unravel the mystery of what ‘Oumuamua was and what it was doing out there in the void of interstellar space.

  To understand this requires an understanding of velocity-position space. This can be a bit tricky to wrap your mind around, but it boils down to appreciating that the position an object holds in space is defined not just by where it is vis-à-vis everything around it but also by what its velocity is vis-à-vis the velocity of everything around it. Imagine a very busy, very wide multilane interstate filled with thousands of cars. All of them are traveling at slightly different rates; some are passing cars, others are falling back, some are well below the speed limit, and others are greatly exceeding it.

  If you averaged these cars’ movements, you would find a few cars that were, relative to all the others, “at rest.” These cars would not be pulling ahead or falling back relative to the rest of the pack. Amid all that motion, those cars would be comparatively still.

  The same applies to the stars. All the stars in the vicinity of the Sun are moving relative to one another. The average of their motions is called the local standard of rest. Amid the motion of all these stars, an object at the local standard of rest, or LSR, is comparatively still. And it is a comparatively rare occurrence.

  Sky path of ‘Oumu
amua as seen from Earth, with the stages of the object’s progress labeled by date. The relative size of each circle gives a schematic sense of the changing distance of ‘Oumuamua along its apparent trajectory. Also shown is the direction of motion of the Sun in the local standard of rest, or LSR (the direction just left of the label “solar apex”). The fact that the object starts from that direction and gets close to us implies that it was initially in the LSR. Between September 2 and October 22, 2017, ‘Oumuamua’s trajectory moved from the local standard of rest to south of the ecliptic plane of the solar system (marked by the thin line).

  Image by Mapping Specialists, Ltd. adapted from Tom Ruen (CC BY 4.0)

  ‘Oumuamua was occupying the LSR.

  Or at least, it was before it accelerated. Around the time that it encountered our Sun, it went from sitting still—relative to the average motion of the stars in our neighborhood, including our own—to moving away from us. Thanks to the kick it received from the Sun’s gravity, it was knocked out of LSR, much as if one of those “still” cars on that multilane highway was violently sideswiped. As a consequence, ‘Oumuamua was broken out of LSR and sent on a path along which it would, like a tennis ball hit by a racket, speedily depart our solar system.

  ‘Oumuamua’s being at LSR was peculiar. Consider that only one in every five hundred stars is as still within the LSR frame as ‘Oumuamua was before it was sideswiped. Our own Sun, for example, is moving at about 45,000 miles per hour relative to this frame, about ten times faster than ‘Oumuamua was moving before the Sun kicked it away from the LSR.