Artists impression of ʻOumuamua courtesy M Kornmesser/ESO



An alien-made artefact or just interstellar debris? What ʻOumuamua says about how science works when data is scarce

by Matthew Bothwell + BIO

Artists impression of ʻOumuamua courtesy M Kornmesser/ESO

There’s an iconic moment, filmed in the shadow of the Very Large Array in New Mexico, that many people who visit this giant telescope try to duplicate. A young astronomer sits cross-legged on the bonnet of her car, the towering line of radio dishes vanishing into the distance behind her. With her laptop in front of her, she’s listening intently to a giant pair of headphones, held upside down so that the strap hangs below her chin. The shot is from the film Contact (1997), and the astronomer, Dr Eleanor Arroway (played by Jodie Foster), is listening, awestruck, to the first signal from an extraterrestrial intelligence. Having worked as a professional astronomer for more than a decade, I’ve met a number of colleagues for whom the film was an important part of their childhood. Many modern astronomers are driven by the ideals that Contact speaks to: the awe of discovery, and the search for company somewhere in this vast and empty Universe.

On 19 October 2017, the astronomer Robert Weryk spotted something rather extraordinary: a splinter of rock, just a few hundred metres across, tumbling through our inner solar system. Not much to write home about, you might think: there are more than 750,000 known asteroids and comets in our cosmic backyard, and countless millions more waiting to be discovered. But this object was very, very special. As his team would soon discover, this piece of flying cosmic debris could only have come from outside of our own solar system. The human race had found its first ever interstellar traveller.

The object was soon named ʻOumuamua: Hawaiian for ‘first distant messenger’ or ‘scout’ (and pronounced the way one might write an ode to a cow: ‘Oh, moo-er, moo-er’). More than three years later, the debate over ʻOumuamua’s true nature has spilled beyond the borders of academic astronomy and into the popular imagination. One reason why is obvious: a visitor from the stars – not in any metaphorical sense, but a real, tangible object right here, in our cosmic backyard – forces us to see ourselves as a small part of a wider Universe that exists far beyond our imaginative shores. There’s another reason, too: in our current space-faring culture, just as we’re launching missions to the planets and dreaming of visiting the stars, it’s inevitable that a tantalising question would arise – what if ʻOumuamua is more than a simple inanimate object?

The existence of extraterrestrial life is one of humanity’s great driving questions. The ancient Greek philosophers before Socrates debated the ‘plurality of worlds’, and who among us hasn’t looked up at the stars at some point and wondered if there wasn’t someone, somewhere, looking back? In the latter half of the 20th century, this enduring fascination crystallised into a systematic scientific search effort, known as SETI: the Search for Extraterrestrial Intelligence. But after more than six decades of hunting for radio signals from nearby stars, we’re as alone as we ever were. When we speculate about life elsewhere in the Universe, we’re doing that most dangerous of things, from a scientific perspective: extrapolating from a single data point, that of human existence. This combination of a profound and universal yearning, undercut by a total absence of evidence, has allowed the question of extraterrestrials to become a cosmic blank canvas, onto which it’s possible to project our hopes and our fears, our deepest insecurities and our loftiest desires. Our answer to the question of whether or not extraterrestrial intelligence might exist often tells us more about the baggage we bring than anything about the Universe as it really is.

Victorians of the late 19th century, living in the era of ambitious engineering, looked at Mars and saw globe-spanning canals – evidence, they believed, of a grand industrial civilisation mirroring their own. In the Cold War 1960s, as millions lived under the shadow of potential nuclear annihilation, ‘neocatastrophism’ – the theory that extraterrestrial civilisations are inevitably wiped out by violent events – emerged as an explanation for our apparent cosmic solitude. The Argentinian Trotskyist J Posadas was convinced that advanced aliens would be socialists; more recently, the Vatican’s then-chief astronomer José Gabriel Funes suggested in 2008 that extraterrestrials might share a close relationship with God. We scientists tend to believe that intelligent extraterrestrials will be builders of technology, fluent in the universal language of mathematics. In Contact, the aliens announce their presence by beaming prime numbers at us, and many of our messages broadcast to the stars consist of physics and mathematics wrapped up in binary code. This perspective on aliens as scientific rationalists underlies most of modern SETI. It’s a viewpoint that I happen to agree with. Then again, I’m a scientist: of course I do.

These labours of imagination aren’t intrinsically bad for science. Indeed, when faced with a new intellectual frontier, lacking in evidence but with plenty of tantalising questions, speculation is inevitable. It allows us to consider ideas that populate an intellectual landscape beyond our evidential horizons. Without speculation, our thinking would never develop and science would be stagnant. Many established scientific theories started out as pure conjecture: the Arabic intellectual Ibn al-Khatib made his suggestion that plagues result from contagion by minute bodies in c1362 CE, hundreds of years before microscopes provided evidence for his ideas. The ancient Greek philosopher Democritus proposed that matter was made of tiny ‘atoms’ more than 2,000 years before any proof arrived. Nicolaus Copernicus’s theory that Earth orbits the Sun was made in 1543, decades before Galileo used his new telescope to show that Copernicus was correct. Speculation and imagination are very much the creative force driving the advancement of scientific knowledge.

The number that caused the uproar was 1.2: this perfectly innocuous number had deeply cosmic implications

But scientists are human, and our flights of imaginative fancy are inevitably influenced by our existing ideas, our politics and our ideologies. Nowhere is this clearer than the topic of extraterrestrial life: when we wonder about the other minds with whom we might share our Universe, we’ve always tended to conjure up reflections of ourselves. And ʻOumuamua, a cosmic traveller flung out from some distant star system, has sparked the latest chapter of this long-running saga.

The discovery that made history came in late 2017. The Panoramic Survey Telescope and Rapid Response System (Pan-STARRS) spotted a faint speck in the sky that didn’t correspond to anything in the near-Earth object catalogues. Astronomers soon realised that this object was in the process of leaving our solar system: it had already swung around the Sun, and would soon pass Mars’s orbit on the way out. The speck – initially given the memorable moniker ‘P10Ee5V’ – was moving fast, flying away from the Sun at around 100,000 kilometres per hour. Over the course of six days, astronomers around the world used telescopes to carefully track its motion: by combining these observations with some basic knowledge of gravity, it was possible to re-trace the object’s steps, mapping out its entire orbit around the Sun. A message circulated around the astronomical world on 25 October 2017, describing the object – by now given the slightly improved name ‘COMET C/2017 U1 (PANSTARRS)’ – as ‘very weird’, and listing a few numbers that described the size and shape of its orbital path. The number that caused the uproar was something called the ‘orbital eccentricity’, measured to be around 1.2. This perfectly innocuous number had deeply cosmic implications.

Animation of ʻOumuamua passing through the Solar System. Courtesy Wikipedia

The shape of a planet’s orbit around a star is something called an ‘ellipse’, a sort of squashed circle. The amount that a planet’s orbit deviates from a perfect circle is called its ‘orbital eccentricity’. Earth’s orbit is only very slightly squashed, with an eccentricity of 0.017. You’d have to look very closely to see that we don’t orbit around the Sun in a perfect circle. Pluto’s orbit, on the other hand, is very squashed indeed: it looks egg-shaped and, at times, Pluto is closer to the Sun than Neptune is. Pluto’s obviously wonky orbit has an eccentricity of 0.25. These numbers can go higher: Eris, the second largest dwarf planet in our solar system has an orbital eccentricity of 0.44, and little Sedna, three times further out than Neptune, comes in at 0.85. Comets have some of the most eccentric orbits of all, spending most of their life out in the far frozen reaches of the solar system and only occasionally dipping close to the Sun. Halley’s Comet has an orbital eccentricity of 0.97, and NEOWISE (which lit up the skies in the summer of 2020) is almost as eccentric as they come, at 0.999.

At this point, you’ll have noticed something. All of these objects, which are part of our solar system, have orbital eccentricities lower than 1. When it comes to orbital eccentricity, 1 is the magic number. Once your orbit hits an eccentricity of 1 (or higher), you’re no longer gravitationally bound to the thing you’re orbiting: strictly speaking, you aren’t ‘orbiting’ at all, and are destined to fly off into the interstellar darkness.

So – back to our weird speck, COMET C/2017 U1 (PANSTARRS). An orbital eccentricity of 1.2 meant that there was no way that this object was in orbit around our Sun. It could only have come from outside our solar system: from interstellar space. What we had found was a visitor from another star system. A more dignified name was in order: it was soon dubbed 1I/ʻOumuamua.

As you might imagine, all this got the world’s attention. Within hours of the announcement, telescopes around the globe were pointed at ʻOumuamua, hoping to learn as much as possible about our visitor before it vanished forever into the dark. We quickly learned that the light reflecting from ʻOumuamua was reddish, similar to some asteroids in our own solar system that are covered in organic silicates, carbon and ice. Any sense of normality stopped there, however.

The first oddity was the light curve – that is, the way ʻOumuamua changed brightness over time. Repeating every eight hours or so, ʻOumuamua got brighter, then dimmed, and then brightened again. This meant that ʻOumuamua must be a spinning asymmetrical object: it looked brightest when its largest surface was pointing towards us, and vice versa. What was surprising was the extreme difference between these two states. Normal, slightly misshapen asteroids get maybe 20 or 30 per cent brighter and dimmer as they spin. The light from ʻOumuamua, on the other hand, was varying by a factor of 10. ʻOumuamua had to be something very long and thin: a cigar-shape, or maybe a flat pancake, a few hundred metres wide but just tens of metres across. Completely different, in other words, from the mostly spherical (or dumpily potato-shaped) asteroids that populate our inner solar system. ʻOumuamua, our first interstellar visitor, was like nothing we’d seen before.

Was ʻOumuamua a chunk of frozen hydrogen, perhaps chipped off a distant star’s equivalent of Pluto?

This is the point at which many people – including some astronomers – started using the ‘A’-word. Artificial. The comparison to a discarded rocket or a science-fiction flying saucer was inescapable: the rocket that took astronauts to the Moon, Saturn V, measured 110m by 10m – eerily similar to our interstellar visitor. Things got weirder still just a few months later. In the summer of 2018, the astronomer Marco Micheli published a paper in Nature announcing that his team had tracked ʻOumuamua’s motion more accurately than ever before. Their headline finding was simple: ʻOumuamua was speeding up. The effect was subtle – they weren’t seeing ʻOumuamua blasting off to the stars – but there was clearly something non-gravitational afoot. Something was pushing ʻOumuamua faster and faster. It wasn’t long before SETI swung into action, and some of the best alien-hunting machines on the planet were turning their attention towards ʻOumuamua. Several radio telescopes focused on the object, hunting for any possible electronic chatter it might be putting out. The result, however, was silence.

The astronomical community rose to the challenge of explaining ʻOumuamua, with more than 200 papers to date discussing our interstellar visitor. Explanations have ranged from the mundane (ʻOumuamua is the middle of a comet) to the esoteric (ʻOumuamua is a nugget of dark matter). Some groups proposed that the acceleration might be due to excess heat radiation: due to its unusual shape, ʻOumuamua could be radiating most of its heat behind it as it travelled. Such ‘radiative anisotropy’ has been known to push things around in deep space (most notably the Pioneer probe, the strange acceleration of which was a mystery for years). Other papers suggested ʻOumuamua’s motion could be explained by ‘outgassing’ – the Sun boiling ice under the surface, producing gas that spurted out like a rocket engine. Comets – and even some asteroids – in our own solar system do this all the time. Observations indicated that ʻOumuamua had no coma – a nebulous envelope of gas – but this excluded only gasses that we can see. If ʻOumuamua was a chunk of frozen hydrogen, perhaps chipped off a distant star’s equivalent of Pluto, all of the weird behaviour gets explained rather neatly.

The problem is that ʻOumuamua is gone. By now it’s billions of kilometres from Earth, and getting farther away all the time. Unless a very expensive chase mission is launched very soon, ʻOumuamua might remain shrouded in mystery forever.

There’s a name that, by now, has become conspicuous by its absence. If you’ve read even one story about ʻOumuamua over the past four years, you’ll almost certainly have come across it: Abraham ‘Avi’ Loeb, professor of science at Harvard University, and his claim that ʻOumuamua is an alien artefact.

Together with his Harvard colleague, Shmuel Bialy, Loeb published the paper ‘Could Solar Radiation Pressure Explain ʻOumuamua’s Peculiar Acceleration?’ (2018). In other words, could the acceleration be caused by the pressure of sunlight? This innocuous hook was the precursor to a startling conclusion: the pressure from sunlight could work, but not if ʻOumuamua was a lump of rock. The strange acceleration could be chalked up to solar radiation pressure if ʻOumuamua was an artificial construction: a solar sail, built to traverse the galaxy on a wind of light.

Loeb has done more than engage in purely dispassionate scientific theorising. What has led to him hitting headlines around the world is his evangelical mission to convince the public that aliens have visited. Loeb has written a long string of popular articles arguing that the best explanation for ʻOumuamua is an alien solar sail. He followed these up with a book, Extraterrestrial: The First Sign of Intelligent Life Beyond Earth (2021), in which he’s even more vehement: in it, he writes ‘ʻOumuamua must have been designed, built, and launched by an extraterrestrial intelligence.’ In a recent interview promoting the book (headlined ‘Astronomer Avi Loeb Says Aliens Have Visited, And He’s Not Kidding’), Loeb highlights what he calls a ‘crisis’ in science, saying that his colleagues in the astronomical community are ‘not using common sense’, and castigating his most vocal critics as ‘mediocre scientists’. In late June 2021, Loeb is scheduled to speak at Contact in the Desert, which describes itself as the ‘world’s largest UFO conference’, where he’ll share a stage with speakers discussing psychic remote viewing, crop circles and alien abduction.

What’s disturbing to many of Loeb’s (many) critics isn’t the ideas themselves, but the distinctly unscientific certainty with which he presents them to the public. He’s said that the alien hypothesis is ‘much more likely’, and that his ideas are ‘not speculative at all’. But as is so often the case, Loeb’s speculations – which is what they are – are informed by his existing ideas and politics. Loeb is heavily involved with the project Breakthrough Starshot, an engineering initiative founded in 2016 by the billionaire Yuri Milner with one goal: to build a solar sail, and send a probe to another star system. Loeb chairs the project’s advisory committee and, as a result, since 2016 has been a vocal public advocate for solar-sail technology. Around a year later, ʻOumuamua came through our solar system, and ever since Loeb has been on a one-man mission to convince the world that we picked up the trail of an alien solar sail.

Zuckerman’s long-term opposition to immigration and his SETI scepticism are two sides of the same coin

Loeb isn’t the only scientist to fall prey to ideologically motivated speculation: in fact, we can see the same dynamic in the arguments of some of his detractors. In February 2021, the astrophysicist Benjamin Zuckerman of the University of California, Los Angeles published the paper ‘ʻOumuamua Is Not a Probe Sent to our Solar System by an Alien Civilization’. Zuckerman’s overarching theory is simple: ʻOumuamua can’t be an alien spaceship, because aliens simply don’t exist. Zuckerman is an ardent SETI sceptic, and the author of the paper ‘Why SETI Will Fail’ (2002), in which he argues that extraterrestrial intelligence must at least be exceedingly rare, because otherwise they would have come to visit us.

Zuckerman is a controversial figure, for reasons that go beyond the astronomical. As well as being a professor emeritus of astronomy, Zuckerman is an ardent and lifelong anti-immigration activist. There’s a clear philosophical thread running through all of Zuckerman’s ideas: his long-term opposition to immigration and his SETI scepticism are two sides of the same coin. This is made clear in a paper Zuckerman wrote in his book Extraterrestrials: Where Are They? (2nd ed, 1995), a book he co-edited with Michael Hart, a white nationalist who openly advocates for partitioning the United States along racial lines. The paper, ‘Stellar Evolution: Motivation for Mass Interstellar Migrations’ (1985), argues that extraterrestrial civilisations, facing the death of their star, will inevitably end up fleeing their home systems for greener pastures. ‘Massive migration seems the most likely possibility,’ he writes. He also worries about the effect these migrant populations might have on our pristine corner of the Universe: at the end of his paper, he concludes ‘in a crisis such as the death of one’s beloved home star, would the affected society worry about preserving “wilderness areas”?’

The link to Zuckerman’s extreme environmentalist anti-immigration sentiments is obvious. His motivating presumption is simple: the place we live is special, and incoming migrant populations will colonise and pollute it. Applying this principle to his native California, he opposes immigration. Applying it to the stars, it guides the way he imagines other beings. Zuckerman doesn’t conceive of extraterrestrials as wise benefactors coming in peace, or even a warlike force hellbent on destruction. For Zuckerman, extraterrestrials are migrants fleeing tragedy, who will permanently and irreversibly colonise our homeland. And so we reach Zuckerman’s philosophical opposition to SETI: because our solar system remains uncolonised, extraterrestrials must therefore not exist.

The wider astronomical community, it’s fair to say, hasn’t responded warmly to Loeb’s ideas. Sceptical opinions began to roll in soon after ʻOumuamua’s discovery. ‘It’s not a spacecraft,’ said Michele Bannister, an astronomer at Queen’s University Belfast, in a late-2017 interview for WIRED magazine. More up-to-date papers have continued this theme: Jonathan Katz, professor of physics at Washington University in St Louis, published a paper in March 2021, the title of which speaks for itself: ‘ʻOumuamua Is Not Artificial’. Scientists on Twitter were less measured: Benjamin Weiner, an astronomer at the University of Arizona, accused Loeb of ‘bad behaviour’ and ‘misleading the public’.

This critique was thrown into sharp relief earlier this year, during an online discussion hosted by the Instituto de Astrofísica in Chile. The astronomer Jill Tarter of the SETI Institute in California objected to Loeb’s sensationalising, pointing out that he’s bringing the field into disrepute, and accusing him of ‘throwing the entire scientific culture under the bus’. It’s worth noting that Tarter is a legendary SETI pioneer, who’s probably done more to further the search for extraterrestrial intelligence than any other astronomer on Earth. Over her 40-year career, she has tirelessly promoted the quest to find alien life, spending more than a decade as the director of the Center for SETI Research. She was even – in part – the inspiration for Dr Arroway in Contact. All this makes what followed in February 2021 particularly shocking: in a Zoom webinar for the general public on astrophysics, Loeb launched into an attack on Tarter, shouting over the top of her and accusing her of not being supportive enough of his SETI efforts (a field in which she is a veteran, and Loeb a relative newcomer). Loeb’s behaviour drew widespread condemnation, after which he offered a somewhat half-hearted apology.

Loeb’s critics have provided a range of counterpoints to his evangelical advocacy. ʻOumuamua didn’t attempt to slow down to get a better look at us, and was utterly radio-silent. The modest telescope you could fit inside ʻOumuamua would be outclassed by even a modest network of dishes in the alien’s home system, making the 100,000-year (minimum) journey to our solar system rather pointless. Many astronomers have pointed out that these interstellar objects are likely very common in our Milky Way, with some estimating that there could be more than a trillion trillion such travellers spread throughout our galaxy. Several of them probably pass through our solar system every year: if this is the case, then ʻOumuamua suddenly starts to look a lot less special. Indeed, in August 2019 a second interstellar object was spotted in our solar system, a comet that was subsequently named 2I/Borisov.

At some point the speculative dust settles and the hard data arrive

This pushback from the scientific community allows us to zoom out a little, and see the role of speculation in better context. Speculation might well be the creative engine of science, but it’s only when flights of imagination are followed up by intellectually honest, rigorous critique that we have a chance of learning more about our world. Many good ideas started off as wild speculation, but so did countless bad ones. Many ancient Greeks believed that light beams originated from our eyes and, in the late 17th century, the astronomer Edmond Halley thought that Earth might be hollow. The critical thing, and the key to the scientific process, is the ability to sift the good ideas from the bad.

This scientific tug-of-war is just the latest chapter in a long-running dynamic – an argument that has happened before, and will happen again. Look back to a century or so ago, when the origin of our Universe was the biggest question in astronomy. In the late 1920s, the astronomical world was shaken by one of the most surprising discoveries of all time: the Universe is expanding. The prevailing cosmology of the era was that the Universe was unchanging and infinitely old, so to suddenly find themselves in a growing, evolving cosmos was a shock to the system for most astronomers. The expanding Universe seemed to suggest some kind of beginning point when everything kicked off – but what that might look like was anyone’s guess. In the absence of evidence, speculation took flight. The Belgian astronomer and Catholic priest Georges Lemaître in 1927 proposed that the Universe was born from a ‘primeval atom’; in 1948, Fred Hoyle, based in Cambridge, proposed a ‘steady-state model’, where new matter is continuously created as space expands, which allowed the Universe to be infinitely old after all.

But at some point the speculative dust settles and the hard data arrive. In 1964, two researchers detected the ‘echo’ of the Big Bang, and the rest is history. These days, we live in an era of ‘precision cosmology’, in which the age, size and shape of the Universe can be measured to a staggering level of accuracy. Territory that was once a playground for the imagination has been mapped in exquisite detail. Our speculative efforts have moved on to the next set of unknowns: the multiverse, dark matter and dark energy are all beyond the current frontier of knowledge, and finding answers will require our imaginations as much as our telescopes.

On the question of extraterrestrial intelligence, and ʻOumuamua in particular, firm proof has not yet arrived. So what of Loeb, the scientist who cried ‘alien’? As most other astronomers have pointed out, he’s very likely to be wrong. The balance of evidence just isn’t on his side. The strength of science, as a method for learning about our world, is the ability to self-correct when the data come in. But this self-correction often applies only to the field as a whole: individual scientists, when their speculations are not borne out by the evidence, sometimes fail to change their minds.

Hoyle remained staunchly opposed to the Big Bang theory until his death in 2001. If he’d lived in the age of Twitter, he would have been front-page news: ‘Cambridge professor denies the Big Bang’ would make for clickbait just as appealing as ‘Harvard professor says aliens have visited’. But Hoyle was wrong, just as Loeb is probably wrong. Ultimately, here’s the lesson we might draw from all this: because the back-and-forth between speculation and self-critique is the heartbeat of science, it’s misleading when only the first half of that dynamic makes headlines. A media landscape that truthfully represented how science works would champion Tarter more than Loeb.

The deepest questions that SETI has set out to answer might never be solved. If we’re truly alone in the Universe, our destiny might well be to search and search and come up empty-handed. But when it comes to ʻOumuamua, some real data could be close. ʻOumuamua itself might be long gone, but it won’t be long until astronomers have an unprecedented ability to scan the sky for other rogue objects. The Vera C Rubin Observatory in Chile is a futuristic and much more advanced version of Pan-STARRS, and is set to open its eyes to the sky around 2022-23. If it turns out that ʻOumuamua was a one-off, a unique visitor unlike anything else out there, then it might be time to fire up our imaginations once again.

But if, as most theories suggest, ʻOumuamua turns out to be just one of many interstellar visitors that regularly pass through our cosmic neighbourhood, then we can be pretty confident that it was indeed just a lump of inert material. This won’t be the end of the story: it will open up a new frontier of knowledge, as astronomers begin the careful process of studying these interstellar visitors, and discovering what secrets they might hold about their distant homes.

This Essay was made possible through the support of a grant to Aeon from the John Templeton Foundation. The opinions expressed in this publication are those of the author and do not necessarily reflect the views of the Foundation. Funders to Aeon Magazine are not involved in editorial decision-making.