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'Oumuamua: Alien Spacecraft or Dark Comet?
Six years after we found it, astronomers finally have an idea of what 'Oumuamua was
At first we thought it was a comet. That made sense: comets usually come from very far away, and this one seemed to have come almost from the stars themselves. Yet the more we looked at it, the stranger things got. This, if it really was a comet, was a comet unlike any other seen: a comet so bizarre and ill-formed that it looked far more like a chunk of solid rock or metal, or, some thought, a piece of alien technology.
The nature of this object, dubbed ‘Oumuamua, has puzzled astronomers for years. When it was first spotted, in 2017, it soon became clear that it had entered our solar system from interstellar space - the first known object to do so. Yet it also became clear that it had a strange shape; looking, by one estimation, like a long thin cylinder, or by another, like a tumbling pancake.
It behaved in a puzzling way too, spinning so fast that researchers struggled to explain why it hadn’t ripped apart, and accelerating away from the Sun in a way that defied gravity. Telescopes trained on ‘Oumuamua saw none of the usual tails or halos a comet would show, especially one so venturing so close to the Sun. Yet an asteroid, if it were one, would have moved differently, more ponderously, through the night sky.
The puzzle of ‘Oumuamua was reflected in its name. When it was discovered, astronomers categorised it under “C”, for comet, labelling it C/2017 U1. But when observations failed to spot any hint of a tail or halo of gas around it, they decided to recategorise it under “A”, for asteroid. C/2017 U1 thus became A/2017 U1, the first object ever to move from one category to another.
Then, frustrated by its uncertain nature, the Minor Planet Centre decided to take things a step further. ‘Oumuamua would not just change letters, it would gain an entire new category all to itself. All objects coming from interstellar space are now labelled “I”, for interstellar. A/2017 U1 thus became the first object to enter this category, taking the designation 1I/2017 U1. At the same time astronomers settled on a less technical moniker for the object, giving it the name ‘Oumuamua, an Hawaiian word meaning something close to scout, or messenger.
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Though ‘Oumuamua likely spent a century or more falling towards the Sun, our window of opportunity to watch it proved short. By the time we found it, in October 2017, ‘Oumuamua was already heading away from us, shooting towards interstellar space. Within weeks it had faded from view, invisible to even our most powerful telescopes.
Those few weeks of observations, however, turned out to be rather controversial. Its brightness suggested its length lay somewhere between one hundred and one thousand metres. Yet that brightness varied radically as it spun; a pattern that could only be explained if ‘Oumuamua had an unusually varied surface - which seemed unlikely - or if it was shaped like a long thin cigar, or, possibly, a pancake.
That strengthened the view that ‘Oumuamua is something exotic. No other comets or asteroids look remotely similar and theories struggle to explain how such shapes could even form. Perhaps, one researcher suggested, it was a long icy shard; a fragment of some long destroyed planet. Another concluded that it was a piece of interstellar “fluff”, a wisp of ice and dust formed in some distant cosmic cloud.
Speculation only got more frenzied when astronomers dropped another bombshell. ‘Oumuamua, they reported, was accelerating away from the Sun, a movement that couldn’t be explained by the laws of gravitation. Some extra force, some mysterious additional thrust, had boosted ‘Oumuamua as it headed back towards the stars.
Normally these kinds of accelerations can be explained in two ways. The first comes from the pressure of sunlight: a slight push that occurs as objects reflect light. This pressure is small, but significant enough that satellite operators must account for it as they plot out orbits. Asteroids and comets are subject to the same pressure. Yet unless ‘Oumuamua was far lighter and more reflective than believed, it didn’t seem enough to explain the motion astronomers were seeing.
The other explanation, however, seemed equally implausible. Comets often accelerate away from the Sun, propelled by ice vaporising from their surfaces and streaming away into space. This acts much like spacecraft thrusters do, giving the comets a slight push in one direction. Yet, of course, observations of ‘Oumuamua saw absolutely no sign of a tail, apparently ruling the idea out.
‘Oumuamua, then, was not only strangely shaped, but somehow moving under an undetected force. How curious, Harvard astronomer Avi Loeb noted, that the first object we happened to spot from interstellar space looked and behaved so oddly. Could there be more to the story than we realised?
What, he posed, if ‘Oumuamua was not a natural object at all, but some kind of artificial creation? How could we tell? What would it look like? Might ‘Oumuamua be a piece of alien technology, just happening to pass through our solar system?
The possibility was slim, he admitted at the end of 2017, but ‘Oumuamua was so strange that it was worth checking out anyway. To that end he directed the Green Bank radio telescope to scan the object, probing it for artificial signals. That, of course, found nothing, but Loeb was far from deterred.
A year later he published a paper arguing that an artificial origin explained how ‘Óumuamua had unexpectedly accelerated away from the Sun. If it were light enough, and made of some kind of reflective material, then the pressure of sunlight alone would have pushed it onto a faster orbit. ‘Oumuamua, he wrote, could have been an alien light sail: a spacecraft designed to use starlight to guide its voyage through the galaxy.
He conceded that the evidence showed ‘Oumuamua was unlikely to be an active spacecraft. It was tumbling, spinning out of control, and on a far from optimal orbit. Instead, he later argued, it was possibly a piece of alien space trash, an ancient relic discarded by an advanced civilization.
Such things are not completely unknown. In 2020 space telescopes picked out another object showing an unexpected acceleration away from the Sun. It too lacked the tail of a comet, and seemed to be unusually reflective. And, sure enough, researchers eventually realised it really was artificial: a piece of an old rocket body, tumbling its way through space. Though that object was human made, Loeb argues that the parallels to ‘Oumuamua are too strong to ignore.
In an article in Scientific American, Loeb compared the survey of interstellar objects to a man walking along a beach. As he walks he picks up pebbles, most of which are obviously natural in shape. But every now and then he finds an old bottle, a relic of our industrial civilization. If space is really teeming with intelligent civilizations, he notes, then similar relics of their presence should be all around us.
Loeb is no crank scientist. As an astronomer he is highly regarded, with an impressive record of research and a senior position at Harvard University. Yet his theory of an alien spacecraft was met with skepticism in the scientific community. There is no strong proof that ‘Oumuamua is really artificial, apart from its rather unusual characteristics. And aliens, most astronomers will tell you, will need extraordinary evidence to prove their presence. An odd looking interstellar comet is far from enough for that.
Yet if ‘Oumuamua is not a normal comet, and not an alien spacecraft, then what is it? One idea, recently boosted by a new paper, is that of a “dark comet”; an object that behaves like a comet, but produces a tail that we cannot see. That would explain its acceleration away from the Sun and, perhaps, how it ended up with such an odd shape.
Though it's a nice idea, astronomers have struggled to come up with plausible ideas for dark comets that could survive millions of years in interstellar space. One idea, nicknamed the hydrogen iceberg theory, suggests that ‘Oumuamua formed in an interstellar cloud from frozen chunks of hydrogen. As it approached the Sun, and began to melt, an invisible plume of hydrogen gas would have streamed out from it, giving it a slight acceleration.
Yet calculations by Loeb showed this was unlikely. Though hydrogen icebergs could possibly form in the coldness of interstellar space, the faint heat coming from distant stars would prove too much for them to last long. ‘Oumuamua, if it were really an iceberg, would never have made it anywhere close to Earth.
Perhaps it was with relief, then, that astronomers recently embraced a new, far simpler, theory of ‘Oumuamua’s origins. The idea, summed up in a paper published in Nature, posits that ‘Oumuamua long ago formed as a normal comet in an alien solar system.
At some point, however, something - perhaps a passing star, perhaps a close encounter with a planet - happened to knock ‘Oumuamua away from its parent star. At first it would have kept its comet-like appearance, but as time went by and it drifted in the harshness of interstellar space, it would have gradually started to change.
Its ice, under bombardment from high-energy radiation, would have begun to break down, forming bubbles of frozen hydrogen. In the coldness of interstellar space, these bubbles would have remained trapped within the comet. But under the heat of the Sun, once ‘Oumuamua entered our solar system, those bubbles would have escaped, streaming out in an invisible tail and thus exerting a slight push on the comet.
That would explain the unexpected acceleration, and - since much of the comet would have eroded away - perhaps its odd shape as well. Even better, the effect may be easy to prove. The paper suggests that it should take place in almost all comets, especially those that spend long periods at the fringes of the solar system. But because it is a small effect and the resulting hydrogen tail is hard to see, we have probably missed it in the past. Careful studies of the next few comets to pass by the Sun should show if such tails really exist or not.
Loeb, however, has not been so receptive. In a response paper, he argues that the study had missed a crucial factor. As hydrogen gas escapes from a comet, it should cool its surface. That, he writes, would reduce the amount of gas escaping and, therefore, limit how much acceleration it can cause. The strange movements of ‘Oumuamua, he concludes, are too big to be explained in this way.
Whatever the truth of the matter, it is unlikely to be settled anytime soon. ‘Oumuamua is too far gone for further observations. It now lies further from Earth than Pluto, a small rock adrift in the vastness of space. Only with the arrival of other interstellar objects - of which many are undoubtedly coming - will we have the opportunity to learn more about their origins. All eyes, surely, will be on the next object to pay us a mysterious visit from the stars.
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