On the evening of 28 February, a bright fireball seen across northern France and southern England delivered a scattering of fresh meteorites to the Gloucestershire market town of Winchcombe. One of them disturbed the peace of a Sunday evening as it thumped into a family’s driveway. They found it the next morning, looking like a crumbled BBQ briquette left over from last summer. Now on display at the Natural History Museum in London, it has been identified as a carbonaceous chondrite, a near pristine member of a rare class of meteorite whose origins date back to the beginnings of the solar system more than four and a half billion years ago. Such objects are fragile: the scientists of the UK Fireball Alliance who came to pick up the pieces used toothbrushes to scrub the dust from the driveway. The meteorite has yet to receive an official name, but the convention is that meteorites bear the name of the nearest post office to the site of the largest recovered piece.
The find is so valuable because the parent body of the Winchcombe meteorites, until the last few moments before impact, had experienced an entirely uneventful life for nearly five billion years. The fragment represents an unblemished record of conditions at the time the Earth and the other planets of the solar system were forming. Most of the millions of asteroids that lie between the major planets are located in two bands: the Kuiper Belt, which includes Pluto, in the outer solar system; and the main asteroid belt, between Mars and Jupiter, where the Winchcombe meteorites probably came from. Others, known as trojans, share Jupiter’s orbit, and a few hundred have orbits that take them close to the Sun.
More than ten thousand asteroids are known to cross the Earth’s orbit, and completing this list of threatening bodies is astronomy’s most urgent task. We won’t go the way of the dinosaurs, surprised by the sudden glare of a planet-changing impactor hitting the atmosphere: all rocks larger than a kilometre across whose orbits make them a threat to Earth are now being tracked. But the hunt for the enormous number of smaller asteroids that are still big enough to flatten a city has led to the construction of instruments such as the Pan-STARRS telescopes, which scan the sky from the peak of the Haleakalā volcano on Maui.
From its clear perch three thousand metres above sea level, Pan-STARRS covers a significant fraction of the sky every night. Among the millions of background stars that dot the images from its two 1.4 gigapixel cameras, an occasional moving object is detected. Once its orbit is determined, it can be added to the official catalogue alongside rocks named for Rembrandt (asteroid 4511), Janeausten (39415 – spaces in names are not allowed), Zappafrank (3834) and a host of astronomers, sports stars and other luminaries. Living politicians are banned.
The most surprising discovery of recent times came on 19 October 2017, when Pan-STARRS identified an object whose trajectory indicated an origin in interstellar space, far beyond the limits of the solar system. This faint point of light, our first confirmed interstellar visitor, was named ‘Oumuamua (Hawai’ian for ‘the scout from really, really far away’). The name conveys something of the object’s fleeting passage: by the time of its discovery it was already on its way out of the solar system and it disappeared from view less than a fortnight later, lost to the darkness of the Milky Way.
‘Oumuamua never appeared as more than a single pixel, but even the faintest point of light can be informative, especially when it flickers: changes in brightness give clues to an object’s size and shape. The possibility was briefly considered that it might be a flying pancake – wide and flat – but the consensus is that ‘Oumuamua is a flying cigar, at least six times longer than it is wide, tumbling end over end. Its colour seemed similar to some of Pluto’s neighbours in the Kuiper Belt, fitting expectations that any interstellar objects would probably be icy, like comets, though there was no sign on ‘Oumuamua of the coma and tail that comets develop as they pass through the inner solar system.
The combination made ‘Oumuamua unique among well-studied objects, though a similar visitor appears in Rendezvous with Rama by Arthur C. Clarke (asteroid 4923), which imagines an apparently abandoned cylindrical spacecraft entering the solar system. In Clarke’s novel, plucky astronauts make a close-up inspection of Rama and confirm its artificial nature, though its origin and the reason for its visit remain obscure. ‘Oumuamua can’t possibly be as big as Rama – which was thirty miles long – but astronomers inspired by the comparison were quick to consider the possibility that it was an interstellar craft.
One of them was Avi Loeb, a cosmologist at Harvard, who is thoroughly convinced that ‘Oumuamua represents humanity’s first encounter with an alien artefact. Loeb puts the chances of a natural origin for it at a trillion to one, based on three observations. First, as it departed the solar system, ‘Oumuamua sped up under the influence of a force other than gravity, just as Clarke’s Rama does. Second, before its encounter with the solar system, ‘Oumuamua was unusually stationary in relation to local stars, ‘like a buoy resting in the expanse of the universe’, and Loeb believes its unusual trajectory is clear evidence of design. Third, he thinks naturally occurring interstellar objects are rarer, by at least two orders of magnitude, than the discovery of even a single visitor to the solar system would imply.
Is an interstellar craft possible? People have long dreamed of ways to travel between the stars, using a chain of atomic bombs to accelerate, or taking a more gentle voyage on a craft equipped with a light-reflecting sail, which could be artificially accelerated using a powerful laser, sending it shooting out of the solar system at a substantial fraction of the speed of light. Loeb chairs the advisory committee of Breakthrough Starshot, a privately funded project to develop a fleet of probes the size of postage stamps to sail to neighbouring stars. ‘Oumuamua looks to him like a sail at least a hundred metres across, a craft under active control: a larger and more sophisticated version of the system his team wants to build.
No amount of further observation or experiment can tell us whether ‘Oumuamua is an alien spaceship. It is now forever beyond the reach of our telescopes, and no craft we could build would find it in the cold reaches of interstellar space. But I think it probably deserves a place on the very long list of unusual astronomical phenomena that turn out not to be aliens. When Jocelyn Bell Burnell found the repetitive signature of what are now called pulsars in data from her Cambridge radio telescope in 1967 – a discovery that later won her thesis supervisor, Antony Hewish, a share of the Nobel Prize – the signal was named ‘LGM1’, since the only possible source for such regular signals was assumed to be little green men. Bell Burnell remembers being annoyed that aliens were getting in the way of her research. In any case, pulsars turned out to have nothing to do with extraterrestrial life: they are rapidly spinning neutron stars, the remnants of spectacular supernovae.
More recently, when volunteers on the Planet Hunters project I lead found that the star KIC 8462852, now known as Boyajian’s Star, was fluctuating wildly in brightness, a paper in the Astrophysical Journal explained the observations by suggesting that a vast fleet of megastructures might be orbiting it – as good a hypothesis as any at the time. The notion that a sufficiently advanced civilisation would surround its star with orbiting solar panels first appeared in Olaf Stapledon’s novel Star Maker (1937), which inspired Freeman Dyson to develop the idea of the Dyson sphere, designed to capture as much of a star’s energy as possible. Alas, the behaviour of Boyajian’s Star turns out to be the result of dust in its solar system. Gamma-ray bursts – first observed in 1967 by US satellites looking out for signs of Soviet nuclear weapons tests in space – are caused by colliding neutron stars in distant galaxies rather than starships engaging their warp drives. Mars’s moons, Phobos and Deimos, are low in density because they contain water ice, not because they are hollow starships. Recent excitement about ‘perytons’, bursts of radio waves detected at Australia’s Parkes Observatory and nowhere else, died down when they were traced to a malfunctioning microwave oven in the observatory’s kitchen.
Systematic efforts by the Search for Extraterrestrial Intelligence (SETI) have generally made use of radio telescopes, on the grounds that radio provides a low-cost way to communicate between the stars, or to announce your presence to the cosmos. Since 2016, Breakthrough Listen, a companion project to Loeb’s Starshot, has been listening for signals from stars with planets that might provide a suitably temperate home for our kind of life, and China’s new FAST radio telescope also has an active SETI program. But fifty years of searching has yielded nothing, a silence that seems to demand explanation.
Even where conditions are right, it’s possible that life is hard to get started, or that if simple life is common the odds of producing intelligent life are nearly impossibly long. Or perhaps we exist in a cosmic nature reserve, with a grand galactic civilisation committed to leaving our primitive selves in peace. Or perhaps, as Cixin Liu’s science fiction trilogy The Three-Body Problem suggests, revealing your presence to a hostile cosmos results in your inevitable destruction, so sending messages into space may be something no sensible civilisation would do. If so, we may come to regret the 2008 advert for Doritos transmitted by one of the most powerful radar systems on the planet towards a star in the constellation of Ursa Major.
Or perhaps we just need to be patient. The universe is large, and signals are presumably rare. By some estimates, we have examined about a quintillionth (10-18) of the Milky Way, the equivalent of exploring the Atlantic by peering into a rock pool. We can increase our chances of finding something by looking not only for radio waves, but for ‘technosignatures’, the effect of intelligent activity on its surroundings. The orbiting solar panels proposed as an explanation for Boyajian’s Star are one example: the waste heat generated by such structures should make them glow brightly in the infrared spectrum. Astronomers have searched for flashes of interstellar laser-powered communications, clusters of satellites around planets and even the dust produced by alien mining fleets, all without success. ‘Oumuamua as a spacecraft would be a fine technosignature. Distant civilisations, even those that have reached only our level of technology, would be aware if they looked at it that our solar system contains a planet with conditions suitable for life. If they’re a little more advanced than us they could detect the oxygen in the Earth’s atmosphere and know that it’s produced by plants, and perhaps the carbon dioxide we’ve pumped out, which would betray the presence and maturity of Earth’s intelligent inhabitants. If they know we’re here, it seems plausible that they would dispatch an interstellar probe to investigate our solar system, which would presumably send back messages about what it finds. Telescopes on Earth listened out for any broadcasts ‘Oumuamua may have made, but there was silence.
Betting on ‘Oumuamua’s artificial nature may encourage us to find our own ways of exploring the cosmos. Loeb argues that we should aspire to a deliberate panspermia, the spreading of life – our kind of life – through the galaxy. The idea was first promoted by another cosmologist who strayed into neighbouring fields, Fred Hoyle (asteroid 8077). In 1946, Hoyle introduced the theory of nucleosynthesis, which explains how stars work. In 1972, he helped found the Institute of Astronomy at Cambridge, but he walked out soon afterwards in a dispute over management, leaving him free to expand his work in new directions. Along with his colleague Chandra Wickramasinghe he was soon arguing not only that life had arrived on Earth from space but that it was still arriving. In Diseases from Space (1979), they tried to show that flu, whooping cough and the common cold could all be explained by an influx of space-borne viruses, using data from school attendance in England and Wales as evidence. (Wickramasinghe is still at it, last year claiming that Sars-CoV-2 came from space.) Unable to find a scientific audience for his fringe ideas, and perhaps in need of income, Hoyle wrote a popular book on panspermia. A few years later he stirred up a row with the Natural History Museum, whose Archaeopteryx fossil he believed, groundlessly, to be fake.
In Extraterrestrial, Loeb doesn’t stray as far as palaeontology, though he does advocate ‘space archaeology’ in the form of an expanded search for technosignatures. But he fails to mention those who led the scramble to observe ‘Oumuamua: people who have spent their entire scientific lives studying asteroids, none of whom thinks it is anything other than a natural phenomenon. From the perspective of observational astronomers, the oddest aspect of ‘Oumuamua is that we know so much about such a tiny thing: few bodies of similar mass in our own solar system have been considered significant enough to study in detail, which makes comparison difficult. Recent work suggests that ‘Oumuamua’s tumbling may be the consequence of a violent beginning in its natal solar system, and the acceleration that Loeb insists is unnatural – summoning the spirit of Galileo (asteroid 697), the patron saint of scorned scientists with fringe ideas (‘And yet it accelerated’) – is just the kind of thing that happens to comets when sunlight warms their icy surfaces. Most important, no one, not even Loeb, has been able to propose a shape for the object that accounts for what was seen from Earth while still allowing it to be a functioning solar sail.
The search for life elsewhere propels rovers across Mars and prompts the construction of vast new telescopes. But ‘Oumuamua doesn’t have to be a spaceship to be fascinating. If it is a natural object, then the simple fact of its arrival suggests that objects like it must be staggeringly common, with at least a trillion trillion (1024) of them in the Milky Way. A second, larger visitor, Borisov, entered our solar system in 2019, though it looked and behaved much more like a normal comet. Given how many interstellar objects we can now assume there to be, one of them is more likely than not to be travelling undetected through the solar system as you read this – a thought that inspired Susanne Pfalzner and Michele Bannister (asteroid 10463) to suggest that such objects will inevitably pass through and be captured by the discs of gas and dust that exist in the early stages of solar system formation. It seems plausible that the planet-creating process is accelerated by this rain of interstellar debris. In which case the existence of Earth may depend on the arrival long ago of ‘Oumuamua’s distant cousins. Reason enough to pay close attention to faint points of light moving slowly among the stars.