'Oumuamua: The First Visitor From Another Star Still Defies Explanation
On the night of 19 October 2017, a robotic telescope on the summit of Haleakala, the dormant volcano that dominates the Hawaiian island of Maui, logged one more faint smudge among the thousands it records on a clear night. When the astronomer Robert Weryk sat down to inspect the Pan-STARRS1 images, the dot refused to make sense. It was moving too fast, and along a path that could not be bent into any orbit closing back on the Sun.
Weryk checked, checked again, then called colleagues. Within days the measurement was confirmed: the object had not come from our solar system. It had fallen in from interstellar space, rounded the Sun around 9 September at roughly 87 kilometres per second, and was already climbing back out, on a trajectory that would never bring it back. Astronomers had expected such visitors for decades, because every young planetary system should throw debris into the galaxy, but until that week not one had ever been caught.
It was very nearly missed. By the time Weryk found it the object was already receding from Earth and fading fast, which left observatories a matter of weeks to gather everything they could. It was catalogued as 1I/2017 U1, the numeral and the letter marking it as the first interstellar object on the books, and the discovery team gave it a Hawaiian name they translated as a scout or messenger reaching out from the distant past: 'Oumuamua.
Even to the largest telescopes it was never more than a point of light, far too small and too distant to resolve into a disk, so its shape had to be read from the way its brightness changed. And it changed dramatically, swinging by a factor of about ten, roughly two and a half magnitudes, as it turned once every seven to eight hours. A body whose glow rises and falls that much as it rotates must be extremely elongated, with one axis at least six times the others and by some estimates closer to ten to one. Estimates of its length ranged from about one hundred to four hundred metres. Nothing among the millions of catalogued asteroids and comets in our own solar system has that profile. Early reconstructions pictured a dark cigar; later analyses of the same light curve favoured a flattened sliver closer to a disk. It was also not spinning cleanly about a fixed axis but tumbling, turning end over end, which usually indicates a collision or a violent origin far in the past.
Its colour was a deep red, the shade that ices and organic compounds acquire after long exposure to cosmic rays, and that is what one expects of a body from deep space. But it showed no other cometary behaviour at all. There was no coma, no tail, no glowing envelope of gas and dust. When NASA turned the infrared eye of the Spitzer Space Telescope toward its predicted position, Spitzer recorded nothing, a non detection sensitive enough to cap any dust production at a trickle. To every instrument that examined it, 'Oumuamua looked like a bare, inert rock.
Then came the measurement that reopened the case. In June 2018 a team led by Marco Micheli published in Nature a careful reconstruction of the object's path, based on ground based data and on Hubble observations that tracked it until January 2018. It had not coasted away on gravity alone. As it receded it was gaining a small extra push outward, an acceleration of about 4.9 millionths of a metre per second squared, which weakened with distance from the Sun roughly as sunlight itself weakens. By the team's reckoning the detection was around thirty sigma, which is about as statistically firm as measurements in this field get.
For a comet, such a push is routine. Sunlight turns ice to vapour, the vapour escapes in jets, and the recoil nudges the nucleus like a very slow rocket. But comets betray themselves while they do it. The escaping gas drags out a visible cloud of dust and leaves a chemical signature that spectrographs can read. 'Oumuamua produced none of it: no visible jets, no dust, no coma, and no carbon based gases where Spitzer would have detected them. A second difficulty sits underneath the first. To produce that acceleration by outgassing, the object would have had to shed a substantial fraction of its own mass as vapour, yet its tumbling remained steady, when jets strong enough to move it should have measurably changed its spin.
By the spring of 2018 the object was beyond the reach of every telescope on Earth and in orbit. It is now travelling outward past the orbits of the outer planets and will never be observed again. Everything anyone will ever know about 'Oumuamua was gathered in about eleven weeks by instruments that were not designed for the job.
Two more interstellar visitors have been found since. Comet 2I/Borisov, discovered by the amateur astronomer Gennadiy Borisov in 2019, looked entirely ordinary: a proper coma, a proper tail, textbook chemistry. A third object, comet 3I/ATLAS, was picked up in 2025. The Vera C. Rubin Observatory in Chile, built to survey the whole southern sky night after night, is expected to turn such discoveries from once in a decade events into routine catalogue entries.
Conclusions and Open Questions
The mainstream position is that 'Oumuamua was a natural body, and several specific natural models compete to explain the acceleration without a tail. Each is credible, and each invokes something never directly observed.
One theory, proposed by Darryl Seligman and Gregory Laughlin, holds that 'Oumuamua was an iceberg of frozen molecular hydrogen, releasing a gas so cold and transparent that no telescope could see it. That would explain the push with no visible tail. Its weakness is that solid hydrogen is hard to form and harder to keep frozen across a journey between stars, and critics argue such a body would evaporate long before arrival.
Alan Jackson and Steven Desch argued in 2021 that it was a shard of nitrogen ice broken off the frozen surface of a Pluto like world in another system, made of the same material that coats Pluto's plains. Its weakness, raised by other researchers, is one of supply: the galaxy may not contain enough Pluto like worlds, or enough violent impacts on them, to produce a fragment of this size and send it our way.
The most economical proposal came in 2023, when Jennifer Bergner and Darryl Seligman published in Nature the idea that 'Oumuamua was an ordinary water ice body, a so called dark comet, in which cosmic rays had converted surface water into trapped hydrogen that seeped out quietly as the Sun warmed it, again producing no dust. Its weakness is that the mechanism, while laboratory plausible, has not been directly observed operating on any object, and the required conditions are specific.
A clearly minority position, argued by the Harvard astrophysicist Avi Loeb with his postdoctoral researcher Shmuel Bialy in The Astrophysical Journal Letters, is that the pressure of sunlight alone could account for the acceleration if the object were extraordinarily thin, a sheet perhaps a fraction of a millimetre thick with a great deal of area and very little mass. Nothing in nature is built that way, but human engineers have proposed exactly that geometry for a light sail. Loeb has been explicit that he has no proof, and argues only that an artificial origin, such as a defunct probe or a discarded fragment of technology drifting on starlight, should not be ruled out by assumption. Most astronomers reject it, and the objections are serious: a wafer thin sail tumbling every eight hours should flex and shift its brightness in ways the light curve does not clearly show, and the prior odds of a dead artefact passing this close are extremely small. Because the object is gone, no direct evidence can now settle it either way.
What remains unexplained is narrow but real. No accepted model yet accounts, with observed processes alone, for an acceleration of that size with no dust, no detected gas and no change in the tumbling. The extreme axis ratio also has no counterpart in our solar system.
The open questions follow from that. Was 'Oumuamua an unusual object, or an ordinary piece of interstellar debris that only looks strange because it is the only one we have seen up close? Would a nitrogen shard or a hydrogen iceberg still seem exotic with a hundred examples to compare? Will Rubin Observatory find more objects that accelerate without a tail, or was this one a genuine outlier? The honest answer for now is that we do not know what passed through the solar system in the autumn of 2017, and that a single, briefly observed example may never be enough to decide. It came from another star, and it took its answer with it.