One night in October 2017, a telescope in Hawaii is carrying out its routine task of scanning the night sky. The following day, one of the researchers discovers a small, and entirely unexpected, point of light in the images. This marks the beginning of an imaginative quest for answers about the mysterious celestial body that has suddenly appeared.
'It quickly became clear that this was a very unusual object', says Jens Hoeijmakers, astrophysicist at Lund University.
Theories and speculation soon gathered pace. Was it a previously unseen comet? An unknown asteroid? A spacecraft built by intelligent extraterrestrials?
Mysterious acceleration
The object appeared to have an odd shape, either very elongated or possibly disc-like. Its size was estimated to be only about one hundred metres. Moreover, the celestial body behaved in a remarkable way, displaying an unexpected acceleration as it moved away from the Sun after passing our planet.
'The object moved like a comet, but did not look like one, as it had no tail of evaporating ice', says Jens Hoeijmakers.
Based on calculations of its speed and trajectory, researchers were soon able to conclude that the object did not originate in our own solar system but was a distant visitor from another system, from some unknown location in the galaxy. An interstellar guest, which was given the name ‘Oumuamua – Hawaiian for 'a distant messenger arriving first'.
The discovery was unique. Never before had humanity been able to study, from close range, an object that had arrived from another solar system.
Jens Hoeijmakers has followed the search for clues about ‘Oumuamua with great interest. Recently, he delivered his associate professorship lecture on this interstellar visitor. In his own research, he studies, among other things, exocomets, comets that exist in other solar systems and therefore orbit stars other than our Sun. A number of known stars have comets that produce extremely bright tails, which Hoeijmakers and his colleagues can study despite the vast distances.
As for ‘Oumuamua, he notes that after nine years of searching for more knowledge about this peculiar visitor, the scientific community still faces many unanswered questions. Hoeijmakers explains that researchers have struggled with different explanations for the object’s unexpected acceleration.
One proposed explanation is based on the general fact that evaporating gas can cause acceleration, as seen in comets when their frozen water surfaces vaporise during close passages by the Sun. However, since ‘Oumuamua lacked a classic comet tail, it could not have contained such evaporating frozen water. Instead, researchers have considered whether the acceleration might have been caused by another gas, one that cannot be detected technically using spectroscopy.
'For example, if the object contains frozen nitrogen or frozen hydrogen instead of frozen water. But those theories have problems for various reasons', says Jens Hoeijmakers.
Another possibility is that ‘Oumuamua’s unexpected acceleration was caused by radiation pressure from our Sun. Hoeijmakers explains that such pressure can arise because sunlight carries both energy and momentum. When light particles strike an object, they transfer energy to it, which may affect its motion. However, this force is very small, and for radiation pressure to have a significant effect, the object must be very light and exposed to the radiation over a long period.
The UFO theory
Theories have also been put forward – even by prominent astronomers – that this interstellar oddity could be a UFO. Could ‘Oumuamua’s acceleration be evidence that the object was manufactured by intelligent extraterrestrials?
'That theory does not hold up very well, as a simple calculation shows that there should in fact be thousands of such interstellar objects visiting our solar system', says Jens Hoeijmakers.
The calculation he refers to is based on a combination of probability theory, the velocities of celestial bodies, and distances in space. When discovered, ‘Oumuamua was travelling at 26 kilometres per second, meaning it takes about ten years to traverse the part of our solar system inside Neptune’s orbit.
Since it is considered likely that, on average, three new interstellar objects enter our solar system every day, this implies that roughly 10,000 such interstellar bodies are continuously present within Neptune’s orbit. This number does not lend support to the UFO theory, according to Hoeijmakers.
'The latest conclusion instead is that ‘Oumuamua is what is known as a dark comet', he says.
Dark comets are celestial objects that were only defined as a scientific class as recently as 2023. A dark comet exhibits the typical behaviour of comets, namely that it accelerates when passing a star, but lacks a comet tail, meaning its increased speed cannot be explained by the evaporation of frozen water but must be due to some other unknown factor. These dark comets are therefore, by definition, not true comets, but rather a distinct group of asteroids, Hoeijmakers explains.
Broadening the field of view
‘Oumuamua played a key role in drawing scientific attention to this group of unusual celestial objects. Over the past three years, several dark comets have been discovered. All of these orbit within our own solar system, unlike ‘Oumuamua, which was merely a visitor from another system and will never return.
'It was purely by chance that ‘Oumuamua passed close enough to our planet to be detected', says Jens Hoeijmakers.
Following its visit, researchers have naturally become eager to discover more distant objects from other solar systems. In recent years, two additional interstellar visitors have been found: 2I/Borisov in 2019 and 3I/Atlas in 2025. Unlike ‘Oumuamua, these newcomers behaved more like typical comets in our solar system, with visible tails.
'We do not know where they originally come from, but we can trace back which stars they have passed', says Jens Hoeijmakers.
The fact that scientists now estimate that thousands of such interstellar objects are temporarily visiting our solar system has led to serious efforts to detect and study more of them. According to Hoeijmakers, new opportunities are now opening up through the recently built Vera Rubin Observatory in Chile.
Located high on a mountaintop, the observatory will use the world’s largest digital camera to take detailed images of the night sky over a ten-year period. In addition to studying dark matter and dark energy, as well as exploding stars and black holes, the observatory will also help scientists discover visitors from other solar systems.
'In the near future, we may be able to speak of interstellar objects as fairly common guests rather than rare ones', says Jens Hoeijmakers.
The Vera Rubin Observatory began operations a year ago, and since February it has been issuing regular alerts about newly detected objects, enabling astronomers around the world to carry out follow-up observations.
For ‘Oumuamua and its interstellar companions, however, all this curiosity and scientific pursuit is of little consequence. These distant visitors continue their journeys through the vast, silent expanse of space, each on its own path – heading towards new galactic adventures.
Where is ‘Oumuamua now?
‘Oumuamua is currently travelling on the outskirts of our solar system, beyond the planet Neptune and the Kuiper Belt. Because the outer regions of the solar system are extremely vast, it will not leave the solar system entirely until 2038.
Its exact position can be followed on the website TheSkyLive, where one can track in real time the growing distance, kilometre by kilometre, between us on Earth and ‘Oumuamua.
Contact
Jens Hoeijmakers, Associate senior lecturer
Department of Physics, Lund University
jens [dot] hoeijmakers [at] fysik [dot] lu [dot] se (jens[dot]hoeijmakers[at]fysik[dot]lu[dot]se)
Jens Hoeijmakers' profile in the Research Portal.
The article was translated using the AI tool Copilot.