The Universe has a special way of surprising us, over and over again. Discovering the interstellar object (ISO) 1I/ʻOumuamua (pronunciation: “oh-moo-ah-moo-ah”) was a rather big one, in large part because it was found so close to home. Such interlopers—rogue bodies of less than a few kilometers in size that originated outside of our Solar System—had been hypothesized and expected for decades. Assuming that the comets and asteroids of our Solar System are not an anomaly, then the Galaxy should host a population of similar objects. Until late 2017, however, increasingly powerful astronomical surveys failed to identify any ISOs. The outlook on finding such objects became bleak, and the predicted number of ISOs in the Galaxy continued to decrease. 

Then, ʻOumuamua barreled past our planet at just a few times the Earth-Moon distance. In contrast to predictions for ISOs, 'Oumuamua didn’t display the brightness-boosting cometary tail that would make it easily detectable. Because this ISO needed to pass through the 1 part in $10^{29}$ of the Milky Way in which we can see such objects, implies that a vast reservoir of similarly dim ISOs exists in the Galaxy. A back-of-the-envelope estimate shows that there should be around 1000 similar objects inside of Neptune’s orbit at any given moment! 

'Oumuamua was only visible to our telescopes for a few months, and its observed physical and dynamical properties are without comparison in the Solar System. Some scientists initially speculated that it could be an artificial object manufactured by an intelligent extraterrestrial civilization. Here Carl Sagan’s remark that “extraordinary claims require extraordinary evidence” comes to mind. That said, the scientific community has yet to reach a consensus on the origin of 'Oumuamua. This ISO is so remarkable that there isn’t even a singular leading hypothesis among the proposed interpretations. 

'Oumuamua’s physical properties 

We know of more than one million objects in the Solar System of similar size to 'Oumuamua: comets and asteroids. None of these previously identified objects resemble 'Oumuamua. First, 'Oumuamua is shaped like a hockey puck, though half as thick. This geometry is more extreme than any known comet or asteroid. In addition, 'Oumuamua’s color was reddish (like many small Solar System objects), but deep images indicated that it was not shedding micron-sized dust as many comets do. Moreover, the Spitzer Space Telescope didn’t find any evidence of sublimating carbon monoxide (CO) or carbon dioxide (CO$_{2}$) that would form a tail if 'Oumuamua were an extrasolar comet. 

Another 'Oumuamua abnormality was that its inbound speed and direction as it approached the Solar System were strongly correlated with two groups of young stars: the Columba and Carina associations. If this correspondence is taken to imply that 'Oumuamua formed with these star clusters, then 'Oumuamua would be younger than the dinosaurs, 100 times younger than the Earth, and 250 times younger than the Milky Way. Provided that similar ISOs form throughout Galactic history, then only two possibilities exist for 'Oumuamua: Either it is an outlier, or objects of its type have short lifetimes. 

When considering just these properties, 'Oumuamua could be classified as an odd, but not extraordinary asteroid. Its most interesting attribute would probably be simply that it originated from outside of our Solar System, meaning that 'Oumuamua could provide an up-close view of another planetary system. However, a detailed analysis of 'Oumuamua’s path through space revealed that its trajectory cannot be explained by gravity alone. Our Solar System’s comets often experience a similar propulsion when evaporating ices act like the exhaust on a rocket. Newton’s Third Law dictates that every action must have an equal and opposite reaction. Comets typically eject this ice toward the Sun because it is the source of heat, and this “outgassing” process forms the comets’ tails and propels them. Because previous observations of 'Oumuamua ruled out the ices commonly found in comets, this ISO must have rocketed out of the Solar System without the usual propellants. Oumuamua’s physical appearance was similar to an interesting asteroid, but its motion through the Solar System was more that of a comet. 

This discrepancy elevated 'Oumuamua’s standing as a minor curiosity to that of a captivating anomaly. It was clear that 'Oumuamua’s character would place it outside of the known astronomical taxonomy. Many ideas suggested to resolve the 'Oumuamua paradox proposed that it was behaving like a comet, but that the evaporating ices were atypical. Because astronomers weren’t trying to find these “exotic ices” in 'Oumuamua, these substances would have been undetected. Alternatively, others advanced the hypothesis that 'Oumuamua’s propulsion was unrelated to the outgassing mechanism of comets and that its physical resemblance to an elongated asteroid was mostly coincidental. 

Further complicating the debate was the detection of another ISO in 2019: 2I/Borisov. While 'Oumuamua’s origin is still unexplained, Borisov was quickly identified as a comet-like object. Borisov conformed to our previously held expectations for ISOs before 'Oumuamua called them into question. Although Borisov’s closest approach to Earth was twelve times farther than 'Oumuamua’s passage, Borisov’s bright cometary tail made it easily detectable. Astronomical surveys had been sensitive to Borisov-types for decades, yet none had been discovered until this one in 2019. It’s probably just a remarkable coincidence that soon after the appearance of 'Oumuamua, the first canonical ISO was found. Nonetheless, Borisov provides a striking contrast with 'Oumuamua and indicates the existence of a diverse Galactic population of ISOs. 

In summary, a good explanation of 'Oumuamua’s formation history should predict its asteroid-like appearance and comet-like trajectory. With only one known example in 'Oumuamua’s class, this interpretation must assume that generic 'Oumuamua-like objects also have an elongated shape, reddish color, young age, and comet-like propulsion without the normal fuel. Furthermore, the population of these ISOs should be sufficiently large such that it is likely for humans to detect one with our current level of technology. 

Possible explanations 

'Oumuamua’s passage close to Earth implies that similarly-bizarre ISOs far outnumber the conventionally considered Borisov-like interstellar comets This zeroth-order inconsistency with expectations indicates that unconsidered processes could be important in forming these 'Oumuamua-like ISOs. Resolving the 'Oumuamua paradox may lead to considerable insight on the workings of the Universe, much as the surprising discoveries of hot Jupiters, fast radio bursts, and dark energy have spurred tremendous progress in other sub-disciplines in astrophysics. 

In a pair of papers recently written with my collaborators at Yale University, the University of Chicago, and the University of Hawaii, we analyzed two scenarios for 'Oumuamua. First, we examined the only physically-allowable interpretation that could reconcile 'Oumuamua with the comets of our Solar System, a “hypothesis of last resort.” CO ice would have to be the dominant propellant in this case, and 'Oumuamua’s comet-like propulsion would need to be small specifically while the Spitzer telescope was imaging it. Although a benign explanation like CO is a priori compelling, this scenario requires a post facto prescription of the outgassing. The CO hypothesis still has other issues, notwithstanding the necessary fine tuning to fit 'Oumuamua’s trajectory. 'Oumuamua’s elongated shape, lack of a comet-like tail, and inferred young age would all be inconsistent with comets in our Solar System.

Second, we analyzed all of the leading exotic interpretations—nitrogen icebergs, hydrogen icebergs, and ultra-porous dust aggregates—and benchmarked them to an order-of-magnitude occurrence model for ISOs. We asked the question, “if 'Oumuamua were actually any of these compositions, then what are the theoretical upper bounds on the galactic population of similar objects? Would 'Oumuamua’s detection still be statistically-favored?” We found that it’s unlikely for 'Oumuamua to be a nitrogen iceberg, and we showed that the hydrogen iceberg and dust aggregate interpretations have their own flaws. Since none of the possible explanations for 'Oumuamua are satisfactory, we must consider the possibility that this ISO is an anomaly. Perhaps something similar will never be seen again. 

Future interstellar objects 

It seems wholly unsatisfying, but our studies concluded that a “wait-and-see” approach is probably the best way to learn more about 'Oumuamua-like objects. Provided that 'Oumuamua wasn’t a fluke, then forthcoming initiatives such as the Vera Rubin Observatory’s Legacy Survey of Space and Time (LSST) should identify around a dozen similar objects in its forthcoming decade-long survey. With planned follow up studies and potential missions to visit these targets (i.e. ESA’s Comet Interceptor), ISO science has an exciting future. While we wait for these findings to illuminate the broader population of ISOs, it is an opportune time for the Metaculus community to make testable predictions. 

First, we ask “When will the third interstellar object be identified?” This question has been live on Metaculus for some time, although this fortified essay provides a good chance to highlight this prompt. 

Next, we ask “how close to Earth will the next 10 interstellar objects pass?” We ask for this answer in astronomical units (“au”), where a value of 1 corresponds to the average Earth-Sun distance. For reference, the closest approaches by 'Oumuamua and Borisov were 0.16au and 1.9au, respectively. 

If 'Oumuamua-like objects are common, then several ISOs are probably lurking slightly beyond the current realm of detectability. These objects could all be discovered in rapid succession at the beginning of a more powerful all-sky survey. Since the Vera Rubin Observatory will increase the volume in which ISOs can be found by at least a factor of fifty, we ask “how many interstellar objects will be identified by the Vera Rubin Observatory during its first year of scientific operations?” 

Finally, we ask “before 2100, will any object in the Solar System be definitively proven to have a non-human, artificial origin?” If ISOs are markers of extraterrestrial intelligence, then they would resolve one of the most pressing and long-standing questions in scientific history. They would represent a true paradigm shift in humanity’s understanding of the cosmos. 

Conclusion 

More than four years after its detection, the scientific community still lacks a widely-accepted explanation of 'Oumuamua. Only time will reveal the nature of these ISOs. Until the deluge of data from forthcoming observational surveys, we must refine our interpretations of the 'Oumuamua paradox with the available information. We hope that the Metaculus community can be an active participant in this scientific dialogue. Happy predicting!