In late 2017, astronomers spotted something unprecedented: an object from another star system zipping through our own. They named it ‘Oumuamua, a Hawaiian word meaning “scout.” Its strange behavior and unusual characteristics defied easy explanation, sparking a fierce debate. Was it a natural, albeit bizarre, interstellar rock, or the first confirmed piece of alien technology? This visitor didn’t linger, and as it sped away, it left a trail of tantalizing questions that scientists are still grappling with today, pushing the boundaries of what we thought we knew about the cosmos.
1. A Messenger From Afar
The name ‘Oumuamua (pronounced “oh-MOO-ah-MOO-ah”) was chosen by the Pan-STARRS telescope team in Hawaii that discovered it. It aptly translates to “a messenger from afar arriving first.” This was the very first interstellar object ever detected passing through our solar system, making it a landmark discovery. Its formal designation is 1I/2017 U1, with the “I” standing for interstellar, a new category created just for it. This naming convention highlights its unique and trailblazing status in astronomical observation. (NASA)
2. A Bizarre Shape
‘Oumuamua’s shape is perhaps its most bewildering feature. Based on its dramatic changes in brightness, astronomers concluded it was highly elongated, possibly ten times longer than it was wide, shaped like a cigar or a flat pancake. This is unlike any known asteroid or comet in our solar system, which are typically much more spherical or potato-shaped. This extreme dimension immediately set it apart and fueled speculation about its artificial origin. (European Southern Observatory (ESO))
3. Speeding Through the System
This interstellar visitor was moving incredibly fast. ‘Oumuamua entered our solar system at a velocity of about 26 km/s (58,000 mph) relative to the Sun. Its trajectory was a hyperbola, meaning it was moving too fast to be captured by the Sun’s gravity. This high speed confirmed it was not from our solar system but was just passing through on its journey through the Milky Way galaxy. (Jet Propulsion Laboratory (JPL))
4. The Missing Coma
Typically, when a comet gets close to the Sun, its ice heats up and creates a visible halo of gas and dust called a coma. Despite passing well within the orbit of Mercury, ‘Oumuamua showed no such activity. Telescopes saw no evidence of outgassing, which was puzzling. If it were an icy comet, it should have had a coma. This lack of a tail suggested it was either a dry, rocky asteroid or something else entirely. (Nature Astronomy)
5. A Mysterious Push
The biggest mystery surrounding ‘Oumuamua was its “non-gravitational acceleration.” After slingshotting around the Sun, it sped up more than the Sun’s gravity alone could explain. This slight but significant push away from the Sun was unexpected. For comets, this push is caused by outgassing acting like a thruster, but as mentioned, ‘Oumuamua had no visible coma. This discrepancy became the central point of the debate about its nature. (Hubble Space Telescope)
6. The Alien Tech Hypothesis
Harvard astronomer Avi Loeb famously proposed a radical explanation for the strange acceleration: ‘Oumuamua was an artifact of an alien civilization. He suggested it could be a solar sail—a thin, lightweight craft propelled by the pressure of sunlight. This would perfectly explain the push it received without any outgassing. While controversial, this hypothesis directly addresses the object’s anomalies in a way that natural explanations struggle to.
7. The Nitrogen Iceberg Theory
One of the leading natural explanations suggests ‘Oumuamua was a chunk of frozen nitrogen, chipped off a Pluto-like “exo-planet” in another star system. As it neared our Sun, the frozen nitrogen would have vaporized, providing the gentle push without creating a visible dust coma (as pure nitrogen gas would be invisible to telescopes). This theory provides a plausible, non-alien explanation for the object’s strange behavior. (Journal of Geophysical Research: Planets)
8. A Hydrogen Iceberg?
Another theory, proposed before the nitrogen iceberg idea, was that ‘Oumuamua was a “hydrogen iceberg.” Made of frozen molecular hydrogen, it would be incredibly lightweight and would sublimate (turn to gas) very quickly when warmed by the Sun, explaining the acceleration. However, this idea has been heavily criticized, as such an object would likely have evaporated long before it ever reached our solar system during its vast interstellar journey.
9. A Tumbling Trajectory
Oumuamua wasn’t gracefully spinning on a single axis. Instead, it was tumoring chaotically through space. This tumbling motion caused its brightness to vary dramatically, by a factor of ten or more, every 7 to 8 hours. This erratic rotation provided the crucial data that led to the conclusion about its highly elongated shape, but the cause of its wild tumble remains unknown—perhaps it was the result of a long-ago cosmic collision. (Nature Astronomy)
10. A Dark, Reddish Hue
Observations revealed that ‘Oumuamua had a dark, reddish color. This is similar to the colors of many objects in the outer solar system, such as those in the Kuiper Belt. This hue is thought to be the result of millions or billions of years of exposure to high-energy cosmic rays, which alter the chemical composition of an object’s surface. This suggested ‘Oumuamua was very, very old.
11. Listening for a Signal
The SETI (Search for Extraterrestrial Intelligence) Institute and the Breakthrough Listen project used powerful radio telescopes, like the Green Bank Telescope, to listen for any potential signals coming from ‘Oumuamua. If it were an alien probe, it might be transmitting something. After scanning across multiple radio bands, they found no evidence of artificial signals, reporting only silence from the mysterious visitor. This lent more weight to the natural origin theories. (Breakthrough Initiatives)
12. A Fleeting Glimpse
Scientists had a very short window to study ‘Oumuamua. It was already speeding away from the Sun when it was discovered, and it was only observable by telescopes for a few weeks before it became too faint to see. This brief encounter meant that many questions, such as its exact shape and composition, could not be definitively answered, leaving plenty of room for speculation and debate. (The Planetary Society)
13. The Cosmic Dust Bunny Idea
Another quirky but plausible theory is that ‘Oumuamua was an extremely porous and lightweight object—a sort of “cosmic dust bunny” or fractal aggregate. Such a low-density object would be easily pushed by solar radiation pressure, explaining its acceleration without outgassing. While this type of object has not been observed before, it could theoretically form in the outer layers of a protoplanetary disk around another star. (University of Oslo News)
14. We Have a Second Visitor
In 2019, just two years after ‘Oumuamua, a second interstellar object was discovered: 2I/Borisov. Unlike ‘Oumuamua, Borisov exhibited the characteristics and behavior typical of a regular comet. It had a visible coma, and its path was explained purely by gravity. The discovery of a “normal” interstellar object made ‘Oumuamua seem even stranger by comparison, highlighting just how anomalous its characteristics really were. (NASA)
15. The Legacy of ‘Oumuamua
Regardless of its true nature, ‘Oumuamua has had a profound impact on astronomy. It proved that interstellar objects do visit our solar system and prompted the development of new survey programs, like the upcoming Vera C. Rubin Observatory, to find more of them. It has challenged our understanding of planet formation and forced scientists to consider more possibilities—both natural and artificial—when studying the cosmos. (Sky & Telescope)