1I/ʻOumuamua

1I/ʻOumuamua is the first confirmed interstellar object ever detected passing through our Solar System. An interstellar object is something that originates from outside our Sun's gravitational influence. This discovery changed our understanding of how objects move between star systems.

While most objects in our neighborhood orbit the Sun, ʻOumuamua was merely passing through. It follows a hyperbolic trajectory, which means its path is not a closed loop. Instead, it travels on a curve that allows it to escape the Sun's pull. It will eventually return to the vastness of interstellar space.

Astronomer Robert Weryk discovered the object on 19 October 2017. He used the Pan-STARRS telescope at the Haleakalā Observatory in Hawaii. At the time of discovery, it was roughly 33 million km from Earth. This distance is about 85 times farther than the Moon. The object had already passed its perihelion, or its closest point to the Sun, on 9 September 2017.

Because it was the first of its kind, the International Astronomical Union gave it a unique designation. It was assigned the number 1I, where the "I" stands for interstellar. The name ʻOumuamua comes from the Hawaiian language. It translates to "first distant messenger."

Scientists observed several unusual characteristics during its visit. First, the object exhibited nongravitational acceleration. This means its speed changed in a way that gravity alone cannot explain. This change in velocity near its closest approach to the Sun summed to about 17 meters per second.

Some researchers suggested this was caused by outgassing. Outgassing occurs when volatile substances inside an object evaporate due to solar heat. However, astronomers did not see a coma, which is the fuzzy cloud of gas usually seen around comets. Another theory is that solar radiation pressure, or the physical push from sunlight, moved the object.

Another mystery involves the physical shape and motion of the object. ʻOumuamua is unusually elongated, meaning it is very long and thin compared to its width. Many models suggest it is much more stretched out than most natural bodies in our Solar System.

Its light curve, which tracks how its brightness changes, suggests it is "tumbling" through space rather than spinning smoothly. This tumbling motion and its reddish color are notable features. While its rotation rate is similar to local asteroids, its extreme shape raised many questions about its origin. Some scientists even compared it to the fictional spacecraft "Rama" from science fiction due to its shape and interstellar nature.

Detailed observations were made using many powerful tools. The Pan-STARRS1 Telescope and the Canada–France–Hawaii Telescope provided early data. Later, the Very Large Telescope and the Gemini South telescope in Chile helped determine its composition. Even space-based telescopes like Hubble and Spitzer joined the effort.

Astronomers also used the Allen Telescope Array to look for radio signals. They searched for any signs of artificial technology, but found no unusual emissions. The data showed no narrowband signals, meaning the object appeared to be a natural, non-technological body.

We can track the exact speed and path of this visitor. ʻOumuamua's orbital eccentricity was measured at 1.20. In astronomy, an eccentricity greater than 1.0 means the object is not bound to the Sun.

Its speed relative to the Sun was much higher than objects from our own Oort cloud. For example, while many local objects move at about 3 km/s, ʻOumuamua traveled much faster. As it passed its perihelion, it reached a maximum speed of 87.71 km/s. This high velocity confirms it could not have been captured by any planet in our system.

There are several theories about where ʻOumuamua came from. It may be the remnant of a disintegrated rogue comet. Another idea is that it is a piece of an exoplanet rich in nitrogen ice, similar to Pluto. In March 2023, some astronomers proposed it was an interstellar comet. They suggested the acceleration came from the release of trapped molecular hydrogen. This would happen if the object was a planetesimal relic, a leftover piece from a distant star system.

Though its exact home is unknown, it serves as a unique window into the chemistry of other worlds.