'Oumuamua (2017): The First Interstellar Object Ever Seen

‘Oumuamua was discovered on 19 October 2017 by Robert Weryk with the Pan-STARRS1 telescope in Hawaii, and Karen Meech’s team showed it came from outside the Solar System: the first interstellar object seen. Their paper described a reddish body with an extremely elongated shape, about ten times longer than wide.

What was known before 2017?

Astronomers had cataloged roughly 750,000 asteroids and comets, and none was thought to come from another star system. That was a puzzle. Models of how the Solar System formed suggest that the giant planets, as they moved, threw a large share of the original planetesimals, the building blocks of planets, out into interstellar space. Other planetary systems should do the same, and the predicted number of icy wanderers suggested surveys should already have caught one. None had been seen.

What the paper found

• An orbit not bound to the Sun. Its path is hyperbolic, an open curve rather than a closed ellipse, with an eccentricity of 1.188 ± 0.016, the highest measured at the time. (An eccentricity above 1 means the object is not gravitationally bound to the Sun.)
• No sign of a comet. The images analyzed showed no cometary activity, even though the object passed within 0.25 au of the Sun (an au is the Earth-Sun distance).
• A red surface. Its spectrum is consistent with comets or organic-rich asteroids in our own Solar System.
• An extreme shape. Large swings in brightness imply an axis ratio of about 10:1 and a mean radius of about 102 meters, assuming a dark surface that reflects 4% of sunlight.
• More visitors than expected. The find suggested earlier estimates of how many interstellar objects pass through were too low.

How they did it

On 19 October 2017, Pan-STARRS1 on Haleakalā spotted an object moving fast across the sky, about 6.2 degrees per day. Images from the previous night also showed it. Follow-up with the Canada-France-Hawaii Telescope on 22 October confirmed its unusual orbit. The team then gathered spectra to measure its color and tracked its brightness over time. An elongated body shows a changing face as it spins, so the size of the brightness swings reveals how stretched it is.

Why it mattered

‘Oumuamua gave astronomers the first physical sample of material from another planetary system, a fragment of someone else’s construction site passing through ours. It also showed that interstellar objects can be found with ordinary sky surveys.

What we know now

NASA notes that ‘Oumuamua was briefly classified as an asteroid until new measurements found it was accelerating slightly, a sign that it behaves more like a comet. Two more interstellar objects have since been found: comet 2I/Borisov in 2019 and comet 3I/ATLAS, discovered on 1 July 2025. In July 2026 ESO reported that 3I/ATLAS likely formed around a star much older than the Sun.

Many more are expected from the Vera C. Rubin Observatory. Its first data preview, small as it was, already held 431 Solar System objects, 93 of them new. Like the 1992 discovery of the Kuiper belt, ‘Oumuamua shows how much remains to be found among the faint, fast-moving small bodies of the sky.

Who wrote it

Karen J. Meech led the characterization team, with Robert Weryk, who made the discovery, Marco Micheli and colleagues. The paper appeared in Nature in November 2017, about a month after the discovery. Semantic Scholar lists about 378 citations (October 2026).

Read the original

Meech, K. J., Weryk, R., Micheli, M., et al. (2017). “A brief visit from a red and extremely elongated interstellar asteroid.” Nature 552, 378-381. doi:10.1038/nature25020

Free to read: full text at PubMed Central.