In simple terms
An interstellar object formed around another star, was thrown out of that system, and drifted between the stars until it reached ours.
It is a visitor, not a resident. The Sun bends its path but never captures it, so the object leaves again and does not return.
How it works
Astronomers identify one from its speed and its path. A body bound to the Sun travels a closed orbit. An interstellar object moves too fast for the Sun’s gravity to hold, so it follows an open, hyperbolic trajectory.
The measure that settles it is orbital eccentricity. A closed orbit has an eccentricity below 1. The first interstellar object found had an eccentricity of about 1.2, which proved it came from outside the solar system.
Three have been confirmed so far. The first was spotted on 19 October 2017 by the Pan-STARRS telescope in Hawaii and named ‘Oumuamua, Hawaiian for a messenger from afar arriving first. 2I/Borisov followed in 2019, and 3I/ATLAS was discovered on 1 July 2025.
The names follow a rule the International Astronomical Union created for them. The number counts confirmed discoveries in order, the capital I stands for interstellar, and the rest records the discoverer or survey. ‘Oumuamua’s formal designation is 1I/2017 U1.
Why it matters
These objects are the only material from other planetary systems that astronomers can observe at close range. Everything else known about other stars is read from light crossing enormous distances.
Their chemistry is therefore a direct sample of how planets assemble elsewhere. When one looks unlike anything in our own solar system, that says something about how varied planet formation can be.
Where you’ll see it
- Sky survey alerts, where fast-moving objects are first flagged.
- Papers reporting the composition of a passing comet.
- Plans for telescopes and missions built to intercept the next one.
Example
‘Oumuamua entered the solar system at about 26 kilometers per second and was up to roughly 400 meters long, with a length about ten times its width, making it one of the most elongated objects of its kind ever seen.
Key facts
- An interstellar object formed in another star system and was ejected into interstellar space.1
- Interstellar origin is confirmed from high velocity and a hyperbolic trajectory, meaning the object does not follow a closed orbital path around the Sun.1
- Three interstellar objects have been confirmed: 1I in 2017, 2I/Borisov in 2019, and 3I/ATLAS, discovered on 1 July 2025.1
- In the designation 1I/2017 U1 the capital I stands for interstellar; the object was discovered on 19 October 2017 by the Pan-STARRS1 telescope in Hawaii, and its name is Hawaiian for a messenger from afar arriving first.2
- Its orbit had an eccentricity of about 1.2, an unbound hyperbolic trajectory, and it entered the solar system at about 26 kilometers per second.3
- Its lightcurve implied a ratio of roughly 10 to 1 between its longest and shortest dimensions, and it measured up to about 400 meters long.23
In the news
Quick checkWhat orbital measurement shows that an object came from outside the solar system?Show answer
An eccentricity greater than 1, meaning the path is hyperbolic and unbound rather than a closed orbit around the Sun.
Sources
- NASA Science. 3I/ATLAS Facts and FAQs. Undated (accessed 14 September 2026)
- NASA Science. Oumuamua. Undated (accessed 14 September 2026)
- European Space Agency, Near-Earth Object Coordination Centre. Oumuamua: the first interstellar object found in our Solar System. Undated (accessed 14 September 2026)
Editorially reviewed by Specialty Digest Editorial TeamLast reviewed September 17, 2026Researched and drafted with AI assistanceReport an issue