Astronomers know an object came from another star by measuring its orbital eccentricity. Anything gravitationally bound to the Sun travels a closed path with an eccentricity below 1. An object arriving from interstellar space moves too fast to be captured, so its path is hyperbolic and its eccentricity exceeds 1.
That single number is the whole test. It is measured from a handful of position observations taken over a few nights, and it is why a confirmation can be announced within days of discovery.
What Eccentricity Actually Describes
Eccentricity measures how stretched an orbit is. A perfect circle has an eccentricity of 0. Planets sit just above that, on mildly stretched ellipses. Long-period comets run out to values close to 1, swinging far beyond Neptune before falling back toward the Sun.
The dividing line is 1. Below it the path closes, and the object returns. Above it the path is open, the object is not bound to the Sun at all, and it will leave and never come back.
The first confirmed interstellar object, ‘Oumuamua, had an eccentricity of about 1.2. It was not a marginal call.
Why Speed Alone Is Not Enough
Speed is the physical reason behind the geometry, but it only means something in context.
The Sun’s gravity can hold onto an object up to a certain velocity at a given distance. Below that, the object is captured into a closed orbit. Above it, the object passes through. ‘Oumuamua entered the solar system at about 26 kilometers per second, and by the time the Sun had pulled it inward it was moving at roughly 196,000 miles per hour as it rounded perihelion in September 2017.
A comet native to the outer solar system can also be fast at close approach. What it cannot do is arrive with excess speed from outside. That excess is what pushes the eccentricity past 1, and it is what the orbital fit detects.
How the Naming System Works
The International Astronomical Union created a designation class specifically for these objects after the first one appeared.
The format is a number, then a capital I, then the discoverer or survey. The number counts confirmed interstellar discoveries in order and the I stands for interstellar. ‘Oumuamua’s formal designation is 1I/2017 U1. Its name is Hawaiian, roughly a messenger from afar arriving first, chosen because the Pan-STARRS1 telescope that found it on 19 October 2017 sits on Haleakala.
What a Few Weeks of Observation Can Reveal
These objects do not linger, so astronomers work fast and extract a surprising amount.
Brightness variation over time gives shape. ‘Oumuamua’s lightcurve swung by almost 2.5 magnitudes, implying a ratio of roughly 10 to 1 between its longest and shortest dimensions. At up to about 400 meters long, that made it one of the most elongated objects of its kind ever observed, unlike anything native to our own solar system.
Spectroscopy gives composition, which is the real prize. These are the only samples of material from other planetary systems that anyone can study at close range. Everything else known about other stars is inferred from light that has crossed enormous distances.
Why Only Three Have Ever Been Found
Three interstellar objects have been confirmed: the first in 2017, 2I/Borisov in 2019, and 3I/ATLAS on 1 July 2025.
The scarcity says more about telescopes than about space. These objects are small, dark and fast, and they are only detectable during a short window near the Sun. Finding them requires surveys that photograph the whole visible sky repeatedly and software that flags anything moving oddly against the stars. That capability is recent, which is why the count went from zero to three in eight years after centuries of nothing.
The expectation is that the number will keep climbing as survey coverage deepens, not that interstellar visitors have suddenly started arriving.