Astronomers know an object came from another star by measuring one number: its orbital eccentricity. Anything gravitationally bound to the Sun travels a closed path with an eccentricity below 1. An object arriving from interstellar space is moving too fast for the Sun’s gravity to ever capture it, so its path is hyperbolic and its eccentricity comes out above 1. That single number is the whole test, and it can be pinned down from a handful of position observations taken over just a few nights, which is why a confirmation can be announced within days of discovery.
What eccentricity actually measures
Eccentricity describes 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 itself: below it, the path closes and the object eventually returns; above it, the path stays open, the object was never bound to the Sun at all, and it will leave and never come back. The first confirmed interstellar object, ‘Oumuamua, came out at an eccentricity of about 1.2, which was not a marginal call.
Why speed alone isn’t the test
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 threshold, the object gets captured into a closed orbit, and above it, the object passes straight through. ‘Oumuamua entered the solar system at roughly 26 kilometers per second, and by the time the Sun’s gravity had pulled it inward it was moving at about 196,000 miles per hour as it rounded its closest approach to the Sun in September 2017. A comet native to the outer solar system can also move fast at close approach, but what it cannot do is arrive already carrying that kind of excess speed from outside the system. That excess is exactly what pushes the eccentricity past 1, and it’s what the orbital fit is built to detect.
How these objects get named
The International Astronomical Union created a designation class specifically for interstellar objects once the first one turned up. 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 for roughly “a messenger from afar arriving first,” chosen because the Pan-STARRS1 telescope that found it, on October 19, 2017, sits on Haleakala.
What a few weeks of observation can reveal
These objects don’t linger near the Sun for long, so astronomers work fast and still manage to extract a surprising amount. Brightness variation over time reveals 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, splitting the object’s light into a spectrum, reveals composition, which is the real prize: these are the only samples of material from other planetary systems that anyone can study at close range, while everything else known about other star systems is inferred from light that has crossed enormous distances.
Why only three have ever been found
Three interstellar objects have been confirmed so far: the first in 2017, 2I/Borisov in 2019, and 3I/ATLAS on July 1, 2025. The scarcity says more about telescope capability than about how much is actually out there. These objects are small, dark, and fast, and only detectable during a short window near the Sun, so finding them requires surveys that photograph the entire visible sky repeatedly plus software that flags anything moving oddly against the background stars. That capability is recent, which is why the count went from zero to three in eight years after centuries of turning up nothing. The expectation among astronomers is that the number will keep climbing as survey coverage deepens, not that interstellar visitors have suddenly started arriving more often.