'Oumuamua was the first interstellar object ever detected

‘Oumuamua, the first object confirmed to have come from another star system, was discovered on October 19, 2017 by the Pan-STARRS1 telescope in Hawaii — already outbound, six weeks after swinging past the Sun on September 9.

Its orbit gave it away. Comets falling in from the solar system’s outer edge sometimes end up on slightly open, hyperbolic paths after a planet nudges them; the most extreme case on record, comet Bowell, reached an eccentricity of 1.057 after a close pass by Jupiter. ‘Oumuamua’s eccentricity was about 1.2, with no planetary encounter to account for it, and it had entered at roughly 26 km/s — far too fast for the Sun to hold. The IAU created a new designation class for it, 1I, the “I” standing for interstellar.

Telescopes had only weeks before it faded. Its brightness swung so sharply as it tumbled that it appeared about ten times longer than wide, up to about 400 metres long — one of the most elongated small bodies ever measured.

Comet, asteroid, then comet again

‘Oumuamua’s label changed twice in its first year:

• October 2017: announced as a comet, until deeper images showed no sign of the dust or gas a comet gives off, so it was reclassified as an asteroid.
• January 2018: it was about 40,000 km ahead of where gravity alone would have put it.
• June 2018: a team combining Hubble and ground-based tracking reported that it had been accelerating slightly.

“Our high-precision measurements of ′Oumuamua’s position revealed that there was something affecting its motion other than the gravitational forces of the Sun and planets.” — Marco Micheli, ESA

The team’s preferred explanation was gas venting from its surface, as on comets, but in amounts too small to see — which is why its nature is still argued over.

It is no longer unique. Amateur astronomer Gennady Borisov found the plainly active comet 2I/Borisov on August 30, 2019, and the ATLAS survey reported 3I/ATLAS on July 1, 2025.