'Oumuamua — first interstellar visitor
The first object confirmed to have come from outside the solar system; its stretched shape and puzzling push sparked years of debate.
Available views: Orbit ✓ · Sky — (not visible in the sky)
The story
On 19 October 2017 the Pan-STARRS telescope on Haleakalā in Hawaii spotted a faint point moving quickly across the sky. When its path was worked out, it was not a closed loop around the Sun but an open curve that passes once and never returns. It was the first object confirmed to have come from outside the solar system. By then it had already made its closest pass by the Sun, at 0.25 AU on 9 September, and was on its way out.
The name ‘Oumuamua is Hawaiian for “a messenger from afar arriving first”. Its brightness rose and fell strongly every few hours. That happens when a long object tumbles, showing its wide side and then its narrow side in turn. So scientists estimated that it was a few hundred metres long and much narrower than that, shaped either like a cigar or like a flat pancake.
Its motion was even stranger. As it left the Sun, a tiny extra push was measured that the Sun’s gravity alone could not explain. A comet gets such a push when its ice turns to gas and jets away, but ‘Oumuamua showed no cloud of gas and no tail. Ideas followed: perhaps it was a chunk of hydrogen ice, or a fragment of nitrogen ice, and Harvard astronomer Avi Loeb even suggested it could be an artificial light sail.
There were only a few weeks to study it. Because it was already heading away when found, it faded from every telescope after Hubble’s last observations in January 2018. It is now leaving the solar system for good at about 26 km/s, toward the constellation Pegasus. The orbit view replays its open path slicing at an angle through the solar system.
When and where to see it
‘Oumuamua can no longer be seen with any telescope. It crossed the orbit of Neptune in 2022 and is now more than 50 AU from the Sun, moving away toward Pegasus. It arrived from the direction of Vega, the bright star in Lyra. It is now far too faint even for the largest telescopes, so it can never be observed again.
Choose ‘Oumuamua in the orbit view’s event list to replay its closest pass by the Sun on 9 September 2017 and its steeply tilted path, which dives through the solar system from above to below. Slow the playback to see its closest approach to Earth on 14 October, at 0.16 AU. In the 3-D interstellar view, the directions it came from and is leaving toward can be compared with the constellations in the real sky.
See this for yourself — replay the moment in the orbit view
Past returns and history
It was first announced with a comet name, C/2017 U1, and then renamed as an asteroid, A/2017 U1, because no gas could be seen coming off it. Later it received ‘1I’, the first number in a new series for interstellar objects. Its surface was reddish, much like the icy bodies in the outer solar system.
In 2023, researchers at the University of California, Berkeley and Cornell University offered a new explanation. Hydrogen formed in its ice during its long journey through space may have been trapped inside, then slowly escaped when the Sun warmed it, giving a gentle push. If so, the object could speed up without a visible gas cloud. The debate is not fully over, but natural explanations are now more widely accepted.
‘Oumuamua spent only a short time near the Sun. At its closest it was moving at about 88 km/s, and it was not found until more than a month later. Had it been spotted while still approaching, astronomers could have checked much more closely whether it gave off gas. That missed chance became a reason to build more sky-survey telescopes and to prepare spacecraft that can chase the next visitor.
Why it matters
‘Oumuamua was the first proof that material from around other stars really does pass through our neighbourhood. The idea that such objects exist was old, but this was the first one ever caught. With Borisov in 2019 and 3I/ATLAS in 2025, the study of interstellar visitors became a field of its own.
The heated debate left something behind as well. The claim that it was an alien probe was not accepted by most scientists. Still, the argument gave weight to a new idea: when the next visitor arrives, do not just watch it through telescopes, but go after it. Seen only as a faint distant dot, its true shape and make-up could never be settled.
That is why the European Space Agency is preparing Comet Interceptor. Planned for launch in 2029, it is designed to wait at a point about 1.5 million km beyond Earth and then set off toward a newly found comet or interstellar object. Scientists expect that, as the Vera Rubin Observatory keeps scanning the whole sky, interstellar objects will be found more often than before.
FAQ
How did astronomers know it came from outside the solar system?
Its path was an open curve rather than a closed loop around the Sun. Its eccentricity was 1.2, while objects bound to the Sun have values below 1.
Is it an alien probe?
Most scientists think not. Natural explanations, such as trapped hydrogen escaping, are more widely accepted, though there is no way to check directly.
How big was it?
It was probably a few hundred metres long and much narrower than that. It was too small and distant to be photographed as more than a point.
Can it be seen again?
No. It has been beyond every telescope since early 2018 and is leaving the solar system for good.
Related
- 2I/Borisov — the interstellar comet
- Halley's Comet perihelion (1986)
- Comet Hyakutake sweeps past Earth
- The Sun — object index
- Pegasus — object index
Times and figures are approximate — verify with official sources when planning travel. Simulation uses J2000 mean elements.