It looks as if the interstellar visitor 'Oumuamua isn't an alien space probe after all – not even a dormant one. An international team of researchers led by Queen's University Belfast (QUB) has confirmed that the 400-meter-long (1,300 ft) spindle is a rocky object containing water ice that is coated with organic-rich substances coming from a planetary system similar to ours and it has been floating through space for millions of years.

When the object `Oumuamua (A/2017 U1) was discovered on October 19, 2017 by the University of Hawaii's Pan-STARRS 1 telescope on Haleakala, it set off alarm bells across the scientific community. Once its trajectory was accurately plotted, the object turned out to be the first ever observed entering the Solar System from interstellar space. Its open-ended hyperbolic course had already brought it to within 0.25 AU (23 million mi, 37 million km) of the Sun in September and it was speeding back into deep space at 95,000 km/h (59,000 mph).

The $64,000 question was, what was 'Oumuamua? A study of its light curve showed that it was a spindle 10 times longer than it was wide, slowly tumbling as it moved through space. At first, it wasn't clear whether it was a comet or an asteroid, but the lack of any outgassing of water vapor indicated that it was an asteroid.

But was there more to it than that? Where did it come from? What was it made of? One group of scientists even checked out the long-shot chance that it was a space probe sent by some distant civilization, though (so far) no radio transmissions have been received from it.

The team was led by Professor Alan Fitzsimmons and Dr Michele Bannister from the School of Mathematics and Physics at QUB and included researchers from Britain, the United States, Canada, Taiwan, and Chile, who endeavored to fill the gaps in our knowledge of 'Oumuamua by means of spectral analysis of sunlight reflected from its surface.

What they have found is that the asteroid is similar in composition to objects in the outer reaches of our own solar system, with a dry crust of organic-rich matter sealing in ice beneath its surface. According to the team, this crust was the result of millions or billions of years of cosmic ray bombardment. So protective is this layer that it even protected the ice inside from vaporizing when it passed close to the Sun.

"We have discovered that the surface of `Oumuamua is similar to small solar system bodies that are covered in carbon-rich ices, whose structure is modified by exposure to cosmic rays," says Fitzsimmons. "We have also found that a half-meter [20-in] thick coating of organic-rich material could have protected a water-ice-rich comet-like interior from vaporizing when the object was heated by the sun, even though it was heated to over 300° centigrade [570° F]."

Other observations showed that the color of 'Oumuamua was the same as the icy minor planets seen in the Kuiper belt beyond Pluto. According to Bannister, this indicates that the object came from a star system similar to our own before it was somehow thrown into interstellar space. However, which star it came from remains a mystery.

"We've discovered that this is a planetesimal with a well-baked crust that looks a lot like the tiniest worlds in the outer regions of our solar system, has a grayish/red surface and is highly elongated, probably about the size and shape of the Gherkin skyscraper in London," says Bannister. "It's fascinating that the first interstellar object discovered looks so much like a tiny world from our own home system. This suggests that the way our planets and asteroids formed has a lot of kinship to the systems around other stars.

"We are continuing our research into `Oumuamua and are hopeful that we will make more discoveries in the near future. Discoveries like this really help to give a little more insight into what's out there and encourages people to look up and wonder."

The findings were published in Nature Astronomy and Astrophysical Journal Letters.