The gigantic space rock, known as “Oumuamua”, is the first from outside the solar system to ever be observed by scientists.
Previously described as “cigar-shaped”, astronomers at Queen’s University, Belfast, and elsewhere have studied the asteroid and now estimate it to be the same size and shape as London’s Gherkin skyscraper – which is about 180m (591ft) tall.
The latest research also suggests the rock, which withstood incredibly high temperatures when it swung through our solar system, has been protected from the sun’s heat by a mysterious insulating crust.
Researchers, including scientists from the US, Canada, Taiwan and Chile, believe they have made “key observations” about what its half-metre thick crust is made of.
Dr Michele Bannister, whose research is published in Astrophysical Journal Letters, said the rock “looks so much like a tiny world from our own home system”.
She said: “This object went past the sun and it was only heated like a good oven cooking a cake. It wasn’t actually particularly hot.
“The surface composition is consistent of a layer of insulating material, so something along the lines of dust and grit, maybe organic compounds.
“We don’t know that the interior does contain ice but if it did contain ice it would have been insulated by the layer on the surface.”
The rock was scanned for radio signals after its unique shape – highly unusual for an asteroid – sparked speculation it could be an alien ship.
However, the scans found “no evidence of artificial signals emanating from the object” and it is not believed to be an alien vessel.
Oumuamua, which is Haiwaiian for scout or messenger, is the first object found in the solar system that is believed to be from another part of the galaxy.
It was spotted by University of Hawaii astronomers in October after it passed the Earth at roughly 85 times the distance of the moon.
Professor Alan Fitzsimmons, who is also based at Queen’s and whose separate research paper is published in Nature Astronomy, said the “half-metre thick coating of organic-rich material could have protected a water-ice-rich comet-like interior from vaporising when the object was heated by the sun, even though it was heated to over 300 degrees centigrade”.
Dr Bannister said: “The nice thing about driftwood is that it tells you that there’s trees that grow on distant shores, so this is a little tangible piece of evidence that the way planets formed and grew in our own solar system, it’s happening the same way in other solar systems far away.”
She added: “None of the astronomers have ever talked about it being anything alien, except that it comes from outside the solar system.”