Harvard researchers say it's likely that a mysterious cigar-shaped object hurtling through space is an alien spacecraft sent intentionally to Earth's vicinity by another civilisation.
Here is everything you need to know about the object and the theories surrounding it.
What is Oumuamua?
Scientists have been puzzling over Oumuamua ever since the extraordinary asteroid-type body was seen tumbling past the sun a year ago.
It was discovered by the University of Hawaii’s telescope and named Oumuamua, which is Hawaiian for “a messenger from afar arriving first.”
Since then, it has ignited a revolution for astronomy and it’s the first known object to enter the Solar System from deep space.
Some experts think its flattened, elongated shape and the way it accelerated on its trajectory through the solar system set it apart from conventional asteroids and comets.
Oumuamua might have been an ancient alien relic, researchers suggested.
After months of exploration of little data collected, scientists in June declared it was a comet.
However, Oumuamua is unlike any comet ever documented in the solar system.
It measures up to 0.4 kilometres wide and is highly elongated, as much as 10 times as long as it is wide, according to NASA, which called its shape “quite surprising, and unlike objects seen in our solar system.”
It was also seen as potential evidence for the theory of lithopanspermia — the transfer of microbial life between planets and stars through comets and asteroids.
Researchers believed such an object — hurtling into our surface as a meteor — could have seeded the building blocks for life on Earth.
Researchers at the Harvard Smithsonian Center for Astrophysics now say that might not be the case.
Could it be an alien spacecraft?
Two researchers from Harvard have theorised this week in a new study that Oumuamua is really an alien spacecraft.
Researchers Shmuel Bialy and Abraham Loeb suggested the object “may be a fully operational probe sent intentionally to Earth vicinity by an alien civilization.”
They're suggesting it sped up as it rounded the sun, rather than slow down because it had a sail.
"We explain the excess acceleration of 'Oumuamua away from the sun as the result of the force that the sunlight exerts on its surface," Harvard researcher Leob explained at a press conference.
What do others say?
Other space scientists have strong doubts about this theory and think there may be other, more ordinary explanations.
They include Professor Matthew Kenworthy at the Leiden Observatory in the Netherlands.
“It's not an alien spacecraft. It looks like a piece of rocky or icy material that has come from outside the Solar system, passed around the Sun, and is now heading back out into interstellar space," he told Euronews.
He added that there's a rule of thumb that means when there are several possible explanations, “the simplest one is almost certainly the correct one and in this case, an entirely natural origin explains the facts we have”.
Kenworthy told Euronews that the Harvard researchers were intrigued by one observation that showed Oumuamua was accelerating away from the Sun very slightly faster than expected.
The researchers ruled out most natural explanations and suggested that jets of gas being released from Oumuamua as it heated up on its trip around the Sun were the cause.
Kenworthy said an artificial sail built by aliens would also show the same acceleration pattern — but stressed that the natural explanation was the most likely one.
If it isn't an alien spacecraft, does it matter?
Kenworthy said the discovery was significant even if there were no links to aliens.
"It’s come from another part of the Galaxy, travelling for at least hundreds of thousands of years in the interstellar cold, only to pass for a few brief bright weeks through our Solar system before returning to the cold for many more eons," he said.
“It almost certainly came from a cloud of ice and rocks that were around another star in our Galaxy”.
The truth of the object may be hard to establish, as Oumuamua has left the solar system and is no longer visible even with telescopes.