Just when we thought the question was settled for good, here we are again: discussing whether the tumbling, cigar-shaped object known as 'Oumuamua that came crashing through our solar system late past year is an alien spacecraft.
In a new paper submitted to the Astrophysical Journal Letters, researchers at the Harvard Smithsonian Center for Astrophysics suggest the possibility the object is actually a "lightsail of artificial origin".
First spotted by the Pan-STARRS 1 telescope in Hawaii in October 2017, the odd, fast-moving object has puzzled astronomers since its discovery.
The estimated 400 metre-long object sped through our inner solar system at a speed of more than 26 kilometres per second before departing in January 2018.
Avi Loeb, Harvard's astronomy department chairman and a co-author of the paper, cautions that it's "impossible to guess the objective behind Oumuamua without more data".
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"Two very capable, very bright astronomers, from a very credible organization, Harvard, have come out with the notion that Oumuamua could be alien in origin", Diamond said.
Not only is it the first-ever interstellar asteroid or comet detected by astronomers while passing through the solar system, its shape is unusual, its acceleration is unexpected, and a cometary tail - the signature trail of particles seen behind shooting stars - is conspicuous in its absence.
The object's "excess acceleration" when it travelled through our solar system and its peculiar trajectory distinguishes it from comets and asteroids, the researchers explained.
The scientists also noted that light-sails with similar dimensions have originated on earth.
It's been dubbed a comet, an asteroid, and a new class of interstellar object. "The lightsail technology might be abundantly used for transportation of cargos between planets or between stars".
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When the object was first spotted scientists believed it might have been travelling through space for hundreds of millions of years.
Comets, in particular, are known to speed-up due to a process known as "outgassing", in which the sun heats up the surface of the icy comet, releasing melted gas.
So far, there is no obvious origin for 'Oumuamua.
The theory, which the scientists call "exotic", comes from a recent paper they wrote which was published online in November. But the object has already left the solar system and is no longer visible, even with telescopes.
The Harvard scientists speculated that solar radiation hitting a solar sail could explain why the mysterious object started moving faster. Not all shells are the same, and similarly only a fraction of the interstellar objects might be technological debris of alien civilizations.
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But Harvard's research team says they're not giving up hope that it's a sign of alien life: they "follow the maxim of Sherlock Holmes: When you have excluded the impossible, whatever remains, however improbable, must be the truth".