In late 2017 when a mysterious object appeared out of interstellar space and zipped through the solar system, there was immediate conjecture of whether it could be an alien technology.
Scientists have since come up with a number of less-fantastical theories of what the cigar-shaped thing dubbed 'Oumuamua might be, but none coming up with anything that fits the data - including its weird shape, its tumbling end-over-end motion and the fact it was speeding up as it went by.
Earlier this year a promising theory was put forward by scientists from Yale claiming it could be a hydrogen iceberg, formed in a "molecular cloud" somewhere in outer space, far beyond the solar system's furthest reaches.
"Even though the hydrogen iceberg thing is a little exotic, it explains every single mysterious thing about 'Oumuamua," Yale researcher Darryl Seligman told Wired in May.
But new research has shot down this hypothesis.
"The proposal by Seligman and Laughlin appeared promising because it might explain the extreme elongated shape of 'Oumuamua as well as the non-gravitational acceleration," said Thiem Hoang of the Korea Astronomy and Space Science Institute.
"However, their theory is based on an assumption that hydrogen ice could form in dense molecular clouds. If this is true, hydrogen ice objects could be abundant in the universe, and thus would have far-reaching implications."
But none others have been seen before, and the new research suggests they simply couldn't survive a journey through space lasting hundreds of millions of years.
"An accepted route to form a kilometre-sized object is first to form grains of micron-size, then such grains grow by sticky collisions," said Dr Hoang. "However, in regions with high gas density, collisional heating by gas collisions can rapidly sublimate the hydrogen mantle on the grains, preventing them from growing further."
Essentially, the friction heats solid hydrogen so it turns back into gas before it can grow to something the size of 'Oumuamua - somewhere between 100m and 1000m long.
Even the minimal heat from stars would be enough to melt any proto hydrogen iceberg, he added.
"This object is mysterious and difficult to understand because it exhibits peculiar properties we have never seen from comets and asteroids in our solar system," said Dr Hoang.
Others have suggested the strange unseen properties 'Oumuamua possesses could be the result of alien technology, but even with the current best guess probably now debunked, Dr Hoang is optimistic the mystery will be solved soon.
"If 'Oumuamua is a member of a population of similar objects on random trajectories, then the Vera C Rubin Observatory (VRO), which is scheduled to have its first light next year, should detect roughly one 'Oumuamua-like object per month. We will all wait with anticipation to see what it will find."
A second interstellar visitor which passed in January, 2I/Borisov, was determined to be a comet.
