A visitor to our solar system became the subject of headlines around the world after the cigar-shaped rock now named `Oumuamua flew past our sun.
It isn’t an alien spacecraft, a new study has suggested - but there’s still something very strange about it, University of Maryland researchers have said.
The space object, a visitor from another star system, was promptly scanned by the alien-hunting SETI project - and UFO fans (naturally) came up with dozens of strange theories.
But while it showed no signs of life in scans in 2017, a mysterious ‘speed boost’ observed by astronomers reignited debate this year.
Oumuamua, Hawaiian for Scout', spins like a coke bottle and accelerates like a comet, but without the gas jets often seen trailing them.
The study's co-author, Dr Matthew Knight, an associate research scientist in the University of Maryland Department of Astronomy, said: 'The alien spacecraft hypothesis is a fun idea, but our analysis suggests there is a whole host of natural phenomena that could explain it.
'We have never seen anything like Oumuamua in our solar system. It's really a mystery still.
'But our preference is to stick with analogues we know, unless or until we find something unique.'
Dr Knight worked with astronomer Dr Alan Fitzsimmons from Queen's University Belfast and 14 experts from the US and Europe.
They analysed data from the Discovery Channel Telescope (DCT) at the Lowell Observatory in Arizona from their base at the International Space Science Institute in Bern, Switzerland.
Dr Knight added: 'We tend to assume that the physical processes we observe here, close to home, are universal.
'And we haven't yet seen anything like Oumuamua in our solar system. This thing is weird and admittedly hard to explain, but that doesn't exclude other natural phenomena that could explain it.'
Scientists think it could have entered our solar system after being ejected by a gas giant planet orbiting another star.
And researchers said Jupiter may have created some of its own interstellar travellers by sneaking some of its icy objects through the sun's gravity field and into foreign solar systems.