A mysterious object that whizzed by the Earth three years ago was probably the remnants of a planet from another solar system, scientists now say.
Since it was first spotted by the Pan-STARRS astronomical observatory in Hawaii in late 2017, 'Oumuamua has defied scientists' best efforts to figure out what it was.
Based on its speed (87km a second) and trajectory (almost straight down from above), scientists quickly figured out 'Oumuamua wasn't from around here - making it the first interstellar object ever detected passing through our solar system. Some figured it was probably a comet, but a very strange one; possibly an 'iceberg' made of frozen hydrogen that used to be a cloud; or even an ancient alien relic, perhaps some kind of space-yacht or machine probe.
None of the explanations held up to closer scrutiny, but scientists at Arizona State University now think they've "probably resolved the mystery of what 'Oumuamua was".
"We can reasonably identify it as a chunk of an 'exo-Pluto', a Pluto-like planet in another solar system," said astrophysicist Steven Desch.
"Until now, we've had no way to know if other solar systems have Pluto-like planets, but now we have seen a chunk of one pass by Earth."
'Oumuamua - which means 'messenger' or 'scout' in Hawaiian - was probably knocked out of its orbit around another star about 400 billion years ago, and reached our part of the galaxy around about 1995. At the time it was probably still fairly round, but lost more than 95 percent of its mass as it swung close to the sun in mid to late 2017, before it was finally spotted by astronomers here on Earth.
Simulations suggest it was likely made of frozen nitrogen - just like the surface of Pluto. The difference is Pluto stays very, very far away from the sun - about 40 times further than Earth, while 'Oumuamua got four times closer.
"Being made of frozen nitrogen also explains the unusual shape of 'Oumuamua," said planetary scientist Alan Jackson.
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"As the outer layers of nitrogen ice evaporated, the shape of the body would have become progressively more flattened, just like a bar of soap does as the outer layers get rubbed off through use."
'Oumuamua's exact shape isn't known - the light it reflects suggests it's tumbling end over end, and likely cigar-shaped or flat like an oval pancake - either way, like nothing we've seen in our own solar system. A new artist's impression of the interstellar visitor bears a remarkable resemblance to the famous Millennium Falcon spaceship from the Star Wars movies.
At its closest, 'Oumuamua was about 24 million kilometres away - while that sounds like a long way, it's only one-seventh of the distance from here to the sun, close by space standards.
Sadly, the Arizona State University team say there's still no evidence 'Oumuamua has anything to do with aliens.
"Everybody is interested in aliens, and it was inevitable that this first object outside the solar system would make people think of aliens," said Dr Desch. "But it's important in science not to jump to conclusions. It took two or three years to figure out a natural explanation - a chunk of nitrogen ice - that matches everything we know about 'Oumuamua. That's not that long in science."
There's an irony in calling 'Oumuamua a "Pluto-like planet", Pluto having lost its planet status in 2006 after astronomers discovered there were loads of similar-sized objects orbiting the sun - rather than make them all planets, the literal definition of planet was changed to cut Pluto out.
