The Harvard scientists who suggested a piece of space rock might actually be an alien spacecraft flying past Earth says the strange visitor "doesn't look like anything we've seen before".
Astronomer Avi Loeb kicked off international intrigue when he co-authored a paper that suggested Oumuamua, which flew past Earth last year, could have been a probe sent by aliens from deep in space to survey Earth. The research explored the unexpected movement of the rock and suggested that it could be a "lightsail", which was sent intentionally by aliens to investigate Earth.
But even if the rock has a natural origin then it is incredibly strange, Professor Loeb told the Today programme. It has a strange shape, doesn't show the characteristics usually observed in comets and asteroids, and is flying through space in a way that scientists wouldn't expect it to, he said.
"Most intriguingly it's moving on a trajectory that implies an extra push in addition to the gravitational force from the sun," he said. "There seems to be another force acting on it."
It is that strange behaviour that led Professor Loeb and a colleague to speculate that the rock could have been thrown out by some explosion, deep in space, from an extraterrestrial civilisation. They even suggested that the object could be a probe intentionally sent through space.
It makes the object fascinating to scientists wherever it came from, he said.
"It's the very first interstellar object that we discovered in the solar system, passing near the earth," he said. "And it's sort of like having a guest for dinner and realising that out of the other guests you have this one is from a foreign country and you can learn about the culture in that country without paying the airline ticket to go there."
Ultimately it will be very difficult to find out more about Oumuamua, because it has flown quickly out of the solar system and away from view. He said he hoped that we would have been able to chase it down – we currently lack the technology to fly fast enough to do so, but that within a decade or two we should be have those kind of rockets.