It wasn’t an asteroid: they’re dull and rocky, and often round, and it was shiny and elongated. It didn’t have a comet’s bright gassy tail. And it seemed to accelerate away from the sun as it left us, propelled in a straight line. Astronomers called it ‘Oumuamua, meaning ‘scout’ – and, says Prof. Avi Loeb of Harvard University, we should take seriously the possibility that it was just that: “a fully operational probe sent intentionally to Earth’s vicinity by an alien civilisation”.

Renowned for his work in the physics of black holes and dark matter, he has now written a challenging book, Extraterrestrial, bringing the ‘Oumuamua data together. “I’m not arguing we know for sure it was an artificial object. I’m saying it’s a reasonable plausibility based on the evidence.”

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