The interstellar asteroid ‘Oumuamua has been making headlines since it was first discovered in 2017. Most controversially, Avi Loeb of Harvard University has suggested that it might be an alien probe, a claim that he has doubled-down on in the face of criticism by other astronomers.

Loeb says no other explanation fits the asteroid’s unusual shape and acceleration, but now Amaya Moro-Martin at the Space Telescope Science Institute in Baltimore has an alternative suggestion: ‘Oumuamua could be a kind of giant interstellar …