Trawling the skies on 19 October 2017, the world's most powerful telescope captured an extraordinary image. Pan-STARRS1, situated in Hawaii, detected a thin, flat object the size of a football pitch moving away from the sun. It did so with a strange tumbling motion, turning end over end every seven hours, varying in brightness as it went.

For the 11 days it was visible, astronomers were mystified. Most concluded it was a strange asteroid or comet, being turned over by jets of evaporating gas on its surface. The only problem was that these jets usually appear as a visible tail. There was no tail to be seen. The team that spotted it named it Oumuamua - Hawaiian for "first scout from a distant place".

For one man, this name had a particular significance. Avi Loeb was convinced that the object had entered our solar system from interstellar space for a purpose.

Loeb is one of the world's most distinguished astronomers. He was a leading figure in the Event Horizon Telescope project, which produced the first photograph of a black hole. He has helped develop new ways of detecting exoplanets, and predicted the existence of stars that shoot between galaxies.

In November 2018, Loeb and his research assistant Shmuel Bialy published an article in the magazine Astrophysical Journal Letters which caused a sensation. His argument - developed in a new book called Extraterrestrial: The First Sign of Intelligent Life Beyond Earth - was that Oumuamua was something humans had never seen before.

Instead, it was "a new class of thin interstellar material, either produced naturally, through a yet unknown process... or of an artificial origin". Loeb believed that it was the latter: in other words, a piece of alien technology.

His theory was partly based on Oumuamua's movement, or lack of it, in relation to the solar system. It appeared to be positioned like a buoy in the ocean, with the sun moving past it and bumping it with its gravitational field.

Even more significant was its highly unusual shape and lack of a comet's tail. Comets also move jerkily, whereas Oumuamua's motion was smooth.

Loeb was understandably disappointed when most of the scientific community rejected his theory. But what worried him more, he says, was a wider problem: their refusal to engage with the idea of alien intelligence at all.

He cites a remark by a colleague at Harvard University: "This object is so weird - I wish it never existed." It was, Loeb insists, "a terrible thing to say for a scientist... you should accept with open arms anything that nature gives you".

The question, he says, is whether humans are ready to confront the idea that life on Earth is not unique, and not even particularly impressive. "I fear the answer is no," he writes in his book, "and that prevailing prejudice is a cause for concern".

Yes: The only other object that has been seen to behave in the same way as Oumuamua turned out to be the remains of a US rocket. So it is logical to suppose that Oumuamua too was manufactured rather than created naturally.

No: Just because we have not encountered something like Oumuamua before, we should not deduce that aliens made it.

Or: It might not be a spacecraft, but it is right to be curious. Science should be about asking questions and discovering new things. To reject this object because it is too mysterious goes against science.