How a Harvard professor became the world's leading alien hunter

Avi Loeb's single-minded search for extraterrestrial life has made him the most famous practising astronomer in the US and possibly the most controversial

- SETH FLETCHER

ON OCT 19, 2017,a telescope in Maui detected someO thing that had entered our solar system from elsewhere in the galaxy. Astronomers named it Oumuamua, Hawaiian for "scout" or "messenger," because it was the first interstellar object they had ever recorded - the only known traveller to have crossed the vast distance between another star system and our own.

Where it came from was only part of its mystery. Oumuamua fit none of the wellunderstood astronomical categories. If it was a rock - an asteroid - it was an extremely strange one. Researchers estimated that it was at least the length of a football field; its shape was hard to determine, but it seemed to be long and thin, like a cigar."No known objects in the Solar System have such extreme dimensions," wrote the group of astronomers who discovered the object.

The more that scientists studied Oumuamua, the weirder it seemed.Analysis of its trajectory showed that, in the weeks before its detection, Oumuamua sped up as it approached the sun, and its acceleration couldn't be explained by the sun's gravity alone. That extra kick would be normal for a comet. Comets are rocky snowballs, and when they get close to the sun, ice within them turns to vapour, releasing gas and giving them a boost. But Oumuamua lacked a comet's signature tail, and none of the telescopes that observed it detected water vapour, carbon monoxide or other telltale signs of sublimating ice. Scientists started inventing wild ideas to explain Oumuamua's observed characteristics, things like hydrogen icebergs and gigantic dust bunnies less dense than air. They were reaching.