Last year, an enigmatic object named Oumuamua startled astronomers when it came streaking past the sun, giving humanity its first close-up look at an object from beyond our solar system. This year, the interstellar visitor did something even more remarkable: It made it respectable to talk about alien spaceships.

The turning point came in November, when Avi Loeb, the head of the astronomy department at Harvard University, co-wrote a paper saying that Oumuamua is so unusual that scientists should consider the possibility that it’s not a far-out comet or asteroid, as his colleagues assumed, but rather an artificial structure.

In other words, maybe it’s an interstellar craft built by extraterrestrials.

Some of Loeb’s colleagues were intrigued. Others were disconcerted. But suddenly mainstream scientists were talking about how to tell if Oumuamua is a natural object or — as Loeb raised as a possibility in his paper — an alien spacecraft designed to capture the force of sunlight (a so-called lightsail).

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