Classification is hard. When we encounter the unfamiliar, we try to classify it according to a scheme we are familiar with. We do this on the basis of features, aspects of the object that somehow match the features of something familiar, to a greater or lesser degree. But which features do we focus on?

When we select features to use in classification, we introduce bias. Satosi Watanabe proposed his Ugly Duckling Theorem to show that our choice of features determines how we classify things. For something totally unfamiliar, like the asteroid Oumuamua, we choose features and, based on those features, assign it to a classification. Oumuamua has two unusual features: it’s shaped like a cigar rather than a sphere and its trajectory isn’t closed. It came from outside the solar system. There was some speculation that Oumuamua was an alien spaceship due to its shape and origin, but there was no evidence of any life aboard the object, everything else fit the classification asteroid, so that’s how it continues to be classified.

Suppose new information became available. Suppose a twin of Oumuamua, christened Agni, enters the Solar System a year later. Are Agni and Oumuamua both relics of the same unguessable extrasolar catastrophe? Or something else?

Occam’s Razor suggests that we choose the simplest hypothesis that fits the facts. But the choice of features to classify an object determines its category. Asteroid or spaceship? My story Watanabe’s Hammer shows how the Ugly Duckling theorem might upend our current understanding of our extrasolar visitor based on disturbing new information.