Eliminate the impossible, as Sherlock Holmes once said, and the truth, no matter how improbable, lies in whatever remains. Good advice for detectives — and, it turns out, for astronomers. Astronomy is often little more than piecing together scattered clues, carefully eliminating possibilities and arriving at an improbable truth. Take, for example, the announcement of an exomoon discovery back in 2017. Observations seemed to show the presence of large moon, one as big as Neptune, orbiting a gas giant in a distant solar system. But, as the authors of the announcement were careful to state, the discovery was far from...