concept

Transit-timing variation

An indirect way to detect a hidden moon (or planet). As a planet transits the face of its star, a moon orbiting it tugs the planet around their shared centre of mass, making the planet arrive at the crossing a little early or late from one transit to the next. Those timing shifts - only seconds to minutes for an Earth-sized moon - are the fingerprint of the hidden body, but sit right at the noise floor of today's best instruments.