Planet Detection: Transit Timing Variation

Planet detection from transit timing variations is a method for determining the existence and properties of a planet from the gravitational perturbations it induces on the orbit of another, transiting planet. In an unperturbed planetary system consisting of a star and a planet, the two bodies revolve around their center of mass in a▶Keplerian orbit. If in this system the planet transits the star, it will cross the disk of the star at same time during each transit. As a result, the time of the middle of the transit will be identical every time the planet completes one orbit. If because of some external perturbation (e.g., a second planet) the motion of the transiting planet deviates from purely Keplerian, the time of the mid-transit will be different from one orbit to another. This is known as transit timing variation (TTV). Figure 1 shows this schematically.