Transit timing variation

Transit timing variation is a change in the intervals between a planet’s successive transits across its star. Gravitational interactions with other bodies can make transits occur earlier or later than predicted.

Transit light curve: A record of a star’s brightness over time that shows the dip caused when a planet crosses its disk. Its repeated dips provide the timestamps whose changing intervals define the variation.
Kepler-9: A planetary system containing two transiting Saturn-sized planets near a 2:1 orbital resonance. Its timing variations provided an early prominent demonstration of detecting planets through mutual perturbations.
Planetary transit: The passage of a planet across the face of its star as seen by an observer. A sequence of planetary transits supplies the recurring events whose intervals are compared.
Radial velocity method: A planet-detection technique that measures periodic shifts in a star’s spectral lines caused by motion toward and away from Earth. Unlike timing variations, it measures the star’s reflex motion directly.
Orbital resonance: A relationship between orbital periods in which orbiting bodies repeatedly align in the same configuration. Near-resonant planets often produce especially large, structured timing variations.
Kepler-11: A compact system of six known transiting planets orbiting a Sun-like star. Transit timing variations helped constrain the masses of its tightly packed planets.
Orbital period: The time a body takes to complete one orbit around another body. A planet’s predicted transit schedule is based on its orbital period.
Astrometry: The measurement of celestial positions and their changes across the sky. Astrometry detects a planet through a star’s sky-plane motion, not altered transit intervals.
Mean-motion resonance: An orbital resonance in which two bodies’ orbital frequencies are close to a ratio of small integers. This specific resonance shapes the long-period timing signals of many interacting planets.
Exomoon: A natural satellite orbiting a planet beyond the Solar System. A moon can perturb its planet’s orbit and produce a timing signal, though confirmation is difficult.

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