Characterization of extrasolar multi-planet systems by Transit Time Variation
Multi-planet systems are crucial for the understanding of the formation and dynamical evolution of planetary systems. Only a small number of confirmed planets with true masses is known (compared to the number of all planetary candidates) because there is not enough telescope time available to cover all pending confirmations, in particular for faint stars. The mass determination for Earth- or Neptune-sized planets by radial velocity is difficult with current telescopes and instrumental technique. Another method to characterize the planetary system is the Transit Time Variation (TTV). TTV is the earlier or later occurrence of a planetary transit relative to the time of a reference transit. TTV is dominantly caused by third body orbit perturbations by attracting forces acting on the transiting planet by at least one another planet in the same system. Gravitational interactions perturb the velocity of the transiting planet in its orbit which manifests in a periodical perturbation of the revolution period. Measurements of the transit times and the identification of differences from a mean transit period may prove the existence of further planets. This method uses only the information from the light curve and can detect, confirm and characterize transiting as well as non-transiting planets in multi-planet systems. Their orbital elements can be estimated if the TTV is sufficiently resolved. Upcoming spaced-based surveys will observe various stars for which the usual planetary mass determination by ground-based radial velocity observation is not always possible.