RV and g'z' transits of KOI-1089b/Kepler-418b

We report detection of an unresolved light source that contributes an additional ~40% to the target star, which would not have been detected without multicolor photometric analysis. The resulting planet-star radius ratio is 0.110+/-0.0025, more than 25% more than the 0.087 measured by Kepler, leading to a radius of 1.20+/-0.16R_jup_ instead of the 0.94R_jup_ measured by the Kepler team. This is the first confirmation of an exoplanet candidate based primarily on the transit color signature, demonstrating that this technique is viable from ground for giant planets.

It is particularly useful for planets with long periods such as Kepler-418b, which tend to have long transit durations. While this technique is limited to candidates with deep transits from the ground, it may be possible to confirm earth-like exoplanet candidates with a few hours of observing time with an instrument like the James Webb Space Telescope. Additionally, multicolor photometric analysis of transits can reveal unknown stellar neighbors and binary companions that do not affect the classification of the transiting object but can have a very significant effect on the perceived planetary radius.