Context :

Aims : We announce confirmation of Kepler-418b , one of two proposed planets in this system .
This is the first confirmation of an exoplanet based primarily on the transit color signature technique .

Methods : We used the Kepler public data archive combined with multicolor photometry from the Gran Telescopio de Canarias and radial velocity follow-up using FIES at the Nordic Optical Telescope for confirmation GTC g ^ { \prime } and z ^ { \prime } photometry and NOT-FIES radial velocity measurements are only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr ( 130.79.128.5 ) or via http : //cdsweb.u-strasbg.fr/cgi-bin/qcat ? J/A+A/ .

Results : We report a confident detection of a transit color signature that can only be explained by a compact occulting body , entirely ruling out a contaminating eclipsing binary , a hierarchical triple , or a grazing eclipsing binary .
Those findings are corroborated by our radial velocity measurements , which put an upper limit of \sim 1 \mathrm { M } _ { \mathrm { Jup } } on the mass of Kepler-418b .
We also report that the host star is significantly blended , confirming the \sim 10 % light contamination suspected from the crowding metric in the Kepler light curve measured by the Kepler team .
We report detection of an unresolved light source that contributes an additional \sim 40 % to the target star , which would not have been detected without multicolor photometric analysis .
The resulting planet-star radius ratio is 0.110 \pm 0.0025 , more than 25 % more than the 0.087 measured by Kepler leading to a radius of 1.20 \pm 0.16 \mathrm { R } _ { \mathrm { Jup } } instead of the 0.94 \mathrm { R } _ { \mathrm { Jup } } measured by the Kepler team .

Conclusions : 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 .