We present simultaneous multi-color optical photometry using ULTRACAM of the transiting exoplanet KIC 12557548 b ( also known as KIC 1255 b ) .
This reveals , for the first time , the color dependence of the transit depth .
Our g ’ and z ’ transits are similar in shape to the average Kepler short-cadence profile , and constitute the highest-quality extant coverage of individual transits .
Our Night 1 transit depths are 0.85 \pm 0.04 \% in z ’ ; 1.00 \pm 0.03 \% in g ’ ; and 1.1 \pm 0.3 \% in u ’ .
We employ a residual-permutation method to assess the impact of correlated noise on the depth difference between the z ’ and g ’ bands and calculate the significance of the color dependence at 3.2 \sigma .
The Night 1 depths are consistent with dust extinction as observed in the ISM , but require grain sizes comparable to the largest found in the ISM : 0.25–1 \mu m. This provides direct evidence in favor of this object being a disrupting low-mass rocky planet , feeding a transiting dust cloud .
On the remaining four nights of observations the object was in a rare shallow-transit phase .
If the grain size in the transiting dust cloud changes as the transit depth changes , the extinction efficiency is expected to change in a wavelength- and composition-dependent way .
Observing a change in the wavelength-dependent transit depth would offer an unprecedented opportunity to determine the composition of the disintegrating rocky body KIC 12557548 b .
We detected four out-of-transit u ’ band events consistent with stellar flares .