Direct Evidence for an Evolving Dust Cloud From the Exoplanet Kic 12557548 B

We present simultaneous multi-color optical photometry using of the transiting exoplanet KIC 12557548 b (also known as KIC 1255 b) ULTRACAM. 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 ± 0.04% in z '; 1.00 ± 0.03% in g '; and 1.1 ± 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 . 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 μ 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.