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Transmission spectroscopy has been successfully used from both the ground andin space to characterise the atmospheres of transiting exoplanets. Thistechnique is challenging from the ground because ground-based spectrographstend not to be designed to be photometrically stable, and effects such asvariable slit losses cause significant systematic uncertainties. An alternativeapproach is to use simultaneous photometric observations in multiple wavebandsto determine wavelength dependent transit depth differences. We report anapplication of this technique to one of the hottest known exoplanets, WASP-12b,using the triple-beam camera ULTRACAM. We obtained simultaneous light curves inSloan u', and two narrow band filters centered on 4169 and 6010 angstroms, withFWHMs 52 and 118 angstroms respectively. We fit these light curves with aphotometric model and determine the planetary radius in the three differentbands. Our data show no evidence for a difference in planetary radius over thewavelength range we study, and are consistent with an atmosphere that isdominated by Rayleigh scattering from a high altitude haze, as well as morecomplicated atmosphere models which include the effects of molecules such asTiO. Our planetary radius measurements have an average precision of 2.6 percent, compared to the ~1.4 - 2.4 per cent radius differences predicted by themodels over this wavelength range. We also find a consistent time of ingressand egress across our three wavebands, in contrast to the early ingress whichhas been reported for this system at shorter wavelengths.

Transmission photometry of WASP-12b: simultaneous measurement of the planetary radius in three bands