Left panel: synthetic spectra and observational data of the emission spectrum of WASP-18b. A description of the different lines is shown in the legend. The observational data by Nymeyer et al. (2011) are shown as black errorbars. The “emission” lines, which can be made out in the blackbody F_Pl/F_∗ spectrum, are absorption lines of the stellar spectrum. The colored circles show the corresponding Spitzer channels for the synthetic spectra. Right panel: p values of the Kolmogorov-Smirnov test of the residuals between the synthetic observation of the three WASP-18b models and the two respective remaining models. The p value is shown as a function of N_transit for data taken with the three different instruments listed in Table 3. In order to avoid distinguishing between the models based on offsets, the residual distributions were shifted to have a mean value of 0. The black dashed line denotes our threshold value of 10^-3. If an instrument is not shown in one of the three sub-panels then it has a p value lower than 10-12 already after one observation.