Top: Hubble/WFC3 transmission spectrum of HD 209458b (Deming et al. 2013; black). Overplotted are the principal components of the four best matching molecules binned to the resolution of the data: H₂O (blue dots), CO₂ (green squares), NH₃ (magenta triangles), and NO (red squares). The H₂O principal component is also shown at a resolution of R = 1000 (light blue). Bottom: the normalized euclidean distance between each individual component and the data. It is clear that H₂O presents the best match to the data with other molecules being significantly worse.