Water and molecular oxygen absorption have different properties because of the different structure of the molecules. Upper panel: transmittance of Earth in the J band. The deep telluric lines are from molecular oxygen. They are well separated, and the transmittance approaches 1 between them. Lower panel: an aerosol-free model of HD189733b in the same region. Water generates a dense forest of lines, such that the exoplanetary signal can be recovered from the ground from in-between the deep oxygen telluric lines.