Right panel: The ratio of observed - to - true transit depths at a handful of specific wavelengths where molecular absorption features appear. (aasnova.org)
The definition uses atmospheric databases called HITRAN (high - resolution transmission molecular absorption) and HITEMP (high - temperature spectroscopic absorption parameters) that characterize planetary atmospheres in light of how both carbon dioxide and water are absorbed. (centauri-dreams.org)
However, the number of known molecular absorption systems seen in millimeter / submillimeter waveband has been very limited: only about 30 in the Milky Way galaxy and a limited number in other galaxies. (sciencedaily.com)