Exoplanets – planets outside of our own solar system – are now detected on a regular basis. As of 2020, over 4’000 worlds with broadly varying temperatures, masses and sizes are known. When the light of the host star shines through a planetary atmosphere and eventually reaches us, we can learn about the structure and composition of the exoplanetary atmosphere. Such observations, termed “transmission spectra”, have to be compared with computer models of atmospheres in order to measure atmospheric properties such as temperature and oxygen abundance. These computer models vary greatly between different studies, but mostly assume that the atmospheric density decreases exponentially with altitude (as on Earth).