The atmospheric compositions of exoplanets are usually determined by the technique of transit spectroscopy: When the planet transits between its host star and Earth, starlight passes through its atmosphere close to local dawn and dusk, imprinting additional absorption lines on the stellar spectrum. Pluriel et al. considered how the results are affected if the star heats the planet enough for atmospheric molecules to be dissociated on the planet’s day side and recombine on the night side. They found that transit spectroscopy then produces highly biased atmospheric compositions differing from the true values by up to three orders of magnitude. More sophisticated methods are needed to determine the atmospheric compositions of hot exoplanets.
Astron. Astrophys. 636, A66 (2020).