"During primary transits, the spectral signatures of exoplanet atmospheres can be measured using transmission spectroscopy. We can obtain information on the upper atmosphere of these planets by investigating the exoplanets’ excess sodium absorption in the optical region."
https://arxiv.org/pdf/1610.01610.pdf
The element they used is sodium:
"During primary transits, the spectral signatures of exoplanet atmospheres can be measured using transmission spectroscopy. We can obtain information on the upper atmosphere of these planets by investigating the exoplanets’ excess sodium absorption in the optical region."