Original 2017 paper in A&A:

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."

> This method can only be used if a planet and its star differ in their chemical compositions

How prevalent is that condition?

A lot of stars are too hot to have much in the way of complex molecules.

The article goes into some detail about the significance of this planet having water and carbon monoxide, but not methane or ammonia.