When was the first confirmed detection of an exoplanet atmosphere made?

In 2001, astronomers detected sodium in the atmosphere of the exoplanet HD 209458 b, marking the first confirmed observation of an atmosphere beyond the Solar System. The measurement used transit spectroscopy, where starlight filters through the planet’s atmosphere during a transit. This was a breakthrough because it proved that exoplanet atmospheres could be studied directly. It opened a major new branch of astronomy focused on planetary atmospheres and habitability.