Which Hubble instrument made the first exoplanet atmosphere detection in 2001?

The Space Telescope Imaging Spectrograph, or STIS, made Hubble’s first exoplanet-atmosphere detection. Its observations of the transiting hot Jupiter HD 209458 b revealed the spectral signature of sodium in light filtered through the planet’s atmosphere.

HD 209458 b passes in front of its star every 3.5 days. During a transit, a tiny fraction of starlight travels through the planet’s atmospheric edge before reaching Earth. STIS separates that light into a spectrum, allowing astronomers to compare the in-transit and out-of-transit measurements and identify absorption caused by sodium near the yellow-orange sodium D lines.

The observations were obtained during four transits in 2000, and the result was announced in November 2001 as the first direct measurement of an atmosphere around a planet outside the Solar System. Later STIS observations detected a much larger, escaping envelope containing hydrogen, oxygen and carbon. WFC3, COS and ACS are also important Hubble instruments, but they did not make this landmark 2001 sodium detection.