First exoplanet atmosphere detection (2001)
Astronomers using the Hubble Space Telescope detected sodium in the atmosphere of HD 209458b, a hot Jupiter 150 light-years away. They saw starlight filter through the planet's atmosphere during a transit and absorb sodium's signature.
Confirmed that exoplanet atmospheres could be studied from Earth-orbit instruments and opened the field of transit spectroscopy.
Created the observational toolkit — transit spectroscopy — that JWST now uses in far greater detail.
Transmission strings is the latest refinement of that same tool: starlight at the edge of the atmosphere, measured at angles to build a 3D picture instead of a single average.
