HD 209458 b

HD 209458 b is a transiting hot Jupiter that illustrates how an unresolved exoplanet atmosphere can be measured. In the sodium experiment reported in 2002, four exoplanet transits were observed around the sodium doublet near $589.3\mathrm{nm}$. The exoplanet transit depth in that narrow band exceeded the depth in neighbouring bands by approximately $2.3\times 10^{-4}$, interpreted as additional sodium absorption in the planetary limb.^[ref]

The observable is a difference in blocked area with wavelength, rather than an isolated planetary emission spectrum. Its interpretation requires radiative transfer through the limb and a model of the illuminating star. The original sodium signal was weaker than a simple clear-atmosphere prediction, already illustrating why exoplanet clouds, composition and photoionization matter.