Transmission spectroscopy of exoplanet atmospheres — looking at the atmosphere of a planet in transit, backlit by the light of its star — is one of the major growth areas in studying WASP planets.

The latest such study is by Patrick Fischer and colleagues, who pointed the Hubble Space Telescope with its STIS spectrograph at WASP-39b in transit.

The plot shows the resulting data compared with three models of WASP-39b’s atmosphere (depending on how clear or hazy it is, and on the metal abundance compared to the Sun).

Unlike some hot Jupiters, which have very hazy atmospheres with few spectral features, WASP-39b shows a clear detection of potassium and sodium, as expected in largely clear skies.

Comparing to the hazier planets HD 189733b and WASP-6b, Fischer et al remark: “These observations further emphasize the surprising diversity of cloudy and cloud-free gas giant planets in short-period orbits and the corresponding challenges associated with developing predictive cloud models for these atmospheres”.