The star HD 189733 is an orange dwarf glimmering at magnitude 7.7 in Vulpecula 63 light-years away. If you're an amateur telescope user, you've probably seen it many times without knowing it. The star lies in the same field as the Dumbbell Nebula, M27, just ¼° to the nebula's east. A Jupiter-mass planet orbits the star very closely every 2.2 days, as revealed by the star's radial-velocity wobble and also by a slight dimming every time the planet crosses in front of its face. (Details).A team of astronomers used a spectrograph on the Hubble Space Telescope to to examine the star with and without the planet in front of it. The difference revealed evidence about the tiny fraction of light that skimmed sideways through the planet's upper atmosphere on its way to us. Spectral evidence of sodium, potassium, and water were expected but not seen.
The team says that the best explanation for this lack of spectral features is haze in the planet's upper atmosphere that keeps light from passing freely through lower layers. Such a haze is indeed expected for hot planets (this one should be about 800° Celsius), consisting of iron, silicates (rock dust) and/or aluminum oxide (sapphire dust). The astronomers present their full report in the Monthly Notices of the Royal Astronomical Society. NASA and the European Space Agency issued a press release this morning.
On the other hand, astronomers using the ground-based but much larger Hobby-Eberly Telescope in Texas did the same project and barely detected sodium absorption lines in the planet's upper atmosphere. Press release.