T. M. Evans of the University of Oxford and nine others used the Hubble Space Telescope to make an observation which is now published in Astrophysical Journal Letters, and claims:
We present a secondary eclipse observation for the hot Jupiter HD189733b across the wavelength range 290–570nm made using the Space Telescope Imaging Spectrograph on the Hubble Space Telescope. We measure geometric albedos of Ag=0.40±0.12 at 290–450nm and Ag<0.12 at 450–570nm. The albedo decrease towards longer wavelengths is also apparent when using six wavelength bins over the same wavelength range. This can be interpreted as evidence for optically thick reflective clouds on the dayside hemisphere, with sodium absorption suppressing the scattered light signal beyond ~450nm as predicted by models of hot Jupiter atmospheres. Our best-fit albedo values imply that HD189733b would appear a deep blue color at visible wavelengths.
The Guardian translates this into:
Astronomers used the ageing Hubble space telescope to determine the true colour of the distant world, the first time such a feat has been achieved for a planet that circles a star other than the sun.
Unlike the pale blue dot that harbours all known life in the cosmos, the “deep blue dot” is an inhospitable gas giant that lies 63 light years from Earth. On HD189733b, as the planet is named, the temperature soars to 1,000C and glassy hail whips through the air on hypersonic winds.
Though the planet is hostile to life as we know it, the same technique could be used to spot potentially habitable worlds, through changes in cloud cover and other features.
Frederic Pont at Exeter University observed the planet before, during, and after it passed behind its star. When the planet was on either side, the telescope collected light from the star along with light reflected from the planet’s surface. But as the planet moved behind the star, the light it reflected was blocked out.
Using an instrument onboard the telescope called an imaging spectrograph, Pont noticed that blue light dimmed sharply as the planet passed behind its star, but brightened again when it emerged on the other side. “As far as I am aware, nobody has had actual results on the colour of an exoplanet,” Pont said. “Now we can say that this planet is blue.”