ananyo writes "A navy-blue world orbiting a faraway star is the first exoplanet to have its colour measured. Discovered in 2005, HD 189733 b is one of the best-studied planets outside the Solar System, orbiting a star about 19 parsecs away in the Vulpecula, or Fox, constellation. Previous efforts to observe the planet focused on the infrared light it emits — invisible to the human eye. Astronomers have now used the Hubble Space Telescope to observe the planet and its host star. Hubble's optical resolution is not high enough to actually 'see' the planet as a dot of light separate from its star, so instead, the telescope receives light from both objects that mix into a single point source. To isolate the light contribution of the planet, the researchers waited for the planet to move behind the star during its orbit, so that its light would be blocked, and looked for changes in light colour. During the eclipse, the amount of observed blue light decreased, whereas other colours remained unaffected. This indicated that the light reflected by the planet's atmosphere, blocked by the star in the eclipse, is blue."