It is a new step for exoplanets discovery. Thanks to Hubble Space Telescope, astronomers managed first to determine the color of a planet outside the solar system. Its name ? HD 189733 b. And the little surprise : this exoplanet is deep blue. You said blue, like Earth ? Yes. But a bit different of blue you can see on a picture of our planet.
Discovered in september 2005 by a team of astronomers from France and Switzerland, HD 189733 b, located at 63 light-years from the Sun, is a gas giant which is 1.26 times Jupiter's size and which orbits around its star (HD 189733, an orange dwarf) in only 2.2 days at a distance of 4.7 million kilometers.
The atmosphere of this planet says a lot : more than 1,000°C (because of its proximity with its star), 7,000 km/h winds. Its composition : water vapor, methane and carbon dioxide. A bit similar to our atmosphere's, but being a gas planet with extreme conditions, HD 189733 b couldn't welcome life.
But the most important is its color. Deep blue. How to find the color of this planet (like any other planet actually) ? Astronomers have observed the exoplanet before, during and after its passage in front of, then behind its star and evaluated each time the quantity and nature of light received, by noting the differences depending on the position. A graph is obtained.
"We saw the light of the whole system falling in the blue part of the spectrum when the planet passed behind the star. From there, we can understand that the planet is blue, because its signal is constant for the other colors measured", said Tom Evans from Oxford University. (Source : maxisciences.com, french article)
An other question : why is Hd 189733 b blue ? For the moment, the blue of the planet could be because of silicate particles in the atmosphere. Thanks to this method, astronomers hope find the color of other exoplanets (whose more than one thousand are officialy cataloged). With beautiful planet wallpapers that we can find on Google Images, yes, we could find blue, red, yellow, orange, green, purple planets. The thing which will be even more important, will be the first real photos of an exoplanet. Patience !