The heavens are home to an alien world that shines a deep cobalt blue in a solar system far, far away from our own. 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."
The deep cobalt colouration is thought to come from a similar process to that which makes Earth look blue from space, namely the scattering of blue light in the atmosphere. On the planet Pont observed, the scattering is probably due to a fine mist of silicate particles that are blown around by 7,000kph winds.
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