The study marked the first time the colour of a planet outside our solar system has ever been accurately measured, with the results suggesting it is a deep shade of blue – the same colour as Earth when viewed from space.
Prof Frédéric Pont, who led the study, published in Astrophysical Journal Letters, said: “This planet has been studied well in the past, both by ourselves and other teams.
“But measuring its colour is a real first — we can actually imagine what this planet would look like if we were able to look at it directly.”
HD 189733b is 63 light years away, meaning it can only be faintly detected as it passes across the surface of its parent star, and its colour can not be directly observed.
But astronomers were able to calculate how it would look to our eyes by measuring the amount of light reflected off its surface.
They analysed the amount of starlight reaching Hubble before, while and after the planet moved behind its parent star, and found that the volume of blue light dropped as it disappeared from view.
Thomas Evans of Oxford University, first author of the paper, explained: “We saw the brightness of the whole system drop in the blue part of the spectrum when the planet passed behind its star.
“From this, we can gather that the planet is blue, because the signal remained constant at the other colors we measured.”
HD 189733b gets its blue colour from its turbulent atmosphere which is thought to be rich in silicate particles, which reflect blue light.
It belongs to a class of planets known as “hot Jupiters“, which are similar in size to gas giants in our solar system but are situated much closer to their star.
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