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The other blue planet: HD 189733b's stunning azure hue revealed by Hubble telescope for first time

Deep cobalt colouring comes from drops of liquid gas raining horizontally in 7,000 km/h winds
  • @SteveAConnor

We are not alone. Scientists have discovered a second blue planet in the Universe, although this one is decidedly inhospitable and unlikely to support life.

Planet HD 189733b lies some 63 light years beyond our Solar System in the constellation Vulpecula is a deep cobalt blue according to data gathered by the Hubble space telescope, but its azure hue is not due to water but drops of liquid glass raining down horizontally in 7,000 kilometre-per-hour winds.

By measuring the wavelengths of light that are lost when the orbiting planet slips behind its star, scientists have been able to calculate the colour that the planet as it would appear if seen by the naked eye.

It is the first time that scientists have been able to calculate the visible colour of an “exoplanet” beyond our own Solar System, said Frederic Pont of the University of Exeter, one of the authors of the study.

“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,” Dr Pont said.

The planet is a gas giant, similar to Jupiter, and orbits very close to its sun, meaning that its temperatures are a scorching 1,000C or higher. Extreme winds pelt silicate particles sideways, which scatter blue light.

It was technically challenging to work out the colour of the planet because the light from its nearby star swamped any reflected light from the planet. However, by measuring the loss of light as the planet disappeared by its sun, the scientists were able to assess the wavelengths that are reflected by HD 189733b.

“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 colours we measured,” said Tom Evans of Oxford University, the lead author of the study.