12 Jul 2013

Planet's colour measured as blue

2:58 pm on 12 July 2013

For the first time, astronomers have determined the true colour of a planet orbiting another star.

HD-189733-b has a deep azure hue - the result of silicate or glass rain in the atmosphere which scatters blue light. It is 63 light-years from Earth. The discovery was made by the Hubble space telescope.

Although it might resemble Earth from a distance, the planet is a huge gas giant which orbits close to its host star.

The BBC reports the temperature of the planet's atmosphere is 1000 degrees Centigrade and it rains glass, sideways, in winds of 7000km per hour.

In order to measure what this planet would look like to our eyes, the astronomers measured how much light was reflected from its surface - a property known as albedo.

As the planet passed behind its host star, the astronomers were able to measure changes in the spectrum as light reflected by the planet was temporarily blocked out.

"We saw the brightness of the whole system drop in the blue part of the spectrum when the planet passed behind its star," said Tom Evans from the University of Oxford.

''From this, we can gather that the planet is blue, because the signal remained constant at the other colours we measured."

The BBC reports Earth looks blue from space because the oceans absorb red and green wavelengths more strongly than blue ones, and reflect the blue-ish hue of our sky.

HD 189733b presented a favourable case for these kinds of measurements as it belongs to a class of planets known as "hot Jupiters".