The Hubble Space Telescope has determined the true colour of an alien planet for the first time – a dazzling blue.
Astronomers using the NASA/ESA telescope said that the planet, known as HD 189733b, is a deep cobalt blue, much like Earth.
But that’s where the similarities end.
The planet is a huge gas giant orbiting very close to its host star and its atmosphere is scorching, with a temperature of over 1,000 degrees Celsius.
It also rains glass – sideways – in howling 7,000 kilometre-per-hour winds.
At a distance of 63 light-years from us, this turbulent alien world is one of the nearest exoplanets to Earth that can be seen crossing the face of its star.
It has been intensively studied by Hubble and other telescopes and its atmosphere has been found to be dramatically changeable and exotic, with hazes and violent flares.
Now, it is the subject of an important first – the first measurement of an alien planet’s visible colour.
Earlier observations using different methods have reported evidence for scattering of blue light on the planet, but these most recent Hubble observations give robust confirming evidence, according to the scientists.
The planet’s colour will be discussed in detail in a paper in Astrophysical Journal Letters.
‘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.’
In order to measure what this planet would look like to our eyes, the astronomers measured how much light was reflected off the surface of HD 189733b, a property known as albedo.
To isolate the planet’s light from starlight, the team used Hubble’s Space Telescope Imaging Spectrograph (STIS) to peer at the system before, during, and after the planet passed behind its host star as it orbited.
As it slipped behind its star, the light reflected from the planet was temporarily blocked from view and the amount of light observed from the system dropped.
This technique also shows how the light changes in other ways and crucially its true colour.
Tom Evans, of the University of Oxford, and another of the paper’s authors, said: ‘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.’
HD 189733b belongs to a class of planets known as ‘hot Jupiters’. These massive planets are similar in size to the gas giants in the Solar System, but instead lie very close to their parent star.
It is this size and proximity to their star make them perfect subjects for exoplanet hunting.
Scientists know that hot Jupiters are numerous throughout the universe and as we do not have one close to home in our own Solar System, studies of planets like HD 189733b are important to help them understand these dramatic objects.
Professor Pont said: ‘It’s difficult to know exactly what causes the colour of a planet’s atmosphere, even for planets in the Solar System.
‘But these new observations add another piece to the puzzle over the nature and atmosphere of HD 189733b. We are slowly painting a more complete picture of this exotic planet.’