Why is the sky blue? It's a simple question but one with a surprisingly complex answer if the sky belongs to a planet outside our solar system.
'If you are on Earth looking up the sky looks blue because other wavelengths of light are scattered by molecules like oxygen and nitrogen in the atmosphere,' explains Tom Evans of Oxford University's Department of Physics. 'If you look at the Earth from space it appears (mostly) blue both because of this effect and because water in oceans and lakes absorb other wavelengths, only reflecting blue light back into space.'
So viewed through an astronomer's eyes colour is much more than a pretty effect: it's an invaluable source of information.
Now, for the first time, scientists have determined the colour of a planet outside our solar system (an 'exoplanet'). Because they are so distant, and so much smaller than stars, seeing exoplanets directly with current telescopes is normally impossible (most of what we know about them comes from indirect observations, for instance of nearby stars).
An international team, led by researchers from Oxford University and Exeter University, took advantage of a secondary eclipse, when a planet disappears behind its star. They used the Hubble Space Telescope to study the moment the exoplanet 'HD 189733b' passed behind its parent star so that they saw both the star's light and light reflected off the planet (its 'albedo') and then, once HD 189733b has disappeared, just the star's light on its own.
From the difference in brightness between these two observations they were able to infer HD 189733b's brightness and by examining the wavelength of light reflected off it they were able to determine that it would appear a deep cobalt blue to our eyes.
But HD 189733b, which is 63 light-years away, isn't blue because it is like Earth.
'It's very different from the planets in our solar system,' Tom Evans, first author of a report of the research in Astrophysical Journal Letters, tells me. 'Unlike Jupiter or Saturn this gas giant orbits very close to its star, so it's bombarded with massive amounts of radiation and its atmosphere can reach a temperature of over 1000 degrees Celsius. The planet is also tidally locked so that one side is permanently facing its star whilst the other side is in eternal shadow.'
Suzanne Aigrain of Oxford University's Department of Physics, also an author of the report, comments: 'Despite these differences the laws of physics are the same, and as every planet with an atmosphere in our solar system has clouds we can infer that HD 189733b has clouds. We suspect that these clouds are made of silicate particles, but we don't know how and where they are formed, and the fact that they could be moving at very high speed (with winds of up to 7000 kilometres per hour) makes observations very difficult.'
The researchers believe that a large part of HD 189733b's blue appearance is down to sodium atoms in its atmosphere, as sodium atoms absorb more light at red wavelengths. 'If it wasn't for sodium absorbing the redder wavelengths, the planet would probably be more of a white colour,' Tom explains.
'This planet has been studied well in the past, both by ourselves and other teams,' says Frédéric Pont of the University of Exeter, leader of the Hubble observing programme and an author of this new paper. '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's system is one of the best studied of all exoplanet systems because its star is bright and close to its planets, making interactions easier to spot. 'It's one of the most favourable systems, there aren't many where we can do the same thing,' comments Suzanne. But its parent star does pose some problems; it's an orange dwarf (or 'K-dwarf'), around four-fifths the size of our Sun, that's very magnetically active so it regularly shoots out flares and star spots that can interfere with observations.
'One of the next questions to answer is just how much of the parent star's light HD189733b is absorbing, because the wavelengths we've measured only account for about 20 per cent of the starlight that falls upon the planet,' Tom adds.
Determining exactly how much energy is being fed into its climate system overall has important implications for the circulation and weather on the planet. To do this, the astronomers will need to extend the measurement at longer wavelengths. This will allow them to confirm that none of the red or near-infrared light can escape from the atmosphere, as they currently suspect.
Other members of the Oxford team, along with collaborators at Bern University, will now begin to feed all the data from the recent observations into a model of the planet's atmosphere. 'A lot of what we do draws on models produced using data from gas giants in our own solar system. These enable us to make some basic predictions, although we know that if we push these models to extremes some of these assumptions break down,' Suzanne tells me.
'We would also like to do similar measurements for other planets, to understand how pervasive clouds are in these 'hot Jupiter' planets,' she adds. 'Currently Hubble is the only telescope we can do this with, and it's not clear how many planets we can do it for (HD189733b is one of the most favourable targets). But in the future we may develop clever techniques that enable us to do some of this from the ground, though it's harder because the Earth's atmosphere gets in the way.'
The hope is that with new, more powerful instruments like the James Webb telescope and especially the proposed European space mission EChO we might be able to get an even better glimpse of the atmospheres (and colours) of this and other exoplanets.