The heavens are home to an alien world that shines a deep cobalt blue in a solar system far, far away from our own.

Astronomers used the ageing Hubble space telescope to determine the true colour of the distant world, the first time such a feat has been achieved for a planet that circles a star other than the sun.

Unlike the pale blue dot that harbours all known life in the cosmos, the "deep blue dot" is an inhospitable gas giant that lies 63 light years from Earth. On HD189733b, as the planet is named, the temperature soars to 1,000C and glassy hail whips through the air on hypersonic winds.

Though the planet is hostile to life as we know it, the same technique could be used to spot potentially habitable worlds, through changes in cloud cover and other features.

Frederic Pont at Exeter University observed the planet before, during, and after it passed behind its star. When the planet was on either side, the telescope collected light from the star along with light reflected from the planet's surface. But as the planet moved behind the star, the light it reflected was blocked out.

Using an instrument onboard the telescope called an imaging spectrograph, Pont noticed that blue light dimmed sharply as the planet passed behind its star, but brightened again when it emerged on the other side. "As far as I am aware, nobody has had actual results on the colour of an exoplanet," Pont said. "Now we can say that this planet is blue."

The deep cobalt colouration is thought to come from a similar process to that which makes Earth look blue from space, namely the scattering of blue light in the atmosphere. On the planet Pont observed, the scattering is probably due to a fine mist of silicate particles that are blown around by 7,000kph winds.

With more advanced technology, the colour of a planet could help astronomers work out which ones might be habitable. Most planets in our solar system are either entirely covered by cloud, or have no clouds at all. Earth is unusual in that roughly half of its surface is obscured by cloud, a product of the water cycle that is essential for life.

"If you could see the colour of an exoplanet change over time it would be very revealing. At first, the cloud cover would be the thing to go for," Pont said.

While future instruments could be sensitive enough to observe the changing hues of alien worlds, there may be no suitable telescopes in space to put them on. The Hubble observatory is more than 20 years old and was last repaired by a dedicated space shuttle mission in 2009. Without Hubble, there are no other space telescopes that could look at planets in visible wavelengths. Hubble's replacement, the James Webb Space Telescope, will pick up infra-red light only.

"Most colleagues in the field focussing on habitability concentrate on the detection of molecules in the infra-red, like water, carbon monoxide and methane," Pont said. "That's useful information of course, but my opinion is that we might be giving too much weight to this compared to visible colour, which gives a different kind of information, but maybe just as crucial to understand the general state of an Earth-like planet."