NASA scientists have discovered a blue planet that possibly rains glass, orbiting a star 63 light-years away, marking the first time an exoplanet’s true colour has been determined.

Astronomers making visible-light observations with NASA’s Hubble Space Telescope spotted the planet HD 189733b which is one of the closest exoplanets that can be seen crossing the face of its star.

Scientists measured changes in the colour of light from the planet before, during and after a pass behind its star.

There was a small drop in light and a slight change in the colour of the light.

“We saw the light becoming less bright in the blue but not in the green or red. Light was missing in the blue but not in the red when it was hidden,” said researcher Frederic Pont of the University of Exeter in South West England.

“This means that the object that disappeared was blue,” said Pont.

Earlier observations have reported evidence for scattering of blue light on the planet. The latest Hubble observation confirms the evidence.

If seen directly, this planet would look like a deep blue dot, reminiscent of Earth’s colour as seen from space.

On this turbulent alien world, the daytime temperature is nearly 1093 degrees Celsius, and it possibly rains glass – sideways – in howling, 7242kph winds.

The cobalt blue colour comes not from the reflection of a tropical ocean as it does on Earth, but rather a hazy, blow-torched atmosphere containing high clouds laced with silicate particles.

Silicates condensing in the heat could form very small drops of glass that scatter blue light more than red light.

Hubble and other observatories have made intensive studies of HD 189733b and found its atmosphere to be changeable and exotic.

HD 189733b is among a bizarre class of planets called hot Jupiters, which orbit precariously close to their parent stars. The observations yield new insights into the chemical composition and cloud structure of the entire class.

Clouds often play key roles in planetary atmospheres.

Detecting the presence and importance of clouds in hot Jupiters is crucial to astronomers’ understanding of the physics and climatology of other planets.

HD 189733b was discovered in 2005. It is only 4.6 million kilometres from its parent star, so close that it is gravitationally locked. One side always faces the star and the other side is always dark.

In 2007, NASA’s Spitzer Space Telescope measured the infrared light, or heat, from the planet, leading to one of the first temperature maps for an exoplanet.

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