A Planet Blue, Where Raining Glass Lends Its Hue

The European Space Agency announced the discovery of another blue planet:

This “deep blue dot” is a huge gas giant orbiting very close to its host star. The planet’s atmosphere is scorching with a temperature of over 1000 degrees Celsius, and it rains glass, sideways, in howling 7000 kilometre-per-hour winds.

The planet—known as HD 189773b—was first discovered in 2005, but we’ve only recently discerned the deep azure blue of its surface. It’s the first time we have been able to tell an exoplanet’s true color.

Scientists made that discovery by measuring the combined light emitted from the star and the light reflected off the planet. They noticed the amount of blue light dropped off when HD 189773b moved behind its star. As it reemerged, the blue light reappeared. All other colors remained constant.

hd 189773b
An artist's impression of HD 189773b

Although its color is similar to Earth’s, HD 189773b’s proximity to its star and its enormous size—a gas giant like Jupiter—make it a very different celestial body. Where Earth’s blue tint comes from light reflected off our oceans, scientists think the exoplanet’s come from silicate particles—small bits of glass, most likely—which are whipping through its atmosphere at tremendous speed. Although HD 189773b seems dangerously inhospitable, determining the color of other exoplanets could help us predict which of them may harbor life.