Ever since our astronomers have gained the ability to see into deep space and discern what lies in distant solar systems, a total of 910 extra-solar planets have been discovered. Of those, only a handful have been confirmed as potentially habitable by Earth scientists. Despite these discovered, it was not until recently that a “blue planet” outside of the Solar System, thanks to NASAs Hubble telescope.
But here’s the kicker: as it turns out, the planet is not blue due to the presence of liquid water. The blue color likely comes from clouds in the atmosphere made of molten glass. The planet is known as HD 189733 b, located roughly 63 light years away from Earth in the constellation of Vulpecula (aka. the Fox). Initially discovered in 2005 by French astronomers who observed it passing in front of its star, HD 189733 b is one of the best-studied exoplanets.
Prior to this new finding, it was already known that the planet was a hot Jupiter — a massive gas giant that orbits very close to its parent star — and that, using polarimetry, it was most likely blue. Since that time, the blue color has been confirmed by a spectrograph aboard Hubble which scanned the planet during an eclipse. As it passed behind its parent star and out of our vision, Hubble recorded less blue light coming from the star, while the other colors remained the same.
This strongly indicates that the light reflected by HD 189733 b’s atmosphere is blue and thus, if we were close enough to directly observe the planet, it would appear blue. This is an apparent first for astrophysicists and astronomers, who wouldn’t normally be able to observe such a fluctuation from 63 light years away. But the size of the planet, plus the amount of light reflecting off it from its very-close-by star, mean that Hubble can do its thing.
As for the cause of the color itself, the current theory is that the planets atmosphere is full of clouds that contain tiny silicate particles, which absorb some light frequencies but reflect and scatter blue light. In the words of NASA, because the surface of the planet is around 815 Celsius (1,500 Fahrenheit), these particles are likely in a molten, liquid state that periodically turn into rain. Yes, you read that right, the planet experiences periods of molten glass rain!
In addition to that, it is also known that its orbital period (length of a year) is only 2.2 days. The planet is also tidally locked, meaning that one side is always facing towards the sun while the other experiences perpetual night. So basically, outside of its blue color, HD 189733 b is about as uninhabitable as it gets.
Ah well, the search for a truly Earth-like exoplanet continues I guess! And in the meantime, enjoy this short video from Hubble ESA – a computer graphic representation of the universe’s other “blue planet”: