HD 189733b: Hubble Measured Its Cobalt Colour In 2013, Revealing Silicate Hazes Instead Of Oceans Beneath
This article was compiled with the help of the following source(s): SpaceDaily.
From a distance, the exoplanet HD 189733b looks like the most reassuring thing in the night sky: a deep cobalt blue, the same shade as Earth seen from orbit.
For a moment, it is easy to imagine oceans, clouds, maybe even something familiar waiting out there 63 light-years away. That comfort does not survive a closer look. The blue on HD 189733b has nothing to do with water.
It comes from a scorching atmosphere hazed with silicate particles, the raw material of glass, in a place where that glass may fall sideways on winds of thousands of kilometres an hour. It is, in other words, an Earth-coloured planet that is about as far from Earth-like as a planet can possibly get.
A Colour That Was Actually Measured, Not Guessed
What makes this story scientifically remarkable is that the blue is not an artist’s impression or a rough estimate. In 2013, astronomers using the Hubble Space Telescope measured the colour directly, by watching the planet’s light as it slipped behind its host star and tracking how the combined light changed during that eclipse.
The dip in certain wavelengths told researchers exactly what the planet itself was reflecting back into space. The answer came back as a deep azure blue, and it marked the first time scientists had ever pinned down the precise visible colour of a planet beyond our solar system.
HD 189733b is classified as a “hot Jupiter,” a gas giant roughly the size of our own Jupiter, but orbiting so close to its star that it completes a full lap in just about two days. That proximity is the entire reason this planet has become one of the most intensely studied worlds outside our solar system.
It is close enough, and crosses in front of its star often enough, that astronomers can interrogate it again and again with each new generation of telescope technology.
Why The Blue Is Misleading
On Earth, the colour blue is the product of two very specific things: sunlight scattering through our atmosphere and that same light bouncing off vast oceans covering most of the planet’s surface.
It is natural, almost instinctive, to project that same explanation onto any blue dot we spot from a distance. HD 189733b breaks that assumption completely. It has no ocean. It has no solid surface at all to speak of, since it is fundamentally a massive ball of gas.
Its blue colour instead comes from high-altitude hazes laced with silicate grains, tiny particles that happen to scatter blue light efficiently while absorbing most other wavelengths. Seen from far enough away, the visual outcome looks nearly identical to Earth.
The underlying cause could not be more different. Same colour. Completely opposite reason.
Glass Rain, Moving Sideways
The conditions responsible for producing those silicate hazes are genuinely brutal by any standard. The planet’s daytime atmosphere regularly exceeds 1,000 degrees Celsius, hot enough that silicate material exists as vapour high in the atmosphere before condensing into tiny molten droplets of glass as it cools and rises further.
Now add the wind into that equation. Atmospheric models put HD 189733b’s wind speeds at around 7,000 kilometres per hour, something in the range of seven times the speed of sound here on Earth. At that velocity, those glass droplets would not fall gently downward the way rain does on our planet.
Instead, they would be driven violently horizontal, ripping sideways through the atmosphere. By some descriptions, a single droplet travelling at that speed would strike with something comparable to the impact energy of a small artillery shell.
That single detail is what has made HD 189733b famous well beyond astronomy circles: a planet where it effectively rains glass, sideways, at supersonic speed.
Two honest caveats deserve to sit alongside that vivid image, though. Because HD 189733b is a gas giant with no solid surface, there is no ground anywhere for that “rain” to actually land on, the way it does here on Earth.
The glass droplets form and move entirely within the planet’s thick atmosphere itself, never striking any surface in the traditional sense. And the sideways-glass description, while based on real atmospheric modelling, is ultimately an inference drawn from observed silicate clouds combined with calculated wind speeds.
Nobody has photographed this phenomenon directly. It is a scientifically reasonable interpretation of genuinely harsh atmospheric data, not a postcard snapshot beamed back from the planet’s surface.
What This Really Teaches Us About Distant Worlds
The most valuable lesson buried inside this story has very little to do with glass or wind speed, and everything to do with how much a single colour can conceal. A blue planet is not automatically a watery, welcoming one. The most Earth-looking dot of light in a telescope can turn out, on closer inspection, to be a roaring furnace where the local weather is quite literally made of molten glass moving at supersonic speed.
HD 189733b remains one of the most closely studied exoplanets precisely because its proximity and frequent transits make it an ideal natural laboratory. Each new round of observation adds another layer to scientists’ understanding of its atmosphere, its chemistry, and its violent weather patterns.
The clearer that picture becomes, the louder the underlying warning grows for anyone scanning the night sky for something familiar: at interstellar distances, appearances are only a starting point for genuine scientific questions, never an answer in themselves.
Check out our previous coverage of space discoveries and exoplanet research here.
