With all the information astronomers have been able to gather about planets orbiting other stars — their mass, the length of their year, their potential for harbouring life and even what's in their atmosphere — for the very first time, they have figured out what colour a planet is, and it's turned out to be a rather familiar shade of blue.
About 63 light years away, orbiting an orange dwarf star in the northern constellation Vulpecula (the Fox), is a massive, scorching-hot gas giant planet named HD 189733b. This 'hot Jupiter' orbits so close to its star that its year lasts only around 2.2 of our Earth-days, and it's 'tidally-locked' with its star, so that the same side of it faces the star at all times. Temperatures on the 'day side' measure around 1,000 degrees Celsius, and those on the 'night side' still reach around 500 degrees. It's estimated that this temperature difference between the two sides of the planet whips up the atmosphere into winds blowing over 7,000 km/h!
HD 189733b was discovered in 2005, using the 'transit method', as astronomers in France saw the light from the star dim repeatedly as the planet whizzed around it every 2 days or so. It was this same transiting, or technically the exact opposite, that allowed them to figure out what colour the planet is.
The planet itself is far too close to the star to see directly. The light from the star just completely washes out our view of everything close to it (thus the 'artist's conception' drawings). However, the light a telescope receives from there includes both the light from the star and, at times, light reflected from the planet's atmosphere. Using that, we can figure out the planet's colour.
So, aiming the powerful Hubble Space Telescope at the star, astronomers collected the light from this system. When the planet is on the same side of the star as we are, we're not going to get much from it. When it swings around the other side and is just about to duck behind the star, we have a brief window of opportunity to pick up both the star's light and the reflected light from the planet. When the planet disappears behind the star, we won't see the reflected light, and then we'll pick it up again right after the planet emerges from the other side of the star.
"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 Prof. Frederic Pont, a member of the research team from the University of Exeter, according to a NASA statement. "This means that the object that disappeared was blue."
This video gives a very cool view of this hot planet, showing what it would look like if we did a fly-by:
Earth appears as a 'blue marble' or 'pale blue dot' (depending on which spacecraft took the pictures), which is due to the water that covers about 70% of the planet's surface. However, in the case of HD 189733b, the colour is not from a surface covered by water — it's far too hot for that — but instead it's likely from an atmosphere that's filled with glass.
According to NASA: "The cobalt blue color 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."
However, as Prof. Pont mentions in a University of Exeter news release: "It's difficult to know exactly what causes the colour of a planet's atmosphere, even for planets in the Solar System, but these new observations add another piece to the puzzle over the nature and atmosphere of HD 189733b. We are slowly painting a more complete picture of this exotic planet."
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Reading about other planetary finds — like the three Kepler worlds announced back in April, and the three potentially-habitable worlds orbiting nearby Gliese 667c — I've always been impressed by the imagination and technical skills of the artists at NASA's Ames Research Center and Caltech's Jet Propulsion Laboratory, who try to give us an idea of what these planets may look like. However, while my feelings towards those artists isn't going to be diminished by this new discovery, this is still an amazing find.
"This planet has been studied well in the past, both by ourselves and other teams," Pont said in the news release. "But measuring its colour is a real first — we can actually imagine what this planet would look like if we were able to look at it directly."
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