Almost everyone has heard Carl Sagan's description of a "pale blue dot" describing planet Earth, and now the Hubble Space Telescope has captured another blue planet. However, this one is anything but Earth-like.
Illustration: Location of HD 189733b. Credit: NASA, ESA, and G. Bacon (STScI)
Located only 63 light-years away, the planet - HD 189733b - is one of the nearest exoplanets, and one of the most inhospitable. If seen from space, it would be a "deep blue dot," but its color doesn't arise from sleepy blue oceans. Like a brilliant blue gas flame, the coloration comes from extreme heat!
HD 189733b was discovered in 2005, orbiting only 2.9 million miles from its parent star. This places the planet so close to a major source of gravity, that it remains "tidally locked." This means one side of the planet always faces the star, while the other remains permanently dark. Scientists figure the planet's daytime atmosphere is heated to nearly 2,000 degrees Fahrenheit... a temperature so extreme that it literally rains molten glass-sideways. The huge differences in temperatures between the sides of the planet also drive intense winds - winds which could reach speeds of up to 4,500 miles per hour.
Why does it sing the blues? HD 189733b is classified as a "hot Jupiter" and its blue coloration may be the product of silicates in its high cloud formations. The condensation temperature of these silicates could form small drops of glass that would scatter blue light more than red. As one of the nearest exoplanets to Earth, this blue baby has been subject to intense scrutiny by the Hubble Space Telescope and other observatories. It can be seen transiting the parent star, revealing its ever-changing and exotic atmosphere. These observations are giving researchers new fuels for study about the chemical compositions of alien worlds and how the "hot Jupiter" class reveals its cloud structure.
Clouds are known to play critical roles in planetary atmospheres, and detecting the presence and importance of clouds in hot Jupiters is crucial, say researchers.
"We obviously don't know much on the physics and climatology of silicate clouds, so we are exploring a new domain of atmospheric physics," said team member Frederic Pont of the University of Exeter, South West England, the United Kingdom.
So, how did the astronomers deduce the "blue" coming from HD 189733b? By employing the Hubble Space Telescope's Imaging Spectrograph (STIS), the science team observed the planet during all phases of transit and eclipse - before, during and after its passages around the parent star. Because the orbit of the planet is seen from Earth as "edge-on", we're able to pick it up as it continually moves both in front and behind the star.
Artist's rendering of HD 189733b. Credit: NASA, ESA, M. Kornmesser
With Hubble eyes, a small decrease in light (about one part in 10,000) is detected as the planet goes into eclipse. This also means a small change in the color of the light as well.
"We saw the light becoming less bright in the blue, but not in the green or the red. This means that the object that disappeared is blue because light was missing in the blue, but not in the red when it was hidden," said Pont.
Earlier observations have reported evidence for the scattering of blue light on the planet, but this most recent Hubble observation gives confirming evidence, said the researchers.
However, Hubble isn't the only space-based telescope checking in on HD 189733b. Just six years ago, NASA's Spitzer Space Telescope took measurements of its infrared light - the signature heat from the planet. This observation was the very first time a temperature map was made of an exoplanet and it revealed that the two radically different sides varied in temperature by about 500 degrees Fahrenheit.
Such extremes would cause intense winds to sweep across the surface from day to night. By combining these information sets, researchers are able to peer past the planet's hot signature and take a closer look at the composition of its atmosphere.
According to Pont, it's not exactly clear what causes the color of a planet's atmosphere - even for planets we can study closely. Although we can literally almost touch Jupiter, its ruddy atmosphere is caused by unknown color-carrying molecules. We've even passed probes through the atmosphere of Venus, and all we know is that it doesn't reflect ultraviolet light, the UV absorber that lurks in its atmosphere remains unknown.
As for Earth? From space, it appears blue because our oceans absorb red and green wavelengths more intensely than blue light. What's more, the oceans also reflect Earth's blue sky... a process known as Rayleigh scattering where the shorter wavelengths of sunlight are selectively scattered by the oxygen and nitrogen molecules in our atmosphere.
The next time you feel like singing the blues, think of a rather large planet not so very far away. It might not be raining rain, but it's raining violets...
Original Story Source: NASA HubbleSite News Release
Tammy Plotner is a professional astronomy author, President Emeritus of Warren Rupp Observatory and retired Astronomical League Executive Secretary. She's received a vast number of astronomy achievement and observing awards, including the Great Lakes Astronomy Achievement Award, RG Wright Service Award and the first woman astronomer to achieve Comet Hunter's Gold Status.