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For the first time, astronomers have detected the color of a planet beyond our solar system: It's blue, but not because there's water on its surface. It doesn't even have a "surface." Instead, the color is thought to come from glassy grains of silicate in its choking atmosphere.
The planet, known as HD 189733b, is a blazing hot gas giant circling a star 63 light-years from Earth in the northern constellation Vulpecula.
"This planet has been studied well in the past, both by ourselves and other teams," Frédéric Pont of the University of Exeter, leader of the Hubble observing program, said in a news release. "But measuring its color is a real first — we can actually imagine what this planet would look like if we were able to look at it directly."
Its cobalt-blue color was determined thanks to some clever observations that were made using the Hubble Space Telescope. The research team monitored the brightness of the star as the planet passed in front of its disk, and then circled behind it. They found that the brightness levels in the blue region of the light spectrum decreased when the planet passed out of view.
"From this, we can gather that the planet is blue, because the signal remained constant at the other colors we measured," said the University of Oxford's Tom Evans, first author of a research paper appearing in the Aug. 1 issue of Astrophysical Journal Letters.
Earth's blue color, as seen from space, comes from the wavelengths reflected by the oceans. The planets Uranus and Neptune are also blue, due to the light reflected by methane in the atmospheres of those ice giants. But Evans and his colleagues say the blue of HD 189733b is most likely to come from silicates in the planet's atmosphere. HD 189733b is a "hot Jupiter," orbiting so close to its parent star that atmospheric temperatures exceed 1,800 degrees Fahrenheit (1,000 degrees Celsius). At that temperature, the silicates could condense into tiny grains of glass, whipping around the planet at 7,000 mph (11,000 kilometers per hour).
"Silicates are not the only possibility, merely the most likely one," Pont told NBC News in an email. "It could be iron grains, or photochemical products like carbon compounds or sulfur compounds. However, other possibilities do not seem to have the right behavior."
A visit to an alien blue planet
Because HD 189733b is a gas giant, slightly bigger than Jupiter, it doesn't have a surface to stand on. But Pont said he could imagine what visitors would see as they approached the planet.
"Visitors would see the planet dark blue looking down," he wrote. "The cloud cover could be uniform like Uranus or Neptune if the formation process is mainly photochemical, or very structured if the formation is thermal. ... Once in the atmosphere, the visitors would see the sky blue, and the star red through it. They would be able to see stars from the night side, provided they are not too deep in the atmosphere. The visitors would also see the red glow of the atmosphere itself on the night side, because the atmosphere is so hot it glows visibly."
Pont said the color detection method depends on having a planet that's large and close to its parent star, so that significant amounts of light are reflected in visible wavelengths. "This particular method will probably not work for planets in an Earthlike orbit, because the amount of reflected light is much smaller compared to the light of the star," he wrote.
However, astronomers could use different methods to determine the composition of alien atmospheres —and thus figure out what those planets might look like from above. Such studies could be done with NASA's Transiting Exoplanet Survey Satellite, a $200 million space telescope due for launch in 2017.
More about planetary peculiarities:
- Visualize an alien sunset on HD 189733b
- Alien planet's weather forecast: Hot and cloudy
- Flash interactive: How to find an exoplanet
- NBC News archive on planets
In addition to Evans and Pont, the authors of "The Deep Blue Color of HD 189733b: Albedo Measurements with HST/STIS at Visible Wavelengths" include D.K. Sing, S. Aigrain, J.K. Barstow, J-M. Desert, N. Gibson, K. Heng, H.A. Knutson and A. Lecavelier des Etangs.
Alan Boyle is NBCNews.com's science editor. Connect with the Cosmic Log community by "liking" the NBC News Science Facebook page, following @b0yle on Twitter and adding +Alan Boyle to your Google+ circles. To keep up with NBCNews.com's stories about science and space, sign up for the Tech & Science newsletter, delivered to your email in-box every weekday. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.