July 11, 2013, 10:16 a.m. ET
Astronomers Reach Conclusion by Analyzing Light Spectrum
By ROBERT LEE HOTZ
AFP/Getty ImagesA photo released on July 10, 2013 shows an illustration depicting HD 189733b, a huge gas giant that orbits very close to its host star HD 189733.
For the first time, astronomers have discovered the true color of an alien planet orbiting another star—a deep azure blue like Earth seen from space—an international research team said Thursday.
Using the Hubble Space Telescope, the researchers analyzed the spectrum of visible light from a planet known officially as HD 189733b, which orbits a faint star about 63 light years away. This immense blue world—among the closest and most well-studied of the hundreds of exoplanets discovered in recent years—is a hazy hothouse swept by blow-torch winds where it regularly rains molten glass.
For their analysis, the scientists used the Hubble's imaging spectrograph to measure visible light from the star and planet together, and then measured the light from the system when the planet passed behind the star and was hidden from observers on Earth. To isolate the planet's light, they subtracted one from the other.
The wavelength of the missing light corresponds to a visible shade of blue, and that is the color of the light reflected by the planet, the astronomers reported in the upcoming August issue of the Astrophysical Journal Letters.
"We inferred the color," said astrophysicist Tom Evans at the U.K.'s University of Oxford, who led the study. By knowing the wavelength, "we can imagine the color the planet would have if we could look at it with our own eyes."
As viewed from space, the planet Earth is itself a vibrant blue because of its oceans, which absorb red and green wavelengths more strongly than blue ones. Some blue wavelengths of sunlight also are selectively scattered by oxygen and nitrogen molecules in the air. Mars, in turn, appears red to the eye because its rusty surface is rich in iron oxide that absorbs the blue and green wavelengths of the light spectrum while reflecting the red wavelengths.
But the blue color of HD 189733b arises solely from the interplay of light in its super-heated atmosphere. Based on earlier measurements, scientists believe that the planet's exotic air is laced with grains of silicate, a common component of beach sand, whipped up in high clouds by winds that average more than 4,000 miles an hour.
In an atmosphere where the temperature appears to hover at 1,000 degrees Celsius, these silicate particles melt to make "raindrops" of glass that scatter visible blue light more than red light, the scientists reported. Consequently, the orb would appear blue because of its reflected light, but the planet is too far away for its hue to be seen directly with existing telescopes.
"We are really pushing the limits of what we can measure," said Mr. Evans.
No comments:
Post a Comment