This infrared data from NASA's Spitzer Space Telescope - called a spectrum
- tells astronomers that a distant gas planet, a so-called "hot Jupiter"
called HD 189733b, might be smothered with high clouds. It is one of the
first spectra of an alien world.
A spectrum is created when an instrument called a spectrograph cracks
light from an object open into a rainbow of different wavelengths.
Patterns or ripples within the spectrum indicate the presence, or absence,
of molecules making up the object.
Astronomers using Spitzer's spectrograph were able to obtain infrared
spectra for two so-called "transiting" hot-Jupiter planets using the
"secondary eclipse" technique. In this method, the spectrograph first
collects the combined infrared light from the planet plus its star, then,
as the planet is eclipsed by the star, the infrared light of just the
star. Subtracting the latter from the former reveals the planet's own
rainbow of infrared colors.
Astronomers were perplexed when they first saw the infrared spectrum
above. It doesn't look anything like what theorists had predicted.
Theorists thought the spectra of hot, Jupiter-like planets like this one
would be filled with the signatures of molecules in the planets'
atmospheres. But the spectrum doesn't show any molecules, and is instead
what astronomers call "flat." For example, theorists thought there'd be a
strong signature of water in the form of a big drop in the wavelength
range between 7 and 10 microns. The fact that water is not detected may
indicate that it is hidden underneath a thick blanket of high, dry clouds.
The average brightness of the spectrum is also a bit lower than
theoretical predictions, suggesting that very high winds are rapidly
moving the terrific heat of the noonday sun from the day side of HD
189733b to the night side.
This spectrum was produced by Dr. Carl Grillmair of NASA's Spitzer Science
Center at the California Institute of Technology in Pasadena, Calif., and
his colleagues. The data were taken by Spitzer's infrared spectrograph on
November 22, 2006.
