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 209458b, 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.
When astronomers first saw the infrared spectrum above, they were shocked.
It doesn't look anything like what theorists had predicted. For example,
theorists thought there'd be signatures of water in the wavelength ranges
of 8 to 9 microns. The fact that water is not detected might indicate that
it is hidden under a thick blanket of high, dry clouds.
In addition, the spectrum shows signs of silicate dust -- tiny grains of
sand -- in the wavelength range of 9 to 10 microns. This suggests that the
planet's skies could be filled with high clouds of dust unlike anything
seen in our own solar system.
There is also an unidentified molecular signature at 7.78 microns. Future
observations using Spitzer's spectrograph should be able to determine the
nature of the mysterious feature.
This spectrum was produced by Dr. Jeremy Richardson of NASA's Goddard
Space Flight Center, Greenbelt, Md. and his colleagues. The data were
taken by Spitzer's infrared spectrograph on July 6 and 13, 2005.
