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Hot, Dry and Cloudy
This artist's concept shows a cloudy Jupiter-like planet that orbits very
close to its fiery hot star. NASA's Spitzer Space Telescope was recently
used to capture spectra, or molecular fingerprints, of two "hot Jupiter"
worlds like the one depicted here. This is the first time a spectrum has
ever been obtained for an exoplanet, or a planet beyond our solar system.
The ground-breaking observations were made with Spitzer's spectrograph,
which pries apart infrared light into its basic wavelengths, revealing the
"fingerprints" of molecules imprinted inside. Spitzer studied two planets,
HD 209458b and HD 189733b, both of which were found, surprisingly, to have
no water in the tops of their atmospheres. The results suggest that the
hot planets are socked in with dry, high clouds, which are obscuring water
that lies underneath. In addition, HD209458b showed hints of silicates,
suggesting that the high clouds on that planet contain very fine sand-like
particles.
Capturing the spectra from the two hot-Jupiter planets was no easy feat.
The planets cannot be distinguished from their stars and instead appear to
telescopes as single blurs of light. One way to get around this is through
what is known as the secondary eclipse technique. In this method, changes
in the total light from a so-called transiting planet system are measured
as a planet is eclipsed by its star, vanishing from our Earthly point of
view. The dip in observed light can then be attributed to the planet
alone.
This technique, first used by Spitzer in 2005 to directly detect the light
from an exoplanet, currently only works at infrared wavelengths, where the
differences in brightness between the planet and star are less, and the
planet's light is easier to pick out. For example, if the experiment had
been done in visible light, the total light from the system would appear
to be unchanged, even as the planet disappeared from view.
To capture spectra of the planets, Spitzer observed their secondary
eclipses with its spectrograph. It took a spectrum of a star together with
its planet, then, as the planet disappeared from view, a spectrum of just
the star. By subtracting the spectrum of the star from the spectrum of the
star and planet together, astronomers were able to determine the spectrum
of the planet itself.
Neither of the parent stars for HD 209458b or HD 189733b can be seen with
the naked eye. HD 209458b is located about 153 light-years away in the
constellation Pegasus, while HD 189733b is about 62 light-years away in
the constellation Vulpecula. Both planets zip around their stars in very
tight orbits; HD 209458b circles once every 3.5 days, while HD 189733b
orbits once every 2.2 days.
Of the approximately 200 known exoplanets, there are 12 besides HD 209458b
and HD 189733b whose orbits are inclined in such a way that, from our
point of view, they pass in front of their stars. At least three of these
transiting exoplanets are bright enough to follow in the footsteps of HD
209458b and HD 189733 and reveal their infrared spectra to Spitzer.
Astronomers hope to use Spitzer's spectrograph in the future to study HD
209458b and HD 189733b again in much greater detail, and to examine some
of the other candidates for the first time.