Exotic Atmospheres

Exotic Atmospheres
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 star is so much brighter than the planet that the total light from the system would appear to be unchanged, even as the planet disappeared from view. To capture spectra of the pla
NASA/JPL-Caltech/T. Pyle (SSC
October 19, 2009
CreditNASA/JPL-Caltech/T. Pyle (SSC)
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The basic chemistry for life has been detected in a second hot gas planet, HD 209458b, depicted in this artist's concept. Two of NASA's Great Observatories - the Hubble Space Telescope and Spitzer Space Telescope, yielded spectral observations that revealed molecules of carbon dioxide, methane and water vapor in the planet's atmosphere. HD 209458b, bigger than Jupiter, occupies a tight, 3.5-day orbit around a sun-like star about 150 light years away in the constellation Pegasus. Planets like this one, which circle stars beyond our sun, are called exoplanets.

The new finding follows the detection of these same organic molecules in the atmosphere of another hot, giant planet, called HD 189733b, by astronomers using Hubble and Spitzer data. Astronomers can now begin comparing the chemistry and dynamics of these two planets, and search for similar measurements of other candidate exoplanets, advancing toward the goal of being able to characterize planets where life could exist.

Neither of the two planets studied is habitable, but they display the same molecules that, if found around a rocky planet in the future, could potentially indicate the presence of life.

The new findings pave the way for future work that will help astronomers shortlist any promising rocky Earth-like planets where the signatures of organic chemicals might indicate the presence of life.

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