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For the first time, astronomers have studied the chemical composition of planets orbiting stars other than the Sun. Two teams used Spitzer Space Telescope to study two “hot Jupiter” planets orbiting stars in the constellations Vulpecula and Pegasus.

Artist’s concept of a cloudy Jupiter-like planet orbiting close to its parent star. (NASA/JPL-Caltech/T. Pyle (SSC))

The planets, called HD 209458b and HD 189733b, are both gas giants like Jupiter, but orbit their parent stars much closer and faster. The Spitzer data indicate that both of the planets are drier and cloudier than predicted. One, HD 209458b, may harbor grains of sand, called silicates, in its atmosphere.

For each star-planet pair, Spitzer monitored the system through an entire orbit of the planet. Spitzer first recorded the star’s spectrum with the planet in front of the star, and then recorded the spectrum of the star alone. Astronomers were then able to subtract the star’s spectrum from the combined star-planet spectrum — leaving them with the planet’s spectrum. That revealed the rough chemical composition of the planet’s atmosphere.

“With these new observations, we are refining the tools that we will one day need to find life elsewhere if it exists,” said Mark Swain of NASA’s Jet Propulsion Lab, who led a team studying HD 209458b.