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Extra warmth. Astronomers can now detect infrared light from an exoplanet (blue, in this artwork) as it emerges from behind its fiery sun.

Alien Worlds Show Their Faces

Extrasolar planets finally have revealed their true faces. At a NASA press conference today in Washington, D.C., astronomers announced that they have detected light from two alien worlds for the first time.

During the last decade, astronomers have inferred the presence of nearly 150 extrasolar planets by looking for subtle oscillating patterns in the light of the stars they orbit. But these patterns only provide information about the planets' masses and orbits. Using infrared light, astronomers can glean direct details about the properties of planets that circle very close to their parent stars because the planets re-radiate torrents of heat. If such planets pass directly behind their stars as seen from Earth, telescopes might see a slight dip in the total infrared light from the star. Then, the planet's own warmth will be evident when it re-emerges from the eclipse.

NASA's Spitzer Space Telescope now has discovered those few extra degrees of warmth from two "hot Jupiters" that whip around their stars once every 3 days. Two independent teams used Spitzer in late 2004 to stare at each star for several hours, spanning the times when each planet was predicted to pass directly behind its sun. Like clockwork, the total infrared light from the stars dimmed by about 0.25% when the planets disappeared and then edged back up again when they emerged. "It was bang-on what we expected," says astronomer David Charbonneau of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts. His team will report on the planet TrES-1 in the 20 June Astrophysical Journal.

More detailed studies using Spitzer's full range of infrared instruments will reveal the planets' temperatures, as well as information about their compositions and atmospheres, forecasts astronomer Drake Deming of NASA's Goddard Space Flight Center in Greenbelt, Maryland. "Spitzer will pin this down beautifully," says Deming, lead author of a paper on the other planet, known as HD 209458b, in this week's online edition of Nature.

"We are moving out of the realm of merely counting planets and knowing their orbital paths," comments planetary scientist Heidi Hammel of the Space Science Institute in Boulder, Colorado. Ongoing searches for other eclipsing planets will lead to a new cottage industry of measuring light from exoworlds, Hammel believes.

Related sites
NASA information on extrasolar planets
Spitzer Space Telescope
David Charbonneau's home page
NASA Goddard Planetary Systems Branch, directed by Deming