• Dick writes about Earth and planetary science for Science magazine.

Water, Water, Not Everywhere

21 February 2007 (All day)


Really that dry?
The first look at the emitted infrared color of a planet circling another star--as seen in this artist's conception--failed to turn up the expected signs of water.

Two giant extrasolar planets remain frustratingly mysterious, thanks to disappointing data returned by the Spitzer Space Telescope. At a media teleconference held by NASA today, three teams of astronomers said they were unable to detect several expected compounds in the planets' atmospheres, including the always sought-after water. Still, with a harder look--and certainly with better telescopes--they say something will turn up.

The two exoplanets involved are "hot Jupiters" that are one-tenth the distance from their stars as Mercury is from the sun. At 1100K, they emit plenty of infrared radiation that can carry the spectral signature--the distinctive "color"--of compounds such as water, methane, and carbon dioxide. But the glare of the star drowns out those planetary emissions, so astronomers watched through Spitzer as each planet orbited in front of, and then behind, its star. They then subtracted the spectra from the two positions to get the output of the planet alone.

All three teams agree that the compounds that theorists confidently believe are abundant in the planets' atmospheres do not reveal themselves in the Spitzer spectra. One team leader, astronomer Carl Grillmair of the California Institute of Technology in Pasadena, ticked off several possible reasons for the nondetection of water. It might not exist--deemed by all to be highly unlikely, as water should have been in the mix when the planets formed. Or it could be obscured by the clouds of dust hinted at in one group's analysis (but not in another group's analysis of the same data). The most likely outcome, according to astrophysicist Adam Burrows of the University of Arizona, Tucson, and the Grillmair team, is that better data will turn up the water. At the very least, says Burrows, the fact that Spitzer was able to pick up a planetary spectrum at all bodes well for future analyses.

"They didn't find water, but that's okay," says astrophysicist Alan Boss of the Carnegie Institution of Washington in Washington, D.C. "These results have started us down the right road," he says. "It's the beginning of a new era."

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