Now astronomer Travis Barman of the Lowell Observatory in Flagstaff, Ariz., says he found the missing water by analyzing light that shined through the atmosphere of one of those planets, called HD 209458 b. "It's not really that surprising," Barman says, because models of such planets' atmospheres predict that water should be mixed in. "Finding it just reassures us that we know what we're doing."
Barman took a second look at Hubble Space Telescope data collected by Harvard astronomers, who measured the light coming from HD 209458 b as it reached the widest part of its 3.5-day orbit around its star. They determined the planet's radius at 10 different wavelength bands, from 300-nanometer ultraviolet light to 1000-nanometer infrared. Its atmosphere absorbs each band by a different amount, so the planet looks larger in some wavelengths than in others.
"I'm able to simulate the passing of starlight through extrasolar planets," Barman says, and, therefore, he knew how much light water would absorb. He checked the wavelengths between 900 and 1000 nanometers and, sure enough, he saw the trace of water molecules, he reports in a paper to be published in The Astrophysical Journal. "Their added opacity makes [the planet] appear roughly 1 percent larger," he says.
The observations reported in February were of infrared light coming from the side of the planet that faced the star. The heat of the daylight could have cooked the atmosphere evenly all the way down, Barman says, which reduces absorption of light by atmospheric molecules. Starlight grazing the planet's limb, where day meets night, does not have that problem, he says.
"I find it quite convincing," says Jonathan Fortney, a planetary scientist at NASA Ames Research Center in Moffett Field, Calif., who has also modeled the atmospheres of extrasolar planets, of which more than 100 are now known. "This is a totally different data set" than the earlier studies, which were challenging to perform, he adds. "It was great that [they] got done, but it was a little patchy.'
The result is important, Fortney says, because "we would like to be able to understand the atmospheres of all planets around other stars. It's really going to be exciting to see how well we understand these extrasolar planets as we move down to smaller things more the size of Earth. It's one of the first steps toward that long path."