(Related: "New Planet Bonanza Offers Clues in Search for Other 'Earths'" [May 29, 2007].)
Because they are so far from Earth, about 40 percent of the exoplanets we can detect are massive hot Jupiters.
Experts are particularly interested in HD 189733b and a handful of other exoplanets that—when detected from Earth—pass in front of and behind their stars as they cruise along their orbits.
One way of finding these planets is to look for stars that dim when orbiting exoplanets pass in front of them.
In the two previous studies, astronomers looked at HD 189733b and HD 209458b just before they passed behind their stars. This is when these planets' day sides are facing Earth and we can see the light they emit.
For their study, Tinetti and colleagues instead looked at HD 189733b as it passed in front of its star.
At this time the bulk of the planet blocks most of the star's light. But some of the light grazes the planet's outer atmosphere, passing through it like a flashlight beam in a fog.
The planet's atmosphere absorbs certain wavelengths of the starlight, depending on the particular chemicals—such as water—that might be present.
The wavelengths that are missing from the light that reaches Earth reveal the chemical fingerprint of the exoplanet's atmosphere.
Really the First?
The new study, several researchers say, confirms leading theories about gas giants and how they might form outside our solar system.
"We're very happy to see this," said Sara Seager of the Massachusetts Institute of Technology in Cambridge. Seager was a co-author of one of the earlier studies that failed to find water.
But there's some disagreement among scientists about who spotted water on hot Jupiters first.
Travis Barman of the Lowell Observatory in Arizona says a study he published earlier this year showed the first sign of water on an exoplanet.
Using images of HD 209458b from the Hubble Space Telescope, Barman found hints of light absorption that may have been a sign of water.
"This claim [was] merely suggestive," said Heather Knutson of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts.
Signs of water do not show up as clearly in the visible light that Barman was looking at as they do in the infrared range Tinetti and colleagues examined.
Knutson, who wrote a review article of the latest study that will also appear tomorrow in Nature, said that "the new data, by contrast, provide solid evidence."
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