After a few false starts, astronomers say they have finally observed water vapor in the atmosphere of a so-called hot Jupiter, a large gaseous planet tightly orbiting a distant star. Using NASA's Spitzer Space Telescope, a research team measured the infrared starlight shining through the atmosphere of planet HD 189733 b as it passed in front of its star 63 light-years away.

The planet soaked up infrared light at several wavelengths in a pattern expected of water molecules, as detailed online today in Nature. "This is the first convincing detection of water in the atmosphere of a planet outside our own solar system," says Heather Knutson, an astronomy graduate student at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., who was not involved in the study.

Researchers expect to find water on many planets outside the solar system, called exoplanets, including Jupiter-size gas giants such as HD 189733 b and HD 209458 b, which orbits a different star. But in February, independent teams of astronomers armed with Spitzer data reported they could not detect water vapor in either planet's infrared glow as it passed behind its star.

To get a different view, astronomer Giovanna Tinetti and her colleagues at the European Space Agency and University College London focused instead on the light grazing the atmosphere of HD 189733 b. Tinetti had predicted that water would absorb more light at the longer wavelength of 5.8 microns (thousandths of a millimeter) than at 3.6 microns, in contrast with other molecules such as methane and ammonia.

The Spitzer data stacked up according to predictions, Tinetti says—especially when combined with eight-micron measurements reported in May by Knutson's team, which used Spitzer to map HD 189733 b's dayside temperature.

"When I saw that was matching so well with what we already got," she says, "I thought, 'hmm, that's extremely good.'"

Tinetti says the earlier studies could be a product of the planets' bright sides cooking to the same temperature throughout, which makes atmospheric molecules less likely to absorb radiation from below.

Knutson adds that an April report of water vapor in HD 209458 b was iffy, because it relied on a weaker signal from the Hubble Space Telescope, which was not designed to study exoplanets.

Detecting water affirms that models of planet formation are on the right track and bolsters astronomers' confidence that they can tackle such challenging measurements, Tinetti says. "It makes you more optimistic about the possibility of repeating the measurement in the future … with a planet that might be more life-friendly," she adds.

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