An artist’s conception of HD 189733b in orbit around its star.

Hot and Steamy

It may not be the waterworld that fields Kevin Costner’s dreams, but the exoplanet HD 189733b has been found to have water vapor in its atmosphere. This observation provides the best evidence to date that water exists on worlds outside our own solar system.

The discovery was made by NASA’s Spitzer Space Telescope, which possesses a particularly keen ability to study nearby stars and their planets. HD 189733b lies 63 light-years away.

“Water is the quintessence of life as we know it,” says Yuk Yung, professor of planetary science and one of the authors of the study published in the July 12 issue of Nature. “It is exciting to find that it is as abundant in another solar system as it is in ours.”

HD 189733b swelters as it zips around its star every two days or so. Astronomers had predicted that planets of this class, termed “hot Jupiters,” would contain water vapor in their atmospheres, yet evidence has been hard to come by. “We’re thrilled to have identified clear signs of water on a planet that is trillions of miles away,” says lead author Giovanna Tinetti, a European Space Agency fellow at the Institute d’Astrophysique de Paris in France and former postdoc at Caltech’s Virtual Planetary Laboratory.

Coauthor Mao-Chang Liang (PhD ’06) of Caltech and the Research Center for Environmental Changes in Taiwan adds, “The discovery of water is the key to the discovery of alien life.”

Wet hot Jupiters are unlikely to harbor any creatures. Previous Spitzer measurements indicate that HD 189733b is a fiery 1,000 degrees Kelvin on average. Ultimately, astronomers hope to use instruments like those on Spitzer to find water on rocky, habitable planets like Earth. “Finding water on this planet implies that other planets in the universe could also have water,” says coauthor Sean Carey of the Spitzer Science Center, which is headquartered at Caltech.

A team of astronomers had found hints of water on another planet called HD 209458b by analyzing visible-light data taken by NASA’s Hubble Space Telescope. The Hubble data were captured as the planet crossed in front of the star, an event called the primary eclipse. Tinetti and her team used changes in the star’s infrared light as the planet slipped by, filtering the starlight through its outer atmosphere. The astronomers noticed that at each of three different wavelengths, a different amount of light was absorbed—a pattern matching that created by water. “Water is the only molecule that can explain that behavior,” Tinetti says. “Observing primary eclipses in infrared light is the best way to search for this molecule.”

The water on HD 189733b is too hot to condense into clouds; however, previous observations by several telescopes suggest that it might have dry clouds, along with high winds and a hot, sun-facing side that is warmer than its dark side.

Other authors of the paper include Alfred Vidal-Madjar, Jean-Phillippe Beaulieu, David Sing, Nicole Allard, and Roger Ferlet of the Institute d’Astrophysique de Paris; Robert Barber and Jonathan Tennyson of University College London in England; Ignasi Ribas of the Institut de Ciències de l’Espai, Spain; Gilda Ballester of the University of Arizona, Tucson; and Franck Selsis of the Ecole Normale Supérieure, France. —RT