Astronomers report they have detected methane for the first time in the atmosphere of a planet outside our solar system. The finding comes from extrasolar planet HD 189733 b, a gaseous "hot Jupiter" locked in a tight orbit around a star 63 light-years away.

The observations "decisively show that methane is present in addition to water," writes the research team from NASA's Jet Propulsion Laboratory in Pasadena, Calif., and University College London. Members of the same team reported last year that they had identified water vapor in the atmosphere of HD 189733 b.

Although Earthlings may associate methane with gassy cows, it is a common and perfectly nonbiological constituent of other atmospheres in the solar system, including those of Mars and Titan as well as the gas giants Jupiter, Saturn, Uranus and Neptune. Researchers believe that methane and water would also be common components of planetary atmospheres outside of the solar system.

Measuring the relative abundances of elements in an atmosphere allows researchers to infer details about planet formation and weather patterns. HD 189733 b is one of few extrasolar planets suited for such measurements. It transits or crosses in front of its parent star every 2.2 days, blocking more than two percent of the star's light as it does so. Every molecule absorbs light most strongly at specific wavelengths, so by measuring the amount of light blocked at different wavelengths by the planet's atmosphere, researchers can infer its composition.

Researchers report in Nature that HD 189733 b has the telltale absorption patterns of both methane and water. Using data from the NICMOS (Near Infrared Camera / Multi-Object Spectrometer) instrument on the Hubble Space Telescope, they compared the levels of infrared light coming from the planet's parent star during transit with the light intensity between transits.

The results imply that a third constituent, carbon monoxide, is waiting to be found in the atmosphere of HD 189733 b, planetary scientist Adam Showman of the Lunar and Planetary Laboratory at the University of Arizona in Tucson says in an accompanying editorial. Planets are presumed to form from the same material as stars. But Showman notes that the intensity of the methane absorption implies that the planet has a relatively low methane-to-hydrogen ratio of no more than 5 parts per 100,000, or only 10 percent that of its parent star.

The scorching temperature of HD 189733 b—around 1,000 kelvins (1,300 degrees Fahrenheit)—may cause the carbon in its atmosphere to prefer to join with oxygen as carbon monoxide instead of forming methane. Finding the possible carbon monoxide and mapping its surface distribution along with that of methane should illuminate the planet's no-doubt exotic weather patterns, Showman says.

"These are exciting times for studies of extrasolar planets," he adds. Researchers "are finally moving beyond simply discovering such planets to truly characterizing them as worlds."