Whiffs From an Alien World

By Phil Berardelli
ScienceNOW Daily News
19 March 2008

Astronomers have detected the organic molecule methane in the atmosphere of an extrasolar planet for the first time and have confirmed earlier observations of water vapor. Alas, the findings don't come close to suggesting that life has emerged on this other world, but they do contribute to a growing body of data about planetary evolution outside our own solar system.

Over about 15 years, astronomers have discovered 277 planets orbiting other stars. They have relied on two techniques, nicknamed "wobble” and "dip," which infer the mass and position of far-off planets from the effect they have on the motion and brightness of their stars. Astronomers can learn a bit more when a planet transits between its star and Earth: Changes in a star's light spectrum may reveal chemicals in a planet's atmosphere.

Using this technique, researchers report in tomorrow's issue of Nature that a 40-minute gaze with the Hubble Space Telescope last May has revealed methane in the atmosphere of HD 189733b, a Jupiter-size planet orbiting close to its very bright parent star located 63 light-years away. The observation also confirmed last year's discovery by the Spitzer Space Telescope of water vapor in the planet's atmosphere (ScienceNOW, 11 July 2007).

Don't go looking for little green men just yet. During a teleconference for reporters today, co-author Mark Swain of NASA's Jet Propulsion Laboratory in Pasadena, California, emphasized that HD 189733b is far too hot--average atmospheric temperature about 1000°C--to support life as we know it. But the presence of methane raises intriguing questions, he said, because the high temperature should have sequestered all of the carbon in the planet's atmosphere in the form of carbon monoxide (CO), not methane (CH4). That suggests a currently unknown chemical process is at work, he said.

Dim prospects for life aside, experts are excited. This work shows "that we can study the chemistry of the atmospheres of planets orbiting other stars," whereas until now, all we could do was locate them, says astrophysicist David Charbonneau of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts. Perhaps within a few years, scientists can apply the same techniques to the largest versions of transiting Earth-like planets, says Charbonneau, who helped pioneer the methods used in the current research. And astrophysicist Jonathan Fortney of the University of California, Santa Cruz, says the observations will begin to allow scientists to discover whether all giant planets are "fundamentally like warmed-up versions of Jupiter and Saturn" or whether some have evolved along different paths. "We can begin to understand giant planets as a class of astrophysical objects," he says.