Pushing the Hubble Space Telescope to its limits, astronomers say they have made the first discovery of the organic molecule methane in the atmosphere of a planet circling a sun-like star.
Although methane can be generated by cows and rotting garbage, scientists say there's little chance that they've stumbled on signs of life on the planet, about 63 light-years from Earth. The Jupiter-size world's atmosphere sizzles at 1,700 degrees Fahrenheit.
But their apparent success in detecting the gas so far away gives them confidence that they'll be able to find it again someday on a smaller, cooler planet circling a different star.
"On a more hospitable planet this would really be something exciting," said Mark Swain of NASA's Jet Propulsion Laboratory in Pasadena, Calif., who led the study. "These measurements are the dress rehearsal for something we hope to be doing in the future."
Swain and his team reported their breakthrough observations in today's edition of the journal Nature. They began last summer by pointing Hubble at a star in the dim northern constellation Vulpecula (the Fox), which stands high overhead on September evenings.
Astronomers so far have discovered 277 planets around such "nearby" stars. The one in Swain's new study is among a handful that give themselves away when they periodically eclipse their stars as seen from Earth, causing the starlight to dim briefly.
Astronomers have been trying to use these eclipses to study planetary atmospherics. This planet circles its star once every 2.2 days. So Swain used Hubble's Near Infrared Camera and Multi- Object Spectrometer to look at starlight filtering through the planet's atmosphere during each eclipse.
By splitting the light into its component colors, he could see tell-tale patterns of absorption by methane. Last summer, another team using NASA's Spitzer Space Telescope reported finding water vapor in the same planet's atmosphere.
A methane molecule is composed of one carbon atom attached to four hydrogen atoms (CH4). It's common in our solar system, and abundant on lifeless Neptune and Uranus. On Saturn's moon Titan, it falls in liquid form, like rain.
But methane is also a "prebiotic" compound. Combined with a few other organic molecules in friendlier conditions, it could form basic amino acids - the building blocks of life.
Swain said its detection in an exoplanet's atmosphere also "lets us understand the atmosphere in a more detailed way."
"These molecules serve as a probe," he said. "Measuring what molecules are present and how much is present, we can start to learn more about the conditions, composition and chemistry going on in the exoplanet's atmosphere."
Swain said more observations are planned to verify his findings. But Sara Seager, an exoplanet researcher at the Massachusetts Institute of Technology who was not part of Swain's study, said she was "cautiously optimistic" the observation will hold up.
"The Hubble Space Telescope was never designed to make measurements like this. They are really pushing the telescope to its limit," she said.
The observations required an extraordinary degree of stability. It was a stretch for Hubble and not yet possible with ground-based observatories.
"It's just amazing," Seager said. "The big breakthrough is being able to detect any molecule in the atmosphere."
As astronomers begin to look at other exoplanets for methane, they can't be sure, even if they find it, that its discovery by itself would indicate life. "We may never be able to attribute methane 100 percent to life," Seager said.
But if they find abundant methane around a smaller, cooler and more Earth-like planet, she said, "we may be able to think it might come from life. Processes that are not biological tend to produce much smaller amounts of methane."
The strongest sign of life would be evidence of both abundant methane and abundant oxygen - two molecules that, without life to constantly replenish them, would combine to form something else and disappear.
"We have a good chance of doing this, if we're lucky, with the James Webb Space Telescope," Seager said. Webb is planned for launch in 2013. Its science program will be managed by the Space Telescope Science Institute in Baltimore.