A gas giant orbiting a sun 150 light years away boasts water, methane and carbon dioxide, the second planet outside our Solar System found to display signs of life's basic building blocks.

Although the planet is not inhabitable – it is a torrid gas planet much larger than Jupiter – if these chemical fingerprints were found around a rocky planet it could indicate the presence of life.

"It's the second planet outside our Solar System in which water, methane and carbon dioxide have been found, which are potentially important for biological processes in habitable planets," says Mark Swain of NASA's Jet Propulsion Laboratory. "Detecting organic compounds in two exoplanets now raises the possibility that it will become commonplace to find planets with molecules that may be tied to life."

Swain and colleagues studied HD 209458b using the Hubble and Spitzer space telescopes, following hot on the heels of their breakthrough discovery of carbon dioxide around a similar planet, HD 189733b, that that already displayed signs of water vapour and methane. The tantalising triplet of molecules were discovered using the method of spectroscopy, whereby light is split into components to reveal the signatures of different chemical species. While Hubble revealed the presence of the molecules, it was Spitzer's job to determine their quantities.

"This demonstrates that we can detect the molecules that matter for life processes," says Swain. The team found that that the relative amounts of water and carbon dioxide in the two planets' atmospheres is similar, but that HD 209458b shows a greater abundance of methane than HD 189733b. "The high methane abundance is telling us something," says Swain. "It could mean there was something special about the formation of this planet."

This type of analysis will eventually be used to shortlist any promising Earth-like planets in the future, although the detection of organic compounds does not necessarily mean the presence of life – traces of methane are also found in the gas planets of our own Solar System, for example. "If we detect organic chemicals on a rocky, Earth-like planet, we will want to understand enough about the planet to rule out non-life processes that could have led to those chemicals being there," says Swain.

The Kepler Space Telescope is likely to find many Earth-like planets in the near future, but it may be some time before the chemical signs life on such far-away worlds are uncovered.

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