An artist's impression of planet HD 189733b. Credit: C. CARREAU/ESA

Carbon dioxide has been spotted in the atmosphere of a gas-giant planet called HD 189733b, which orbits a star 63 light years from Earth.

Along with water and methane, which have already been detected in the atmospheres of other extrasolar planets, the presence of carbon dioxide could give astronomers clues about whether a planet might support life.

Mark Swain of NASA's Jet Propulsion Laboratory in Pasadena, California, reported his team's findings on 19 November at the Paris conference on Molecules in the Atmospheres of Extrasolar Planets.

The observations were made using the NICMOS near-infrared camera on the Hubble Space Telescope. The detection of carbon dioxide is particularly surprising, because other forms of carbon, such as carbon monoxide and methane, would be expected to dominate the planet's hot, hydrogen-rich atmosphere.