Organic molecules – in the form of methane – have been detected on a planet outside our solar system for the first time. The giant planet lies too close to its parent star for the methane to signal life, but the detection offers hope that astronomers will one day be able to analyse the atmospheres of Earth-like worlds.
Astronomers Mark Swain and Gautam Vasisht of Caltech in Pasadena, US, and Giovanna Tinetti of University College London, UK, used the Hubble Space Telescope to observe the giant planet HD 189733b, which is slightly more massive than Jupiter and lies 63 light years from Earth.
Because the planet crosses the face of its parent star as seen from Earth, some starlight is periodically filtered through the planet's atmosphere, where different chemicals absorb particular wavelengths.
The observations confirm an earlier tentative detection of water vapour and reveal the presence of methane gas.
"Initially, that is surprising," says Sara Seager of MIT in Cambridge, US, who was not involved in the study. Because HD 189733b orbits very close to its parent star – just 10% of Mercury's distance from the Sun, it is very hot, with atmospheric temperatures of about 700° Celsius. "When the temperature is this high, the dominant form of carbon should be carbon monoxide, not methane," says Seager.
The authors suggest that some ill-understood chemical process might be responsible, either concentrating the methane in cooler parts of the atmosphere, or generating extra methane directly. Alternatively, the methane might simply mean that the planet happens to be very rich in carbon, Seager says.
This combination of water and organic molecules would be a promising one for life if it were found in a less hostile spot than the atmosphere of a searing gas giant.
Eventually, astronomers hope to be able to analyse the atmospheres of smaller planets more akin to the Earth, and the new study is a big step in that direction, says Seager. "The path that we're on is towards rocky planets," she told New Scientist. "I'm really excited about this."