... no, that's not my social security number, it's a jupiter-sized planet which does us the favour of circling its sun in a plane that also includes us, i.e. it passes in front of its star once in every complete orbit. Therefore, scientists can use spectroscopic methods to find out which parts of the star's spectrum the planet atmosphere swallows up more effectively than others, and from this conclude what kinds of molecules form the atmosphere.
With this kind of measurements, performed with the Hubble space telescope, Mark Swain et al. have now for the first time presented evidence for carbon molecules in the atmosphere of any extrasolar planet (Nature 2008, 452, 329). Their spectroscopic results reveal methane as a component of HD189733b's atmosphere, and confirm the presence of water vapour. Indirect evidence suggests there might be carbon monoxide as well.
While HD189733b itself is not a very promising candidate for extraterrestrial life (way too hot), it is really exciting that detailed analysis of the atmospheres of extrasolar planets is now possible. In theory, the same kind of technology could be used to detect signs of disequilibrium, i.e. life, on more suitable planets.