By Helen Briggs
Science reporter, BBC News
A carbon-containing molecule has been detected for the first time on a planet outside our Solar System.
The planet is a "hot Jupiter" blasted by starlight
The organic compound methane was found in the atmosphere of a planet orbiting a star some 63 light years away.
Water has also been found in its atmosphere, but scientists say the planet is far too hot to support life.
The discovery, unveiled in the journal Nature, is an important step towards exploring new worlds that might be more hospitable to life, they say.
Methane, made up of carbon and hydrogen, is the simplest possible organic compound.
Located 63 light years from Earth, in the constellation Vulpecula, the little fox
About the size of Jupiter but orbits closer to the parent star in its Solar System than Mercury does in our own
Temperatures reach 900 degrees C, about the melting point of silver
Under certain circumstances, methane can play a key role in prebiotic chemistry - the chemical reactions considered necessary to form life.
Scientists detected the gas in the atmosphere of a Jupiter-sized planet known as HD 189733b.
Co-author Giovanna Tinetti from University College, London, told BBC News: "This planet is a gas giant very similar to our own Jupiter, but orbiting very close to its star.
"The methane here, although we can call it an organic constituent, is not produced by life - it is way too hot there for life."
Dr Tinetti, and co-authors Mark Swain and Gautam Vasisht, from Nasa's Jet Propulsion Laboratory in Pasadena, California, found the tell-tale signature of methane in the planet's atmosphere using the Hubble Space Telescope.
The observations were made as the planet passed in front of its parent star, as viewed from Earth. As the star's light passed briefly through the planet's atmosphere, the gases imprinted their chemical signatures on the transmitted light.
A method known as spectroscopy, which splits light into its components, revealed the chemical "fingerprint" of methane.
The researchers also confirmed a previous discovery - made by Nasa's Spitzer Space Telescope - that the atmosphere of HD 189733b also contains water vapour.
It shows that Hubble, Spitzer and a new generation of space telescopes yet to be launched can detect organic molecules on other extrasolar planets using spectroscopy, they say.
Dr Swain said: "This is a crucial stepping stone to eventually characterising prebiotic molecules on planets where life could exist."
Dr Tinetti said the technique could eventually be applied to extrasolar planets that appear more suitable for life than HD 189733b.
She said: "I definitely think that life is out there. My personal view is it is way too arrogant to think that we are the only ones living in the Universe."
The number of known planets orbiting stars other than our own now stands at about 270.
For most of them, scientists know little more than the planet's mass and orbital properties.
Adam Showman of the Department of Planetary Sciences at the University of Arizona, US, said scientists were finally starting to move beyond simply discovering extrasolar planets to truly characterising them as worlds.
Dr Showman, who was not part of the study, said: "The discovery does not by itself have any direct implications for life except that it proves a technique which might potentially be useful for characterising the atmosphere of rocky planets when we finally start discovering them."
Excitement about finding other Earth-like planets is driven by the idea that some might contain life; or that perhaps, centuries from now, humans might be able to set up colonies on them.
The key to this search is the so-called "Goldilocks zone", an area of space in which a planet is "just the right distance" from its parent star so that its surface is not-too-hot or not-too-cold to support liquid water.