Astronomers have found water vapour and carbon monoxide, but no methane, in the atmosphere of an alien planet orbiting a star 129 light years away.

The star, known as HR 8799, is at the centre of the first planetary system beyond our solar system to be imaged directly, in 2008. The star has at least four gas giants orbiting it. One of them, HR 8799c, is seven times the size of Jupiter that orbits at roughly the same distance Pluto does the sun in our own solar system. The light from the HR 8799c can be distinguished from its star, partly due to its distant orbit.

Having the light from the planet itself means that astronomers can see the planet's atmosphere in unprecedented detail.

Quinn Konopacky, of the Dunlap Institute for Astronomy and Astrophysics, Toronto, Canada, and her colleagues used one of the Keck telescopes in Hawaii to get the most detailed look at its light yet. They then analysed that to get the chemical composition of the distant planet's atmosphere. The data came from 5.5 hours of observations, made up of 33 ten minute exposures.

Carbon monoxide and water both have a "very distinctive chemical fingerprint", said Travis Barman, second author on the Science paper out today, from the Lowell Observatory in Flagstaff, Arizona, US. "We’ve seen it in many other objects and it has a very recognizable pattern," Barman said in a press teleconference. "So it was very easy to see right away in our in our data."

But the researchers found no methane in the planet's atmosphere. "The fact that we see carbon monoxide and no methane in this object that’s cool enough to allow methane to exist tells us that the mixing of the atmosphere is relatively efficient," said Barman.

The planetary system is young, with an estimated age of 30 million years. HR 8799c could be as hot as 1000C, and its gravity around 10 times that of Earth, Konopacky and her colleagues found.

The results announced today give astronomers an insight into how the planetary system formed. There are currently two competing scenarios. Either planets form in a top down sort of way, as a disk of dust and star stuff collapses because of gravity, or they form from the bottom up, as gas slowly builds up on a planetary core.

HR 8799c's atmosphere suggests that it formed from the bottom up, from a debris disk containing plenty of ice that gave rise to the water in its atmosphere today. Ice and dust would have clumped together to make the planet's core, then once it was big enough gas would have started to surround the core, making the deep atmosphere that envelopes Jupiter-like planets.

"Once this core grew large enough to maybe about 10 times the size of the Earth, then it had sufficient gravity to start attracting the gas that was in the disk and it eventually acquired enough gas where it made this large gaseous atmosphere that we see and observe today," said Konopacky.

Almost a thousand exoplanets are now known, and NASA's Kepler mission alone has found nearly three thousand exoplanet candidates. But HR 8799 and its planets are one of only a handful of systems beyond our own solar system that we've seen directly. Most exoplanets are found through indirect methods. Kepler, for example, detects planets by looking for dips in brightness of a planet's host star.

In time, probing the atmospheres of other exoplanets directly, like Konopacky and her colleagues did here, will give astronomers much more information about the conditions beyond our solar system. "Each time we’re able to divide an exoplanet’s light up into even smaller increments, we learn more and more about the planet’s atmosphere, composition and other basic properties," said Barman.