The planet is a ball of gas with surface temperatures of 600C, but future studies of alien atmospheres may reveal signs of life

Astronomers have detected water vapour in the atmosphere of a planet that orbits a star far beyond our solar system.

Observations of the Neptune-sized planet, which lies 120 light years from Earth in the constellation of Cygnus, revealed that its atmosphere was mostly hydrogen with around 25% made up from water vapour.

Until now, researchers have been frustrated in their efforts to study the atmospheres of planets much smaller than Jupiter because their skies were thick with clouds. The problem was so persistent that astronomers had begun to think that all warm, small planets formed with substantial cloud cover.

But writing in the journal Nature, scientists in the US describe how they found a Neptune-sized planet with cloud-free skies, enabling them to make detailed measurements of a small planet’s atmosphere for the first time.

The planet, named HAT-P-11b, is about four times the diameter of Earth. It orbits so close to its star that surface temperatures reach more than 600C and a year passes in five Earth days. Like our own Neptune, the planet lacks a rocky surface – it’s a ball of gas – and is thought to be lifeless.

Scientists from the University of Maryland used Hubble’s wide field camera to analyse light from HAT-P-11b’s host star through the planet’s atmosphere. They found that light with a wavelength of 1.4 micrometres was absorbed, matching the absorption spectrum of water molecules.

“Although this planet is not classically habitable, it reveals to us that when we find Earth 2.0, we will be able to use this technique, transmission spectroscopy, to understand its atmosphere and determine the quality of life available on its shores,” said Jonathan Fraine, a graduate student and first author on the study.

If cloud cover were widespread on smaller planets beyond the solar system, astronomers would need radically different approaches or far more advanced technology to probe their atmospheres. “Now we know that not all warm Neptunes form with high-altitude clouds, we can again explore the diversity of planet formation and gain greater context for our own creation,” said Fraine.

Future studies of alien atmospheres may detect proportions of gases that point to life below. On Earth, methane, ammonia and nitrous oxide are produced mostly by bacteria, while oxygen comes from plants and other photosynthesising organisms. Because the gases are not made in large amounts by anything else, they are considered “biosignatures”, or signs of life.

“Biosignatures are much harder to find, but with bigger, exoplanet-specific telescopes and precise instruments, we should be able to start looking for them too,” said Fraine. “We may be far from analysing an Earth analogue, but now we know that our train is on the right tracks.”

In 2018, Nasa is due to launch its successor to Hubble, the James Webb Space Telescope. The observatory has been designed to pick up signals much fainter than Fraine’s team spotted with the Hubble camera.

In an accompanying article, Eliza Kempton at Grinnell College in Iowa praised the breakthrough. “Searching for water vapour absorption in the atmosphere of an exoplanet passing in front of its host star is akin to looking for a tiny insect passing in front of a bright coastal lighthouse lamp,” she wrote.

“By first pinpointing and studying those planets that provide a clear window into their atmospheres, researchers will ultimately be able to extend the search for water and other molecules to smaller planets, perhaps even Earth-sized planets, with the James Webb telescope and beyond.”