An international team of astronomers has discovered that helium atoms are causing the atmosphere of a "warm-Neptune" exoplanet to expand like a party balloon before escaping into the cold expanse of space. The discovery could help scientists understand the extreme processes taking place in the atmospheres of the hottest exoplanets discovered to date.

Helium is the second most abundant element in the universe, behind hydrogen. It is formed in vast quantities in the hearts of massive stars, and in our own solar system makes up a significant portion of the atmospheres of massive gas giants such as Jupiter and Saturn.

Helium has long been predicted to form a significant part of many exoplanet atmospheres, but until very recently astronomers had been unable to detect it in the gaseous shells of distant worlds. Helium's ability to hide in plain sight is a side effect of the method used by astronomers to search for the elements that make up distant cosmic objects. This involves the use of instruments known as spectrographs.

By attaching a spectrograph to a telescope, astronomers are able to split apart the light emitted by a distant source into its component parts – it's similar to how a prism divides sunlight into a rainbow of colors. The greater the resolution of the spectrograph, the more colors that can be defined from the source.

Within these rainbows of light, or spectra, are the signatures of the elements. Helium's spectral signature is particularly susceptible to absorption by the environment that exists in the gulf between stars. Because of this, the element was only discovered for the first time in the atmosphere of an exoplanet last year by astronomers using the Hubble Space Telescope.

The exoplanet at the heart of the new discovery, HAT-P-11b, is located 123 light years from Earth in the Cygnus constellation. The team observed the distant world using the Carmenes spectrograph mounted on the 4-metre (13-ft) telescope located in Calar Alto in Spain.

The resolution of the Carmenes spectrograph allows the instrument to identify over 100,000 colors in the infrared part of the electromagnetic spectrum that would ordinarily be invisible to the human eye.

Using this instrument, astronomers tracked the location and speed of helium atoms in the atmosphere of HAT-P-11b, and with the help of advanced computer simulations, they were also able to track the trajectory of the atoms.

They discovered that the gaseous shell around the planet was expanding like a balloon as helium is heated and ejected from the exoplanet by the parent star's radiation. The rising helium and hydrogen then forms an extended cloud that eventually escapes into space.

"Helium is blown away from the day side of the planet to its night side at over 10,000 km [6,214 miles] an hour," explains Vincent Bourrier, co-author of the study and member of the European project FOUR ACES that provided computer modelling for the study. "Because it is such a light gas, it escapes easily from the attraction of the planet and forms an extended cloud all around it."

Fittingly, the pressure from the star has inflated the helium to form a balloon-like shape.

The astronomers hope that their research will help shed light on the extreme atmospheric processes taking place around super hot exoplanets. It could also reveal the types of planets that boast hydrogen/helium atmospheres, and how long they can maintain them before the gaseous shells bleed away into space.

A paper detailing the discovery has been published in the journal Science.