A distant planet with a helium-filled atmosphere that has swollen into the shape of a balloon has been discovered by astronomers.
Researchers who made the breakthrough said they detected the inert gas escaping the planet's atmosphere in a cloud "just as a helium balloon might escape from a person's hand".
The planet is 124 light years from Earth and was discovered by an international team of researchers led by academics at the University of Geneva in Switzerland.
Dr Jessica Spake, from the University of Exeter's physics and astronomy department, said: "This is a really exciting discovery, particularly as helium was only detected in exoplanet atmospheres for the first time earlier this year.
"The observations show helium being blasted away from the planet by radiation from its host star.
"Hopefully we can use this new study to learn what types of planets have large envelopes of hydrogen and helium, and how long they can hold the gases in their atmospheres."
The planet, which is around the same size as Neptune, has been named HAT-P-11b and is in the Cygnus constellation.
As part of the study, researchers used a spectrograph in Spain to measure how much light the planet blocked from its host star when it passed in front of it.
The instrument then pulled apart the star's light into its component colours, like a rainbow.
As helium absorbs light of a specific wavelength, the researchers detected a large cloud of the gas surrounding the planet blocking out much more light than the planet itself.
Computer simulations were used to track the trajectory of the helium atoms.
Vincent Bourrier, who led the computer simulation, said: "Helium is blown away from the day side of the planet to its night side at over 10,000kph.
"Because it is such a light gas, it escapes easily from the attraction of the planet and forms an extended cloud all around it."
It is this phenomenon that gives it the balloon shape.
The planet's upper atmosphere is 20 times closer to its star than the Earth is from the Sun.
Romain Allart, a University of Geneva PhD student and first author of the study, said: "We suspected that this proximity with the star could impact the atmosphere of this exoplanet.
"The new observations are so precise that the exoplanet atmosphere is undoubtedly inflated by the stellar radiation and escapes to space."
The research team believe the study could lead to a greater understanding of extreme atmospheric conditions around the hottest exoplanets.
Helium was only successfully found in the atmosphere of an exoplanet, which is a planet outside our solar system, earlier this year in a study led by University of Exeter researchers.