An Earth-like planet can "regenerate" its atmosphere, NASA scientists have found.
Astronomers admitted they thought GJ 1132 b exo-planet would be "pretty boring" when the Hubble Space Telescope spotted it.
It appeared to be a rocky world that had been stripped of its hydrogen atmosphere due to its close orbit to its star.
But they were left stunned when they found a "secondary atmosphere" was present above its volcanic surface 41 light-years from us.
It was made of hydrogen, hydrogen cyanide, methane and a haze thought to be similar to smog on Earth.
The scientists reckon the hydrogen in its atmosphere was the same as that thought lost from its original atmosphere.
Their models found it had been absorbed into the planet's molten magma mantle and is now slowly being released by volcanic activity.
Study co-author Raissa Estrela of NASA's Jet Propulsion Laboratory (JPL) said: "It's super exciting because we believe the atmosphere that we see now was regenerated, so it could be a secondary atmosphere.
"We first thought that these highly irradiated planets could be pretty boring because we believed that they lost their atmospheres. But we looked at existing observations of this planet with Hubble and said, 'Oh no, there is an atmosphere there.'"
The planet, GJ 1132 b, is thought to have begun as a gaseous world with a thick hydrogen blanket of atmosphere and a diameter several times larger than Earth's.
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The so-called "sub-Neptune" appears to have lost its early hydrogen and helium atmosphere until it was the size of Earth due to the intense radiation of the star it orbits.
Lead author Mark Swain of JPL added: "How many terrestrial planets don't begin as terrestrials? Some may start as sub-Neptunes, and they become terrestrials through a mechanism that photo-evaporates the primordial atmosphere. This process works early in a planet's life, when the star is hotter.
"Then the star cools down and the planet's just sitting there. So you've got this mechanism where you can cook off the atmosphere in the first 100 million years, and then things settle down. And if you can regenerate the atmosphere, maybe you can keep it."
GJ 1132 b and Earth share similar densities, sizes and surface atmospheric pressure.
Both are also about 4.5 billion years old, started out with a hydrogen-dominated atmosphere and were hot before they cooled down.
But that's where the similarities end.
Earth's orbit is much further from the Sun, and was not formed as the surviving core of a suspected sub-Neptune planet.
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GJ 1132 b is so close to its red dwarf star that it completes an orbit around it once every day and a half and is "tidally locked" – always showing the same face to its star like our Moon does to the Earth.
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