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Astronomers may have discovered something truly unusual around a nearby Earth-sized exoplanet. Observations of the atmosphere of GJ 1132 b suggest that this might not be its original atmosphere. Researchers believe that it lost the layer of gas around its rocky core long ago, and what we can see now is a second atmosphere produced by volcanic activity on its surface. This is the first time a "secondary atmosphere" has been found on another world outside our Solar System.
Reporting in The Astronomical Journal, the team studied the planet’s atmosphere using the Hubble Space Telescope, modeling how much of its atmosphere the planet is losing into space, which painted quite the peculiar picture.
First discovered in 2015, astronomers thought studying GJ 1132 b would help us learn more about rocky planets like Earth and Mars. It has a radius 20 percent bigger than Earth and a similar density and age to our planet. But it has some big differences too. It probably started as a sub-Neptune planet, shrouded in a large and thick atmosphere of hydrogen and helium.
This world is much closer to its star than even Mercury is to the Sun, making a full orbit in just 1.6 Earth days, meaning it is also scorching hot. This proximity is not good for an atmosphere. Due to intense radiation from its star, the team believes that most of its hydrogen and helium atmosphere blew away into space leaving its rocky core exposed. From volcanic activity in that core, a new atmosphere is thought to have emerged.
"It's super exciting because we believe the atmosphere that we see now was regenerated, so it could be a secondary atmosphere," said study co-author Raissa Estrela of NASA's Jet Propulsion Laboratory in a statement. "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 study suggests that the new atmosphere is made of molecular hydrogen, hydrogen cyanide, and methane. This noxious composition is also accompanied by what looks like an aerosol haze, which researchers believe to be similar to our Earthly smog. It is also interesting that some of the original hydrogen in the atmosphere was likely absorbed in the molten rock and is regularly re-released into the air, which has a similar pressure to the air on our planet.
The team thinks given GJ 1132 b's hot interior, the planet's cooler crust must be very thin, perhaps only hundreds of feet thick. This is not strong enough to support volcanic terrain. Instead, its surface may be cracked, allowing hydrogen and other gases to be released.
"Some [planets] 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," lead author Mark Swain, also from JPL, said. "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 is located about 41 light-years from Earth. It is the first low-mass rocky exoplanet with an atmosphere discovered and it continues to play a crucial role in our understanding of planets.
