This is an artist’s impression of Earth-sized rocky exoplanet GJ 1132 b, located 41 light-years away around a red dwarf star. Scientists using NASA’s Hubble Space Telescope have found evidence that this planet may have lost its original atmosphere but gained a second one containing a toxic mix of hydrogen, methane and hydrogen cyanide. Hubble discovered the “fingerprints” of these gases as light from the parent star filtered through the exoplanet’s atmosphere. The planet is too far away and too dark to be photographed by Hubble. This illustrates what astronomers think is going on in this remote world. Under the smoggy, hazy atmosphere of the planet, there may be a thin crust just a few hundred meters thick. Molten lava beneath the surface is constantly seeping through volcanic fissures. Gases seeping through these cracks seem to constantly fill the atmosphere, which would otherwise be stripped away by sweltering radiation from the planet’s nearby star. Gravity from another planet in the system likely fractures the surface of GJ 1132 b to resemble a cracked eggshell. This is the first time that a so-called “secondary atmosphere” has been detected on a planet outside our solar system. Credit: NASA, ESA, and R. Hurt (IPAC / Caltech)
Orbiting a red dwarf star 41 light-years away is an Earth-sized rocky exoplanet called GJ 1132 b. In some ways GJ 1132 b has intriguing parallels to Earth, but in other ways it’s very different. One difference is that the smoggy, hazy atmosphere contains a poisonous mixture of hydrogen, methane and hydrogen cyanide. Scientists using NASA’s Hubble Space Telescope have found evidence that this is not the planet’s native atmosphere, and that the former was blown away by blistering radiation from the nearby parent star of GJ 1132 b. The so-called “secondary atmosphere” is believed to be formed as molten lava beneath the planet’s surface, constantly seeping through volcanic fissures. Gases seeping through these cracks seem to constantly replenish the atmosphere, which would otherwise be stripped away by the star as well. This is the first time that a secondary atmosphere has been detected on a world outside of our solar system.
Scientists using NASA’s Hubble Space Telescope have found evidence that a planet orbiting a distant star may have lost its atmosphere but gained a second one due to volcanic activity.
The planet, GJ 1132 b, is believed to have started out as a gaseous world with a thick hydrogen atmosphere. Starting at several times the diameter of the Earth, this so-called “sub-Neptune” is believed to have rapidly lost its original hydrogen and helium atmosphere due to the intense radiation from the hot young star it orbits. In a short time such a planet would be stripped down to a bare core the size of the Earth. That’s when things got interesting.
To astronomers’ surprise, Hubble observed an atmosphere that, according to their theory, is a “secondary atmosphere” that is now present. Based on a combination of direct observational evidence and inference via computer modeling, the team reports that the atmosphere is made up of molecular hydrogen, hydrogen cyanide, methane, and also contains an aerosol haze. Modeling suggests that the aerosol haze is based on photochemically produced hydrocarbons, similar to smog on Earth.
Scientists interpret the current atmospheric hydrogen in GJ 1132 b as hydrogen from the original atmosphere that was absorbed into the planet’s molten magma mantle and is now slowly being released by volcanic processes to form a new atmosphere. The atmosphere we see today is believed to be constantly replenished to balance the hydrogen escaping into space.
“It’s super exciting because we believe the atmosphere we’re seeing now has been regenerated, so it could be a secondary atmosphere,” said study co-author Raissa Estrela of NASA’s Jet Propulsion Laboratory (JPL) in Southern California. “We thought at first that these highly irradiated planets could be pretty boring because we thought they were losing their atmosphere. But we looked at existing observations of this planet with Hubble and said, ‘Oh no, there’s an atmosphere there.'”
The findings could affect other exoplanets, planets outside of our solar system.
How many terrestrial planets start out as terrestrial planets? Some can start out as sub-Neptune, and they become terrestrial planets through a mechanism that photo-evaporates the original atmosphere. This process works early in a planet’s life, when the star is hotter , ”said lead author Mark Swain of JPL. Then the star cools down and the planet just sits there. So you have this mechanism that allows you to boil the atmosphere for the first 100 million years, and then things settle down. And if you can regenerate the atmosphere, maybe you can keeping it. “
This plot shows the spectrum of the atmosphere of a rocky exoplanet the size of Earth, GJ 1132 b. The orange line represents the model spectrum. In comparison, the observed spectrum is shown as blue dots representing mean data points along with their error bars. This analysis is consistent with the fact that GJ 1132 b is predominantly a hydrogen atmosphere with a mixture of methane and hydrogen cyanide. The planet also has aerosols that cause light scattering. This is the first time that a so-called “secondary atmosphere”, which was replenished after the planet lost its original atmosphere, has been detected on a world outside our solar system. Credit: NASA, ESA, and P. Jeffries (STScI)
In some ways, GJ 1132 b, located about 41 light-years from Earth, has tantalizing parallels to Earth, but in some ways it’s very different. Both have similar densities, dimensions and ages, being about 4.5 billion years old. Both started with a hydrogen-dominated atmosphere, and both were hot before cooling. In fact, the team’s work suggests that GJ 1132 b and Earth have similar surface atmospheric pressure.
But the planets have completely different origins. Earth is not believed to be the remaining core of a sub-Neptune. And the Earth rotates at a comfortable distance from our sun. GJ 1132 b is so close to its red dwarf star that it orbits its host star once every day and a half. This extremely short distance keeps GJ 1132 b neatly locked and always shows the same face to its star – much like our moon keeps a hemisphere permanently pointed towards Earth.
“The question is, what keeps the mantle warm enough to remain fluid and vigorous volcanism?” Swain asked. “This system is special because it has the potential for a great deal of tidal heating.”
Tidal heating is a phenomenon that occurs through friction, when energy from a planet’s orbit and rotation is dissipated as heat within the planet. GJ 1132b is in an elliptical orbit and the tidal forces acting on it are strongest when it is closest or furthest from the host star. At least one other planet in the host star’s system also pulls gravity on the planet.
The consequences are that the planet is compressed or stretched by this gravity “pumping”. This tidal heating keeps the mantle liquid for a long time. A nearby example in our own solar system is Jupiter’s moon Io, which shows continuous volcanic activity as a result of a tidal tug of war from Jupiter and the neighboring Jupiter moons.
Given the GJ 1132b’s hot interior, the team believes the planet’s cooler, overlying crust is extremely thin, perhaps only hundreds of feet thick. That’s far too weak to support anything resembling volcanic mountains. The flat terrain can also crack like an eggshell due to tidal bending. Such cracks can release hydrogen and other gases.
NASA’s upcoming James Webb Space Telescope has the ability to observe this exoplanet. Webb’s infrared vision allows scientists to look at the planet’s surface. “If there are magma pools or volcanism, those areas will be hotter,” explains Swain. “That will generate more emissions, and so they may be looking at the actual geologic activity – which is exciting!”
The team’s findings will be published in an upcoming issue of The Astronomical Journal
Study sheds light on the fate of distant planetary atmospheres
Detection of an atmosphere on a rocky exoplanet. arXiv: 2103.05657 [astro-ph.EP] arxiv.org/abs/2103.05657
Provided by ESA / Hubble Information Center
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