Exoplanet in second garment?

Artist’s impression of the planet GJ 1132b with what is probably the second atmosphere in its evolutionary history. (Image: NASA, ESA, and R. Hurt (IPAC / Caltech)

The Earth-like exoplanet GJ 1132b is believed to be wrapped in a newly formed second atmosphere, astronomers report. The first gas envelope “blown away” his star, but then he created a new atmosphere through volcanism. This scenario is suggested by research results based on data from NASA’s Hubble Space Telescope. There may be numerous cases of such a history of evolution of rocky planets in space, say the scientists.

The focus is on the system of the red dwarf star Gliese 1132 in the constellation Segel. It is only around 41 light years away from the sun and therefore in our cosmic neighborhood. A few years ago astronomers discovered the planet GJ 1132b there. According to the data, it is a rocky planet 1.4 times the radius of the earth. However, due to its proximity to its parent star, it is about as hot as Venus. The celestial body attracted particular attention, as astronomers were able to prove in 2017 that GJ 1132b has an atmosphere whose transparency becomes apparent as the planet passes in front of the star.

Insights into the atmosphere

The current study by the international team of astronomers now follows on from these research results. It is based on data from the Hubble Space Telescope, which in turn is based on the transit spectroscopy method. The spectrum of light that shimmers through the shell of the exoplanet when it passes in front of its central star is analyzed. The features can then provide clues to specific substances in the atmosphere. As the researchers report, their results and models now show that the atmosphere of GJ 1132b consists of molecular hydrogen, hydrogen cyanide and methane. It also contains an aerosol vapor. The models suggest that this veil is based on photochemically generated hydrocarbons, similar to the smog on earth.

As the scientists explain, the characteristics of the gas envelope suggest that it is a secondary atmosphere. They therefore suspect that GJ 1132b was initially a “sub-Neptune” – a planet with a thick hydrogen atmosphere that was several times the diameter of the earth. However, due to the intense radiation from the nearby parent star, it then lost its original hydrogen and helium atmosphere. This left its hot rock core about the size of the earth.

Evidence of neoplasm

The scientists interpret today’s hydrogen in the atmosphere of GJ 1132b as an indirect remnant from the original atmosphere: the gas was once bound in the molten magma of the planet and then slowly released again through volcanic processes. From this, the majority of today’s atmosphere was formed. It is likely to be preserved because the hydrogen escaping into space is constantly being replenished by volcanism, the scientists suspect.

“It was initially thought that heavily irradiated planets were pretty boring because they were believed to have lost their atmospheres,” says co-author Raissa Estrela of NASA’s Jet Propulsion Laboratory (JPL) in Pasadena. But now it is becoming increasingly clear that this is not necessarily the case. Her colleague Mark Swain says: “It is possible that many planets were not Earth-like from the start. Some may initially have been sub-Neptunes who, through the loss of their original atmosphere, became Earth-like planets. Some of them could then maybe regenerate and keep their atmosphere ”, says the lead author of the study.

Volcanism due to tidal forces

But why could GJ 1132b have such a strong volcanism that enables it to regenerate the atmosphere? The researchers also have an explanation for this: tidal warming. It is a phenomenon that is based on internal friction that can arise from the orbit of a celestial body. “In GJ 1132b, there is particularly intense potential for this,” says Swain. Because the planet is in an elliptical orbit. Particularly strong tidal forces arise when it periodically moves away from its star and comes close to it again. In addition, at least one other planet in the system could pull the planet, kneading it downright. The resulting frictional heating probably keeps the mantle largely fluid and causes volcanism, the researchers explain. They suspect that the crust of GJ 1132b is extremely thin – maybe only a few hundred meters thick. The flat terrain could therefore be criss-crossed with cracks from which hydrogen and other gases are released, the scientists explain.

They are now hoping for further opportunities to examine the exciting celestial body from the sharp view of NASA’s planned James Webb space telescope, which is due to launch into space this year. Its infrared vision could enable astronomers to see down to the surface of the planet. “If there are magma pools or volcanism there, we may be able to see those areas,” says Swain.

Source: NASA, technical article: The Astronomical Journal, doi: arxiv.org/abs/2103.05657

Video: NASA Goddard Space Flight Center

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