This exoplanet has lost its original atmosphere, but has developed a second

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Observations made by the Hubble telescope, coupled with computer simulations, suggest that GJ 1132 b, a nearby exoplanet, has lost its original atmosphere to develop a second thanks to its volcanic activity.

In some ways, GJ 1132 b, found 41 light years away, displays intriguing similarities to Earth. The two planets indeed have similar densities, sizes and ages. Both also started with a hydrogen dominated atmosphere and both were hot worlds before they cooled down. On the other hand, in other respects these two worlds are also very different.

On the one hand, we indeed have the Earth orbiting at a relatively comfortable distance from its Sun. And on the other, there is GJ 1132 b which is so close to its red dwarf that it circles it in a day and a half. This proximity keeps the planet “locked” in its orbit, ultimately showing only one face to its star like the Moon with the Sun.

This incredible proximity has had other consequences for the planet. In a recent study, researchers used direct observations from Hubble and computer models to probe the atmosphere of GJ 1132 b. According to the study, it would now be composed of a toxic mixture of hydrogen, methane and hydrogen cyanide. However, for researchers, this is not its original atmosphere.

A regenerated atmosphere

A few million years after his birth, GJ 1132 b had a very different face. According to the team, this was essentially a gaseous world much bigger than Earth enveloped in a thick layer of hydrogen. This “sub-Neptune” would then have quickly lost its primordial atmosphere because of the intense radiation of his still very hot young star. Stripped of its atmosphere, GJ 1132 b would then have been reduced to its bare core, the size of the Earth.

Then, a “secondary atmosphere” would then have formed when molten lava beneath the planet’s surface leaked through cracks in its crust. The gases that seep through these cracks seem to be constantly replenishing this new atmosphere which would otherwise be stripped by the star as well.

This new discovery could have implications for other exoplanets. “How many terrestrial planets do not begin as such?“, Asks Mark Swain of JPL. “Some can indeed be born as “sub-Neptunes” before transforming“.

Illustration of the rocky exoplanet GJ 1132 b. Credits: NASA, ESA and R. Hurt (IPAC / Caltech)

A gravitational tug of war game

It remains to be understood what keeps the mantle of GJ 1132 b hot enough to remain liquid and fuel this volcanism. According to the authors, everything would be a question of warming tides, a phenomenon that occurs by friction when energy from a planet’s orbit and rotation is dispersed as heat within the planet.

Here, GJ 1132 b moves in an elliptical orbit and the tidal forces acting on it are strongest when it is closest to or farthest from its host star. However, there is at least one other planet within this system which also gravitationally “pulls” on the planet.

As a result, GJ 1132 b is found compressed or stretched by this gravitational “pumping”. This tidal heating would thus keep the mantle of the planet in liquid form long enough. The same is happening in our Solar System with the volcanic moon Io, caught in a “gravitational tug of war” between Jupiter and the other nearby moons.

Details of the study are published in The astronomical journal.

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