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The atmosphere that most planets begin with is often not the same that they end with. Most of the gas present in the formation of a solar system will be hydrogen and helium. But a look at the rocky planets in our Solar System shows three very different atmospheres (and one very tenuous), with hydrogen and helium being relatively minor components. And, as we gain the ability to observe exoplanets’ atmospheres, we should have a broader perspective of all the ways that atmospheres can change as their planets age.
This week, an international team of astronomers reported finding an atmosphere on a planet where it would not be expected to exist. And astronomers suggest that it is actually the second atmosphere on the planet, generated by volcanic activity after the first evaporated early in the planet’s history.
Imaging atmospheres
In general, we currently do not have the technology to image exoplanets unless they are very large, very young, and at a considerable distance from the star they orbit. However, we can still get an idea of what is in its atmosphere. To do that, we need to observe a planet transiting through the line of sight between Earth and its star. During a transit, a small percentage of the star’s light will travel through the planet’s atmosphere on its way to Earth, interacting with the molecules present there.
These molecules leave a signature in the spectrum of light that reaches the Earth. It is an extremely faint signature, as most of the star’s light doesn’t even see the atmosphere. But by combining data from several days of observation, it is possible to make this signature stand out from the noise.
That’s what scientists have done with GJ 1132 b, an exoplanet orbiting a small star about 40 light-years from Earth. The planet is about the size of Earth and about 1.5 times its mass. It also orbits very close to its host star, completing a full orbit in just 1.6 days. That’s close enough to guarantee that, despite the small, faint star, GJ 1132 b is extremely hot.
In fact, it’s so close and hot that researchers estimate it’s currently losing about 10,000 kilograms of atmosphere per second. As the host star was expected to be brighter early in its history, the researchers estimate that GJ 1132 b would have lost an atmosphere of reasonable size in the first 100 million years of its existence. In fact, over the life of the planet, the researchers estimate that it could have lost an atmosphere weighing roughly five times the planet’s current mass – the kind of thing that could be seen if the remaining planet were the core of a mini. -Neptune. .
(There are some uncertainties in these figures, based on how often your star sends out high-energy particles and the strength of the planet’s magnetic field. But they are not large enough to keep an atmosphere in place for the 5 billion complete planet. year of history.)
Therefore, the researchers were probably surprised to find that based on the Hubble data, the planet appears to have an atmosphere.
How did you get here?
One possible explanation for this is that the planet formed at a cooler distance from the star and then migrated inward. But that would mean we’ve captured GJ 1132 b in a relatively narrow time window: between getting close enough to the star to lose its atmosphere, but before all that atmosphere has warmed up to space. The planet is more likely to have formed close to where it is and generated a second atmosphere after the first was lost.
Fortunately, the data provided by Hubble could provide some clue as to what is in the atmosphere. The signature left in starlight by molecules in the atmosphere provides some indication of what they might be. These indications are complicated, since there are many molecules that have signatures that partially overlap in some areas of the spectrum, but not in others, and generate more complications. But it is possible to observe the signal from the planet’s atmosphere and identify combinations of molecules that are compatible with that signal.
The researchers find that there are likely some aerosols in the atmosphere. And its composition really wouldn’t be surprising on another planet: mostly methane, ethane, hydrogen, and hydrogen cyanide. But remember, the only reason this atmosphere is interesting is because the planet should have lost its atmosphere early in its history, and all the hydrogen should have gone with it.
Magma
However, the research team suggests a possible solution to this puzzle. At the beginning of the planet’s history, it should have had a hydrogen-rich atmosphere and a surface that was an ocean of magma. Recent studies have suggested that a large amount of hydrogen can potentially end up stored in magma and, as the planet cools, become trapped beneath the crust.
But potentially not trapped forever. Astronomers suggest that the planet should be hot in part due to the large amounts of radiation it picks up from its extremely nearby star, but also due to the tidal forces that the star’s gravity exerts on its crust. This should be enough to keep the crust thin and flexible, allowing for large-scale volcanism. So, they suggest, today’s atmosphere can be formed and replenished by volcanic activity, with hydrogen-rich magma creating its distinctive composition.
Obviously, that won’t be the simplest thing to confirm, although the arrival of the James Webb Space Telescope will open up new areas of the spectrum to provide independent verification of the estimated composition of the atmosphere. But the best check will simply be to find that this type of secondary atmosphere appears on other exoplanets. And, given the interest in imaging their atmospheres, we may not have to wait long for that.
The arXiv. Abstract number: 2103.05657 (About arXiv). It will be published in The Astronomical Journal.
