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The atmosphere that most planets start with is often not the same as the one they end up with. Most of the gases present in the formation of a solar system will be hydrogen and helium. But a glance through the rocky planets of our solar system shows three very different (and one very tenuous) atmospheres, with hydrogen and helium being relatively minor components. And, as we gain the ability to look at the atmospheres of exoplanets, we should have a greater perspective on all of the ways atmospheres can change as their planets age.
This week, an international team of astronomers reported finding an atmosphere on a planet you wouldn’t expect to exist. And astronomers suggest that this is actually the second atmosphere on the planet, generated by volcanic activity after the first was evaporated early in the planet’s history.
Imaging atmospheres
In general, we currently don’t have the technology to image exoplanets unless they are very large, very young, and at a considerable distance from the star they orbit. Yet we can still get an idea of what is in their atmosphere. To do this, we need to observe a planet that crosses 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 on the spectrum of light that reaches Earth. This is an extremely weak signature, as most of the star’s light never even sees the atmosphere. But by combining data from several days of observation, it is possible to bring out this noise signature.
This is what scientists did with GJ 1132b, an exoplanet that orbits a small star about 40 light years from Earth. The planet is roughly the size of Earth and about 1.5 times its mass. It also orbits extremely close to its host star, completing a full orbit in just 1.6 days. This is close enough to ensure that, despite the small dark star, the GJ 1132b is extremely hot.
It’s so close and so hot, in fact, that researchers estimate it is currently losing about 10,000 kilograms of atmosphere per second. Because the host star was expected to be brighter early in its history, researchers estimate that GJ 1132b would have lost a reasonably sized atmosphere during the first 100 million years of its existence. In fact, over the life of the planet, researchers estimate that it could have lost an atmosphere weighing about five times the current mass of the planet – the kind of thing you could see if the remaining planet was the core of the planet. ‘a mini-Neptune.
(There are uncertainties in these numbers, depending on how often its star sends out high-energy particles and the strength of the planet’s magnetic field. But they are not large enough to hold an atmosphere in place for the entire 5 billion planet. story of the year.)
So, researchers were probably surprised to find that, based on Hubble’s data, the planet appears to have an atmosphere.
How did it happen here?
One potential explanation for this is that the planet formed at a cooler distance from the star, and then migrated inward. But that would mean we caught GJ 1132 b in a relatively narrow window of time: between getting close enough to the star to lose its atmosphere, but before all that atmosphere has warmed up in space. Chances are better that the planet formed near where it is located and generated a second atmosphere after the first was lost.
Fortunately, the data provided by Hubble was able to give an idea of what is in the atmosphere. The signature left in starlight by molecules in the atmosphere gives an indication of what they might be. These indications are complicated – because there are many molecules whose signatures partially overlap in some areas of the spectrum but not in others – and they complicate even more. But it is possible to look at the signal from the planet’s atmosphere and identify combinations of molecules compatible with this signal.
Researchers find that there are probably aerosols in the atmosphere. And its make-up really wouldn’t be surprising on another planet: mostly methane, ethane, hydrogen, and hydrogen cyanide. But remember, the reason this atmosphere is interesting is that the planet should have lost its atmosphere early in its history – and all the hydrogen should have gone with it.
Magma
The research team suggests a potential solution to this conundrum, however. At the beginning of the planet’s history, it should have had both 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, get trapped under the crust.
But potentially not trapped forever. Astronomers suggest the planet should be hot in part because of the large amounts of radiation it picks up from its extremely nearby star, but also because of 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, the current atmosphere can be formed and replenished by volcanic activity, with the hydrogen-rich magma creating its distinct composition.
Obviously, that won’t be the easiest thing to confirm, although the arrival of the James Webb Space Telescope will open up new areas of the spectrum to provide independent control over the estimated composition of the atmosphere. But the best control will simply be to see that this type of secondary atmosphere appears on other exoplanets. And, given the interest in imagery of their atmospheres, we may not have to wait long.
The arXiv file. Abstract number: 2103.05657 (About arXiv). To appear in The Astronomical Journal.
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