Researchers think a planet lost its original atmosphere, built a new one
Gas with a past —
Imaging the atmosphere finds a combination of chemicals that shouldn’t be there.
The atmosphere most planets start with is often not the same as the one they end up with. Most of the gas present at the formation of a solar system will be hydrogen and helium. But a look through the rocky planets of our Solar System shows three very different atmospheres (and one very tenuous one), with hydrogen and helium being relatively minor components. And, as we gain the ability to look at the atmospheres of exoplanets, we should get a greater 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 one wouldn’t be expected to exist. And the astronomers suggest that it’s actually the planet’s second atmosphere, generated by volcanic activity after the first was boiled off early in the planet’s history.
In general, we don’t currently have the technology to image exoplanets unless they’re very large, very young, and a considerable distance from the star they orbit. Yet we can still get some sense of what’s in their atmosphere. To do that, we need to observe a planet that transits across 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.
Those molecules leave a signature on the spectrum of light that reaches the Earth. It’s an extremely faint signature, since most of the star’s light never even sees the atmosphere. But by combining the data from a number of days of observation, it’s possible to get this signature to stand out from the noise.
That’s what scientists have done with GJ 1132 b, an exoplanet that orbits a small star about 40 light years from Earth. The planet is roughly Earth’s size and about 1.5 times its mass. It also orbits extremely close to its host star, completing a full orbit in only 1.6 days. That’s close enough to ensure that, despite the small, dim star, GJ 1132 b is extremely hot.
It’s so close and hot, in fact, that the researchers estimate that it’s currently losing about 10,000 kilograms of atmosphere every second. As the host star was expected to be brighter early in its history, the researchers estimate that GJ 1132 b would have lost a reasonable-sized atmosphere within 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 in at about five times the planet’s current mass—the sort of thing you might see if the remaining planet were the core of a mini-Neptune.
(There are some uncertainties in these figures, based on how often its star sends out high-energy particles and how strong the planet’s magnetic field is. But they’re not large enough to keep an atmosphere in place for the planet’s entire 5 billion-year history.)
So, researchers were probably surprised to find that, based on data from the Hubble, the planet seems to have an atmosphere.
How’d that get 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’ve caught GJ 1132 b in a relatively narrow time window: between it getting close enough to the star to lose its atmosphere, but before all that atmosphere had been heated off into space. The odds are better that the planet had formed near where it is and has generated a second atmosphere after the first was lost.
Fortunately, the data the Hubble provided was able to provide some hint of what’s in the atmosphere. The signature left on starlight by the molecules present in the atmosphere provides some indication of what they might be. These indications are complicated—there are many molecules that have signatures that partially overlap in some areas of the spectrum but not others. But it’s possible to look at the signal from the planet’s atmosphere and identify combinations of molecules that are compatible with that signal.
The researchers find there’s likely to be some aerosols aloft in the atmosphere. And its composition really wouldn’t be surprising on another planet: mostly methane, ethane, hydrogen, and hydrogen cyanide. But remember, the whole 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 away with it.
The research team, however, suggests a potential solution to this conundrum. Early in the planet’s history, it should have had both a hydrogen-rich atmosphere and a surface that was a magma ocean. Recent studies have suggested that a large amount of hydrogen can potentially end up stored in magma and, as the planet cools down, find itself trapped beneath the crust.
But potentially not trapped forever. The astronomers suggest that 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 present atmosphere may be formed and replenished by volcanic activity, with hydrogen-rich magma creating its distinct composition.
Obviously, that’s not going to 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 an independent check on the estimated composition of the atmosphere. But the best check will simply be finding that this sort of secondary atmosphere shows up on other exoplanets. And, given the interest in imaging their atmospheres, we may not have long to wait for that.
The arXiv. Abstract number: 2103.05657 (About the arXiv). To be published in The Astronomical Journal.