Among the most eagerly awaited results from the early observations of the James Webb Space Telescope (JWST) is whether or not the seven rocky planets of the TRAPPIST-1 system have atmospheres.
The TRAPPIST-1 planets are close to us (40 light-years away), are all solid rather than gaseous, and they orbit a cool and small sun that makes the planets easier to observe and measure. Not surprisingly, all seven of the planets in the system will be observed during the first year of JWST observations and the results have begun to come in.
And so far, at least, the planets do not have atmospheres, or at least don’t have substantial atmospheres.
The planet closest to its sun, TRAPPIST 1-b was determined to be devoid of an atmosphere earlier this year and last week a Nature paper reportsed that TRAPPIST 1-C also does not appear to have anything more than a very thin atmosphere.
Neither findings was completely surprising, since these are the two planets closest to the star. But even so, the results indicate that gaseous atmospheres require a particular collection of planetary, stellar and system conditions that may be difficult to achieve.
“TRAPPIST-1 c is interesting because it’s basically a Venus twin,” explained co-author Laura Kreidberg, co-author of the paper and and Director at Max Planck Institute for Astronomy in Germany. “It’s about the same size as Venus and receives a similar amount of radiation from its host star as Venus gets from the Sun.”
Before the observations, she said “we thought it could have a thick carbon dioxide atmosphere like Venus.” But now, “we can definitely rule out a thick and Venus-like atmosphere.”
In a Max Planck Institute release, the large international team studying TRAPPIST 1-C reports that while faint signals of a possible thin atmosphere were detected, that data did not come from a detection of atmospheric chemicals. A barren rock with a surface layer of material weathered from stellar irradiation, it reported, could explain the observations equally well as a thin atmosphere would.
All seven planets that orbit around the red dwarf star TRAPPIST-1 could easily fit inside the orbit of Mercury, the innermost planet of our solar system. In fact, they would have room to spare.
TRAPPIST-1 also is only a fraction of the size of our Sun; it isn’t much larger than Jupiter. So, the TRAPPIST-1 system’s proportions look more like Jupiter and its moons than those of our solar system.
Nonetheless, the TRAPPIST-1 solar system is seen as a possible home to habitable worlds, making it a favorite site for early JWST observations trying to determine whether the planets have atmospheres, and if so what they are they like.
But while the TRAPPIST-1 planets are similar in size and mass to the inner, rocky planets in our own solar system, other factors known and unknown might leave them devoid of atmospheres.
During the first billion years of their lives, M dwarf or red dwarf star emit bright X-ray and ultraviolet radiation that can easily strip away a young planetary atmosphere — especially since the planets are tidally locked and one side always faces the star. In addition, there may or may not have been enough water, carbon dioxide, and other volatiles available to make substantial atmospheres when the planets formed.
To search for an atmosphere on TRAPPIST-1 c, the team used MIRI (Webb’s Mid-Infrared Instrument) on four separate occasions as the planet moved behind the star, a phenomenon known as a secondary eclipse.
By comparing the brightness when the planet is behind the star (starlight only) to the brightness when the planet is beside the star (light from the star and planet combined) the team was able to calculate that the temperature of the planet.
The results showed that the dayside temperature of TRAPPIST 1-c was about 225 degrees Fahrenheit. This was the lowest temperature ever recorded on a rocky exoplanet and showed just how powerful and versatile the JWST can be.
But when it came to determining whether the planet might have an atmosphere, the temperature was considerably too high to maintain a thick atmosphere rich in carbon dioxide at all similar to that of Venus.
That headline result, plus the finding of no atmosphere on TRAPPIST 1-b, has led some to wonder whether any TRAPPIST-1 planets could be habitable.
First there are all the problems associated with that early tempestuous flaring of a red dwarf star. But scientists also concluded that, based on comparisons of observations with models of the planet’s possible chemistry, TRAPPIST 1-c would have had very little water when it formed.
Together, that inferred paucity of water at the planet’s birth and the absence of a thick carbon dioxide atmosphere today suggest that TRAPPIST-1 c may well never have had ingredients and conditions that would support habitability.
Might that be the case with other TRAPPIST-1 planets further from their Sun? It’s a possibility.
Reacting to that prospect, Sebastian Zieba, an exoplanet researcher at the Max Planck Institute and lead author of the Nature paper, said that if planets in red dwarf systems continue to be found to have no atmospheres or only limited atmospheres “that would definitely reduce the amount of planets which might be habitable.” And by a lot, because planets orbiting red dwarf stars are by far the most common in the galaxy.
But there are other perspectives and voices to be heard.
Victoria Meadows, a co-author of the TRAPPIST 1-c paper and head of the University of Washington’s pioneering Virtual Planetary Lab, is working with colleague and co-author Andrew Lincowski to dig deeper into the make-up of that possible faint atmosphere on the planet and into other data from the TRAPPIST 1-c observations.
Based on the data so far, Meadows said, “We cannot rule out or in a thin atmosphere” on TRAPPIST 1-c.
Joshua Krissansen-Totton, also of the University of Washington, has written a paper as well that will appear soon in the Astrophysical Journal Letters that argues that the results from Trappist 1 b and c do not necessarily dim prospects for finding atmospheres on Trappist-1 e and f. These two planets are further out from their Sun and are in a habitable zone — where temperatures potentially allow for the presence of liquid water
In his abstract he wrote, “Naturally, it is still possible that all Trappist-1 planets formed volatile-poor and are all airless today. But the airlessness of b (and c) does not require this, and as such, JWST transit spectroscopy of e and f remains the best near-term opportunity to characterize the atmospheres of habitable zone terrestrial planets.”
While observations of the TRAPPIST-1 system by JWST are proceeding with some speed, the analysis and reanalysis of the data takes much more time. Meadows said that with all the new data coming in about the TRAPPIST system, it will likely take some years for scientists to reach consensus views about what the system is really like.
And that data, whatever it ultimately shows, is unprecedented.
Sebastian Zieba put it this way in a release: “We want to know if rocky planets have atmospheres or not. In the past, we could only really study planets with thick, hydrogen-rich atmospheres. With Webb we can finally start to search for atmospheres dominated by oxygen, nitrogen, and carbon dioxide.”
In other words, atmospheres that could potentially surround a habitable planet.
Many Worlds 
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