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Catching Up with TRAPPIST-1
Let’s have a look at recent work on TRAPPIST-1. The system, tiny but rich in planets (seven transits!) continues to draw new work, and it’s easy to see why. Found in Aquarius some 40 light years from Earth, a star not much larger than Jupiter is close enough for the James Webb Space Telescope to probe the system for planetary atmospheres. Or so an international team working on the problem believes, with interesting but frustratingly inconclusive results.
As we’ll see, though, that’s the nature of this work, and in general of investigations of terrestrial-class planet atmospheres. I begin with news of TRAPPIST-1’s flare activity. One of the reasons to question the likelihood of life around small red stars is that they are prone to violent flares, particularly in their youth. Planets in the habitable zone, and there are three here, would be bathed in radiation early on, conceivably stripping their atmospheres entirely, and certainly raising doubts about potential life on the surface.
Image: Artist’s concept of the planet TRAPPIST-1d passing in front of the star TRAPPIST-1. Credit: NASA, ESA, CSA, Joseph Olmsted/STScI.
A just released paper digs into the question by applying JWST data on six flares recorded in 2022 and 2023 to a computer model created by Adam Kowalski (University of Colorado Boulder), who is a co-author on the work. The equations of Kowalski’s model allow the researchers to probe the stellar activity that created the flares, which the authors see as deriving from magnetic reconnection that heats stellar plasma through pulses of electron beaming.
The scientists are essentially reverse-engineering flare activity with an eye to understanding how it might affect an atmosphere, if one exists, on these planets. The extent of the activity came as something of a surprise. As lead author Ward Howard (also at University of Colorado Boulder) puts it: “When scientists had just started observing TRAPPIST-1, we hadn’t anticipated the majority of our transits would be obstructed by these large flares.”
Which would seem to be bad news for biology here, but we also learn from Kowalski’s equations that TRAPPIST-1 flares are considerably weaker than supposed. We can couple this result with two papers published earlier this year in the Astrophysical Journal Letters. Using transmission spectroscopy and working with JWST’s Near-Infrared Spectrograph and Near-Infrared Imager and Slitless Spectrograph, the researchers looked at TRAPPIST-1e as it passed in front of the host star. A third paper, released on December 5, examines these data and the possibility of methane in an atmosphere. Here we run into the obvious limitations of modeling.
The most recent paper is out of the University of Arizona, where Sukrit Ranjan and team have gone to work on methane in an M-dwarf planet atmosphere. With an eye toward TRAPPIST-1e, they note this (italics mine):
We have shown that models that include CH4 are viable fits to TRAPPIST-1e’s transmission spectrum through both our forward-model analysis and retrievals. However, we stress that the statistical evidence falls far below that required for a detection. While an atmosphere containing CH4 and a (relatively) spectrally quiet background gas (e.g., N2) provides a good fit to the data, these initial TRAPPIST-1 e transmission spectra remain consistent with a bare rock or cloudy atmosphere interpretations. Additionally, we note that our “best-fit” CH4 model does not explain all of the correlated features present in the data. Here we briefly examine the theoretical plausibility of a N2–CH4 atmosphere on TRAPPIST-1 e to contextualize our findings.
Should we be excited by even a faint hint of an atmosphere here? Probably not. The paper simulates methane-rich atmosphere scenarios, but also discusses alternative possibilities. Here we get a sense for how preliminary all our TRAPPIST-1 work really is (and remember that JWST is working at the outer edge of its limits in retrieving the data used here). A key point is that TRAPPIST-1 is significantly cooler than our G-class Sun. As Ranjan points out:
“While the sun is a bright, yellow dwarf star, TRAPPIST-1 is an ultracool red dwarf, meaning it is significantly smaller, cooler and dimmer than our sun. Cool enough, in fact, to allow for gas molecules in its atmosphere. We reported hints of methane, but the question is, ‘is the methane attributable to molecules in the atmosphere of the planet or in the host star?…[B]ased on our most recent work, we suggest that the previously reported tentative hint of an atmosphere is more likely to be ‘noise’ from the host star.”
The paper notes that any spectral feature from an exoplanet could have not just stellar origins but also instrumental causes. In any case, stellar contamination is an acute problem because it has not been fully integrated into existing models. The approach is Bayesian, given that the plausibility of any specific scenario for an atmosphere has an effect on the confidence with which it can be identified in an individual spectrum. Right now we are left with modeling and questions.
Ranjan believes that the way forward for this particular system is to use a ‘dual transit’ method, in which the star is observed when both TRAPPIST-1e and TRAPPIST-1b move in front of the star at the same time. The idea is to separate stellar activity from what may be happening in a planetary atmosphere. As always, we look to future instrumentation, in this case ESO’s Extremely Large Telescope, which is expected to become available by the end of this decade. And next year NASA will launch the Pandora mission, a small telescope but explicitly designed for characterizing exoplanet atmospheres.
More questions than answers? Of course. We’re pushing hard against the limits of detection, but all these models help us learn what to look for next. Nearby M-dwarf transiting planets, with their deep transit depths, higher transit probability in the habitable zone and frequent transit opportunities, are going to be commanding our attention for some time to come. As always, patience remains a virtue.
Here’s a list of the papers I’ve discussed here. The flare modeling paper is Howard et al., “Separating Flare and Secondary Atmospheric Signals with RADYN Modeling of Near-infrared JWST Transmission Spectroscopy Observations of TRAPPIST-1,” Astrophysical Journal Letters Vol. 994, No. 1 (20 November 2025) L31 (full text).
The paper on methane detection and stellar activity is Ranjan et al., “The Photochemical Plausibility of Warm Exo-Titans Orbiting M Dwarf Stars,” Astrophysical Journal Letters Vol. 993, No. 2 (3 November 2025), L39 (full text).
The earlier papers of interest are Glidden et al., “JWST-TST DREAMS: Secondary Atmosphere Constraints for the Habitable Zone Planet TRAPPIST-1 e,” Astrophysical Journal Letters Vol. 990, No. 2 (8 September 2025) L53 (full text); and Espinoza et al. “JWST-TST DREAMS: NIRSpec/PRISM Transmission Spectroscopy of the Habitable Zone Planet TRAPPIST-1 e,” Astrophysical Journal Letters Vol. 990, No. 2 (L52) (full text).
As we’ll see, though, that’s the nature of this work, and in general of investigations of terrestrial-class planet atmospheres. I begin with news of TRAPPIST-1’s flare activity. One of the reasons to question the likelihood of life around small red stars is that they are prone to violent flares, particularly in their youth. Planets in the habitable zone, and there are three here, would be bathed in radiation early on, conceivably stripping their atmospheres entirely, and certainly raising doubts about potential life on the surface.
Image: Artist’s concept of the planet TRAPPIST-1d passing in front of the star TRAPPIST-1. Credit: NASA, ESA, CSA, Joseph Olmsted/STScI.
A just released paper digs into the question by applying JWST data on six flares recorded in 2022 and 2023 to a computer model created by Adam Kowalski (University of Colorado Boulder), who is a co-author on the work. The equations of Kowalski’s model allow the researchers to probe the stellar activity that created the flares, which the authors see as deriving from magnetic reconnection that heats stellar plasma through pulses of electron beaming.
The scientists are essentially reverse-engineering flare activity with an eye to understanding how it might affect an atmosphere, if one exists, on these planets. The extent of the activity came as something of a surprise. As lead author Ward Howard (also at University of Colorado Boulder) puts it: “When scientists had just started observing TRAPPIST-1, we hadn’t anticipated the majority of our transits would be obstructed by these large flares.”
Which would seem to be bad news for biology here, but we also learn from Kowalski’s equations that TRAPPIST-1 flares are considerably weaker than supposed. We can couple this result with two papers published earlier this year in the Astrophysical Journal Letters. Using transmission spectroscopy and working with JWST’s Near-Infrared Spectrograph and Near-Infrared Imager and Slitless Spectrograph, the researchers looked at TRAPPIST-1e as it passed in front of the host star. A third paper, released on December 5, examines these data and the possibility of methane in an atmosphere. Here we run into the obvious limitations of modeling.
The most recent paper is out of the University of Arizona, where Sukrit Ranjan and team have gone to work on methane in an M-dwarf planet atmosphere. With an eye toward TRAPPIST-1e, they note this (italics mine):
We have shown that models that include CH4 are viable fits to TRAPPIST-1e’s transmission spectrum through both our forward-model analysis and retrievals. However, we stress that the statistical evidence falls far below that required for a detection. While an atmosphere containing CH4 and a (relatively) spectrally quiet background gas (e.g., N2) provides a good fit to the data, these initial TRAPPIST-1 e transmission spectra remain consistent with a bare rock or cloudy atmosphere interpretations. Additionally, we note that our “best-fit” CH4 model does not explain all of the correlated features present in the data. Here we briefly examine the theoretical plausibility of a N2–CH4 atmosphere on TRAPPIST-1 e to contextualize our findings.
Should we be excited by even a faint hint of an atmosphere here? Probably not. The paper simulates methane-rich atmosphere scenarios, but also discusses alternative possibilities. Here we get a sense for how preliminary all our TRAPPIST-1 work really is (and remember that JWST is working at the outer edge of its limits in retrieving the data used here). A key point is that TRAPPIST-1 is significantly cooler than our G-class Sun. As Ranjan points out:
“While the sun is a bright, yellow dwarf star, TRAPPIST-1 is an ultracool red dwarf, meaning it is significantly smaller, cooler and dimmer than our sun. Cool enough, in fact, to allow for gas molecules in its atmosphere. We reported hints of methane, but the question is, ‘is the methane attributable to molecules in the atmosphere of the planet or in the host star?…[B]ased on our most recent work, we suggest that the previously reported tentative hint of an atmosphere is more likely to be ‘noise’ from the host star.”
The paper notes that any spectral feature from an exoplanet could have not just stellar origins but also instrumental causes. In any case, stellar contamination is an acute problem because it has not been fully integrated into existing models. The approach is Bayesian, given that the plausibility of any specific scenario for an atmosphere has an effect on the confidence with which it can be identified in an individual spectrum. Right now we are left with modeling and questions.
Ranjan believes that the way forward for this particular system is to use a ‘dual transit’ method, in which the star is observed when both TRAPPIST-1e and TRAPPIST-1b move in front of the star at the same time. The idea is to separate stellar activity from what may be happening in a planetary atmosphere. As always, we look to future instrumentation, in this case ESO’s Extremely Large Telescope, which is expected to become available by the end of this decade. And next year NASA will launch the Pandora mission, a small telescope but explicitly designed for characterizing exoplanet atmospheres.
More questions than answers? Of course. We’re pushing hard against the limits of detection, but all these models help us learn what to look for next. Nearby M-dwarf transiting planets, with their deep transit depths, higher transit probability in the habitable zone and frequent transit opportunities, are going to be commanding our attention for some time to come. As always, patience remains a virtue.
Here’s a list of the papers I’ve discussed here. The flare modeling paper is Howard et al., “Separating Flare and Secondary Atmospheric Signals with RADYN Modeling of Near-infrared JWST Transmission Spectroscopy Observations of TRAPPIST-1,” Astrophysical Journal Letters Vol. 994, No. 1 (20 November 2025) L31 (full text).
The paper on methane detection and stellar activity is Ranjan et al., “The Photochemical Plausibility of Warm Exo-Titans Orbiting M Dwarf Stars,” Astrophysical Journal Letters Vol. 993, No. 2 (3 November 2025), L39 (full text).
The earlier papers of interest are Glidden et al., “JWST-TST DREAMS: Secondary Atmosphere Constraints for the Habitable Zone Planet TRAPPIST-1 e,” Astrophysical Journal Letters Vol. 990, No. 2 (8 September 2025) L53 (full text); and Espinoza et al. “JWST-TST DREAMS: NIRSpec/PRISM Transmission Spectroscopy of the Habitable Zone Planet TRAPPIST-1 e,” Astrophysical Journal Letters Vol. 990, No. 2 (L52) (full text).
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