Understanding the atmospheres of terrestrial exoplanets is crucial for assessing their habitability. The TRAPPIST-1 system, with its seven rocky planets orbiting a nearby ultracool dwarf star, offers a unique opportunity to investigate this. Recent advancements, particularly the launch of the James Webb Space Telescope (JWST), have enabled the detection of atmospheric constituents like carbon dioxide (CO2) on exoplanets. Previous JWST observations of TRAPPIST-1 b indicated a lack of a CO2 atmosphere. This study focuses on TRAPPIST-1 c, aiming to determine its atmospheric characteristics through direct observation of its thermal emission using JWST's MIRI instrument. The study's importance lies in its contribution to our understanding of the formation and evolution of rocky exoplanets and their potential for habitability, particularly in the context of M dwarf systems, where atmospheric escape is a significant factor. The research question revolves around characterizing the atmosphere (or lack thereof) of TRAPPIST-1 c and determining its implications for planetary formation models.

Previous studies using JWST have focused on characterizing the atmospheres of other terrestrial exoplanets. Observations of LHS 3844 b and GJ 1252 b revealed dayside temperatures consistent with a lack of atmospheric absorption from carbon dioxide, suggesting a lack of significant atmospheres. These findings motivate further investigation of cooler planets, which might retain atmospheres more effectively. Theoretical models have explored the effects of volatile inventories, outgassing, atmospheric escape, and stellar properties on the atmospheric composition of exoplanets. Planets around M dwarfs, like TRAPPIST-1 c, are predicted to be particularly susceptible to atmospheric loss due to the host star's high XUV radiation during its pre-main-sequence phase. This study builds on these existing works by investigating a planet within the habitable zone of its star, potentially providing further insights into the volatile content and atmospheric evolution of planets in this region.

Four eclipses of TRAPPIST-1 c were observed using JWST's MIRI instrument with the F1500W filter (centered at 15 µm, a strong CO2 absorption band). Each observation lasted approximately 192 minutes, covering the planet's eclipse duration and out-of-eclipse baseline. Four independent data reductions were performed using publicly available software (Eureka!) and custom pipelines, employing aperture photometry to extract the light curve of TRAPPIST-1. The light curves were fitted with an eclipse model, accounting for instrumental systematic noise using polynomial fits, exponential ramps, and decorrelation against the point spread function. Markov Chain Monte Carlo (MCMC) methods were used to estimate eclipse depths, with the final depth calculated as the mean and uncertainty from the four reductions. From the eclipse depth, the dayside brightness temperature was derived. Atmospheric models were constructed and compared to the measured eclipse depth to constrain atmospheric properties. The models included a range of cloud-free O2/CO2 mixtures with varying surface pressures and CO2 concentrations, as well as pure CO2 atmospheres. A cloudy Venus-analogue atmosphere was also modeled. Additionally, various bare-rock models with different surface compositions and considering space weathering were compared with the observations. Finally, atmospheric evolution models were used to link the observed atmospheric properties to the planet's initial water inventory and the XUV flux from the host star.

The study found a planet-to-star flux ratio of 421 ± 94 ppm for TRAPPIST-1 c at 15 µm, corresponding to a dayside brightness temperature of 380 ± 31 K. This temperature is significantly cooler than previously measured for other small, rocky exoplanets. This high dayside temperature is inconsistent with a thick CO2-rich atmosphere. Specifically, the data rule out cloud-free O2/CO2 atmospheres with surface pressures ranging from 10 bar (with 10 ppm CO2) to 0.1 bar (pure CO2). A Venus-analogue atmosphere with sulfuric acid clouds is also disfavored at 2.6σ confidence. Thinner atmospheres or bare-rock surfaces are compatible with the observations. The absence of a thick CO2 atmosphere suggests a relatively volatile-poor formation history, with less than 9.57 Earth oceans of initial water. Comparison with physically motivated forward models also rules out thick atmospheres. Comparisons between measured flux and bare-rock models with various compositions (basaltic, feldspathic, Fe-oxidized, granitoid, metal-rich, and ultramafic) showed consistency across all surface types. Atmospheric evolution models further support a low initial water inventory for TRAPPIST-1 c. The upper limit on the surface pressure (10-100 bar) implies an initial water abundance of approximately 4–10 Earth oceans.

The absence of a thick CO2 atmosphere on TRAPPIST-1 c challenges expectations for planets around M dwarfs. The findings suggest that these planets may form with a lower volatile inventory or experience greater atmospheric loss than their counterparts around Sun-like stars. The intermediate brightness temperature hints at either moderate heat redistribution by a thin atmosphere or a non-zero Bond albedo for a rocky surface. The consistency of bare-rock models with the observations suggests a lack of significant atmospheric components. The low volatile inventory implied by the results could have implications for the habitability of other planets in the TRAPPIST-1 system, especially those within the habitable zone. Future observations of other TRAPPIST-1 planets are crucial to determine if a low volatile abundance is a common characteristic of the system. The study's results underscore the importance of direct atmospheric observations for characterizing exoplanets and refining planetary formation models.

This study demonstrates that TRAPPIST-1 c lacks a thick CO2-rich atmosphere, indicating a volatile-poor formation history. The findings suggest that rocky planets around M dwarfs might have a smaller volatile inventory or experience more atmospheric loss than those around Sun-like stars. Future research should focus on observations of other TRAPPIST-1 planets to determine the system's overall volatile budget and assess its implications for habitability.

The study primarily focuses on the 15 µm band, which limits the ability to detect other atmospheric constituents. The analysis assumes cloud-free atmospheres, which might not accurately represent the true atmospheric conditions. Further, the atmospheric evolution models rely on assumptions about the star's XUV flux and the planet's escape efficiency, introducing uncertainties in the estimated initial water inventory.