Promise and Peril: Stellar Contamination and Strict Limits on the Atmosphere Composition of TRAPPIST-1c from JWST NIRISS Transmission Spectra (2409.19333v3)

Abstract: Attempts to probe the atmospheres of rocky planets around M dwarfs present both promise and peril. While their favorable planet-to-star radius ratios enable searches for even thin secondary atmospheres, their high activity levels and high-energy outputs threaten atmosphere survival. Here, we present the 0.6--2.85\,$\mu$m transmission spectrum of the 1.1\,$\rm R_\oplus$, $\sim$340\,K rocky planet TRAPPIST-1\,c obtained over two JWST NIRISS/SOSS transit observations. Each of the two spectra displays 100--500\,ppm signatures of stellar contamination. Despite being separated by 367\,days, the retrieved spot and faculae properties are consistent between the two visits, resulting in nearly identical transmission spectra. Jointly retrieving for stellar contamination and a planetary atmosphere reveals that our spectrum can rule out hydrogen-dominated, $\lesssim$300$\times$ solar metallicity atmospheres with effective surface pressures down to 10\,mbar at the 3-$\sigma$ level. For high-mean molecular weight atmospheres, where O$_2$ or N$_2$ is the background gas, our spectrum disfavors partial pressures of more than $\sim$10\,mbar for H$_2$O, CO, NH$_3$ and CH$_4$ at the 2-$\sigma$ level. Similarly, under the assumption of a 100\% H$_2$O, NH$_3$, CO, or CH$_4$ atmosphere, our spectrum disfavors thick, $>$1\,bar atmospheres at the 2-$\sigma$ level. These non-detections of spectral features are in line with predictions that even heavier, CO$_2$-rich, atmospheres would be efficiently lost on TRAPPIST-1\,c given the cumulative high-energy irradiation experienced by the planet. Our results further stress the importance of robustly accounting for stellar contamination when analyzing JWST observations of exo-Earths around M dwarfs, as well as the need for high-fidelity stellar models to search for the potential signals of thin secondary atmospheres.

• The paper analyzes TRAPPIST-1c's atmosphere using JWST NIRISS transmission spectra, addressing challenges posed by stellar contamination via models accounting for effects like the Transit Light Source Effect (TLSE).
• Analysis places stringent constraints on atmospheric composition, ruling out thick, low-metallicity hydrogen-dominated atmospheres for TRAPPIST-1c with over 3σ confidence due to the absence of clear molecular absorption features.
• The study highlights the critical need for enhanced precision in stellar modeling to accurately distinguish planetary atmospheric signatures from the significant variability of active M dwarf host stars.

Analysis of Stellar Contamination and Atmospheric Composition of TRAPPIST-1c Using JWST NIRISS

The paper conducted by Radica et al. provides an insightful examination into the atmospheric properties of TRAPPIST-1c, a rocky exoplanet located within the TRAPPIST-1 system, utilizing transmission spectra gathered via the James Webb Space Telescope (JWST) NIRISS instrument. This investigation addresses the complexities introduced by stellar contamination and breathtakingly explores the atmospheric limits through high-resolution spectroscopic observations.

TRAPPIST-1c presents a unique opportunity for atmospheric characterization due to its transit across an ultracool M dwarf star, where the planet-to-star radius ratio amplifies potential atmospheric signals. However, the high activity level associated with its host M dwarf poses a significant challenge for atmospheric retention and the discernment of planetary signals amidst stellar noise.

Key Findings and Methodological Observations

1. Stellar Contamination Observations: The research employs advanced models to account for the effects of the Transit Light Source Effect (TLSE), observing that stellar heterogeneities such as spots and faculae substantially impact the transmission spectra of TRAPPIST-1c. The paper retrieves parameters consistent between two distinct observational visits, highlighting the importance of recognizing stellar influences in transmission spectroscopy.
2. Transmission Spectrum Analysis: The 0.6–2.85 μm transmission spectrum of TRAPPIST-1c points to the absence of clear molecular absorption features that would indicate the presence of a substantial atmosphere. Importantly, stringent constraints are placed on the presence of thick, low-metallicity hydrogen-dominated atmospheres, ruling them out with more than 3σ confidence.
3. Potential Atmospheric Scenarios: Efforts to account for potential CO₂-rich or CH₄-dominated atmospheres suggest these scenarios remain plausible within certain statistical thresholds. However, the likelihood of these atmosphere types is reduced when considering photochemical degradation of CH₄ and atmospheric escape mechanisms for CO₂, driven by historical high-energy stellar irradiation.
4. Theoretical Implications: While the stellar contamination model robustly addresses the invariance of stellar activity effects between the two visits, it underscores the ongoing necessity for accurate stellar models. This is particularly relevant for the characterization of exoplanets orbiting highly active M dwarfs where stellar variability can obscure atmospheric signatures.

Implications and Future Directions

This work signifies an important step in addressing the methodological challenges posed by stellar contamination in exoplanetary atmospheric studies. It emphasizes the need for enhanced precision in stellar modeling to resolve the impact of host star activity on measurements, especially in multi-epoch observations using next-generation telescopes like the JWST.

The paper also highlights the importance of integrating photometric and spectroscopic data within a comprehensive atmospheric retrieval framework, allowing for more definitive conclusions about the presence and composition of thin, high-mean molecular weight atmospheres—potentially providing critical insights into the formation and evolution pathways of terrestrial exoplanets.

The findings of Radica et al. set the stage for more targeted observational campaigns and theoretical work aimed at better constraining the atmospheres of planets within the habitable zones of M dwarfs. Future observations with additional instruments, such as JWST's NIRSpec, could potentially leverage alternative spectral features to delineate clearer insights into the atmospheric compositions, and thereby increase the detectability prospects of secondary atmospheres for exoplanets around M-type stars.

In conclusion, this paper methodically demonstrates the dual challenges and possibilities inherent in the search for atmospheric constituents around rocky exoplanets in proximity to active host stars, carving a path forward for both observational strategies and theoretical research in this dynamic field of astrophysical sciences.