No thick carbon dioxide atmosphere on the rocky exoplanet TRAPPIST-1 c (2306.10150v1)

Abstract: Seven rocky planets orbit the nearby dwarf star TRAPPIST-1, providing a unique opportunity to search for atmospheres on small planets outside the Solar System (Gillon et al., 2017). Thanks to the recent launch of JWST, possible atmospheric constituents such as carbon dioxide (CO2) are now detectable (Morley et al., 2017, Lincowski et al., 2018}. Recent JWST observations of the innermost planet TRAPPIST-1 b showed that it is most probably a bare rock without any CO2 in its atmosphere (Greene et al., 2023). Here we report the detection of thermal emission from the dayside of TRAPPIST-1 c with the Mid-Infrared Instrument (MIRI) on JWST at 15 micron. We measure a planet-to-star flux ratio of fp/fs = 421 +/- 94 parts per million (ppm) which corresponds to an inferred dayside brightness temperature of 380 +/- 31 K. This high dayside temperature disfavours a thick, CO2-rich atmosphere on the planet. The data rule out cloud-free O2/CO2 mixtures 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 disfavoured at 2.6 sigma confidence. Thinner atmospheres or bare-rock surfaces are consistent with our measured planet-to-star flux ratio. The absence of a thick, CO2-rich atmosphere on TRAPPIST-1 c suggests a relatively volatile-poor formation history, with less than 9.5 +7.5 -2.3 Earth oceans of water. If all planets in the system formed in the same way, this would indicate a limited reservoir of volatiles for the potentially habitable planets in the system.

Analysis of the Atmosphere of TRAPPIST-1c Using JWST Observations

The paper conducted on TRAPPIST-1c, a rocky exoplanet orbiting the nearby dwarf star TRAPPIST-1, provides significant insights into the planetary atmosphere characteristics using data from the James Webb Space Telescope (JWST). One of the primary findings of this research is the absence of a thick carbon dioxide (CO₂)-rich atmosphere on TRAPPIST-1c, inferred from thermal emissions detected through the Mid-Infrared Instrument (MIRI) on JWST. The planet-to-star flux ratio is measured at 421 ± 94 ppm, correlating with a calculated brightness temperature of 380±31 Kelvin.

Key Findings

• Atmospheric Thickness: The high dayside temperature of TRAPPIST-1c suggests a lack of a thick CO₂ atmosphere. The data accumulatively disfavors certain atmospheric models such as cloud-free O₂/CO₂ mixtures with surface pressures from 10 bar (10 ppm CO₂) to 0.1 bar (pure CO₂). A Venus-like atmosphere with sulfuric acid clouds is also ruled out with a confidence level of 2.6σ.
• Volatile Composition: The absence of a thick CO₂ atmosphere indicates a formation scenario with limited volatile inventory, potentially comprising less than 9.5 Earth oceans of water. If applicable across the entire TRAPPIST-1 planetary system, this implies restricted volatile reservoirs for other planets, potentially including those in the habitable zone.
• Observational Techniques and Data Analysis: Four independent data reductions were applied using both public and custom software pipelines. Techniques included aperture photometry, polynomial fits, and Markov chain Monte Carlo (MCMC) methods, ensuring consistent results across methodologies with variations in residuals ranging within 938 - 1,079 ppm. The final eclipse measurement of 421ppm was determined, accounting for systematic uncertainties through robust fitting methodologies.

Implications for Exoplanet Research

The paper critically advances the understanding of exoplanet atmospheres, particularly for terrestrial planets around ultra-cool dwarfs. The reliance on thermal emission observations, rather than indirect methods, underscores the importance of direct observational techniques for accurately characterizing atmospheric composition. The constraints provided by the absence of a thick CO₂ layer have broad implications for models on planetary formation and atmospheric evolution, suggesting less atmospheric retention than initially theorized.

Prospects for Further Research

This investigation opens the discourse on how planets in similar orbits might evolve atmospherically in constrained volatile inventory settings. Further scrutiny of the TRAPPIST-1 planetary system is recommended, focusing on the potentially habitable planets to validate the formation hypothesis of limited volatile abundance. Moreover, extending JWST observational capabilities to other exoplanet systems with similar characteristics could greatly enhance the understanding of atmospheric dynamics in various environmental and orbital contexts.

The constraints derived from TRAPPIST-1c's dayside temperature encourage ongoing exploration of cooler planets which might more feasibly retain their atmospheres, potentially providing vital clues to atmosphere formation mechanisms and loss processes across small rocky planets. Ultimately, enhanced models integrating observational data will aid in evaluating the prospects for habitability in diverse planetary systems.