No thick carbon dioxide atmosphere on the rocky exoplanet TRAPPIST-1 c

Sebastian Zieba (), Laura Kreidberg, Elsa Ducrot, Michaël Gillon, Caroline Morley, Laura Schaefer, Patrick Tamburo, Daniel D. B. Koll, Xintong Lyu, Lorena Acuña, Eric Agol, Aishwarya R. Iyer, Renyu Hu, Andrew P. Lincowski, Victoria S. Meadows, Franck Selsis, Emeline Bolmont, Avi M. Mandell and Gabrielle Suissa
Additional contact information
Sebastian Zieba: Max-Planck-Institut für Astronomie
Laura Kreidberg: Max-Planck-Institut für Astronomie
Elsa Ducrot: Université Paris-Saclay, Université Paris Cité, CEA, CNRS, AIM
Michaël Gillon: University of Liège
Caroline Morley: University of Texas at Austin
Laura Schaefer: Stanford University
Patrick Tamburo: Boston University
Daniel D. B. Koll: Peking University
Xintong Lyu: Peking University
Lorena Acuña: Max-Planck-Institut für Astronomie
Eric Agol: University of Washington
Aishwarya R. Iyer: Arizona State University
Renyu Hu: California Institute of Technology
Andrew P. Lincowski: University of Washington
Victoria S. Meadows: University of Washington
Franck Selsis: Université de Bordeaux, CNRS, B18N
Emeline Bolmont: Observatoire Astronomique de l’Université de Genève
Avi M. Mandell: NASA Goddard Space Flight Center
Gabrielle Suissa: University of Washington

Nature, 2023, vol. 620, issue 7975, 746-749

Abstract: 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 System1. Thanks to the recent launch of the James Webb Space Telescope (JWST), possible atmospheric constituents such as carbon dioxide (CO2) are now detectable2,3. Recent JWST observations of the innermost planet TRAPPIST-1 b showed that it is most probably a bare rock without any CO2 in its atmosphere4. 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 µm. We measure a planet-to-star flux ratio of fp/f⁎ = 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 2.6σ 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 $${9.5}_{-2.3}^{+7.5}$$ 9.5 − 2.3 + 7.5 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.

Date: 2023
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DOI: 10.1038/s41586-023-06232-z

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