Abstract EANA2025-1

The TRAPPIST-1 system has already been intensely studied by the JWST in the recent years, and the first successful observations hint at that the innermost planets of the system may not even possess an atmosphere [1,2], or if then only in combination with photochemical hazes [3].

To evaluate the range of possible atmospheric pathways for the different rocky planets in the TRAPPIST-1 system and compare them with the most recent observations, we conduct a large parameter study in which we first model the potential interior thermal evolution of the TRAPPIST-1 planets using 2D mantle convection simulations, by employing planetary compositions and interior structures as derived in [4] assuming that all seven planets are rocky planets (i.e. without large fractions of volatiles), which is still in line with the observed masses and radii [4]. We take into account initially super-heated cores from the planet accretion phase, as well as heating by radiogenic decay, tidal heating and induction heating from the star [5]. Melting in the interior leads to volcanic activity, with outgassing depending on the mantle redox state, melting temperature, melt extrusion efficiency, gas speciation in the melt, solubility of volatiles in the melt, and atmospheric chemical evolution (assuming chemical equilibrium in the atmosphere). The evolving atmospheric compositions are modelled under various atmospheric escape scenarios to assess their potential survivability.

The absence of an atmosphere for the innermost planets may then be explained most easily if they possess a low redox state in their interior mantles, comparable to Mercury, leading to low-mean-molecular-weight and low-pressure atmospheres [6] that are less stable against atmospheric escape. Here we show the full range of possible secondary outgassed atmospheres in the TRAPPIST-1 predicted by our coupled thermo-chemical interior-atmosphere model.

 

[1] Greene, T.P., Bell, T.J., Ducrot, E. et al. (2023). Thermal emission from the Earth-sized exoplanet TRAPPIST-1 b using JWST. Nature 618, 39–42.

[2] Zieba, S., Kreidberg, L., Ducrot, E. et al. (2023). No thick carbon dioxide atmosphere on the rocky exoplanet TRAPPIST-1 c. Nature 620, 746–749.

[3] Ducrot, E., Lagage, P.O., Min, M. et al. (2024). Combined analysis of the 12.8 and 15 μm JWST/MIRI eclipse observations of TRAPPIST-1 b. Nature Astronomy, doi: 10.1038/s41550-024-02428-z.

[4] Carone, L., Barnes, R., Noack, L., et al. (2025). From CO2-to H2O-dominated atmospheres and back-How mixed outgassing changes the volatile distribution in magma oceans around M dwarf stars. Astronomy & Astrophysics, 693, A303.

[5] Kislyakova, K. G., Noack, L., Johnstone, et al. (2017). Magma oceans and enhanced volcanism on TRAPPIST-1 planets due to induction heating. Nature Astronomy, 1(12), 878-885.

[6] Brachmann, C., Noack, L., Baumeister, P.A., Sohl, F. (2025). Distinct types of CHON atmospheres and surface pressures depending on melt redox state and outgassing efficiency. Icarus, 429, 116450.