Predictions for Observable Atmospheres of Trappist-1 Planets from a Fully Coupled Atmosphere–Interior Evolution Model

Krissansen‐Totton, Joshua; Fortney, Jonathan J.

doi:10.3847/1538-4357/ac69cb

Cited by 24 publications

(21 citation statements)

References 124 publications

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“…For the composition of the atmospheres, in addition to a 100% CO 2 atmosphere, we choose to vary the abundance of trace CO 2 , at 1 ppm, 100 ppm, and 1%, against background gases of N 2 , O 2 , and H 2 O. Moreover, we also consider atmospheres containing a range of other trace gases plausible in secondary atmospheres (Turbet et al 2020;Krissansen-Totton & Fortney 2022;Whittaker et al 2022), which may not necessarily absorb at 15 μm but may be detected via observations at other wavelengths. For this purpose, we adopt the same trace abundance grids (i.e., 1 ppm, 100 ppm, 1%) for CO, CH 4 , H 2 O, and SO 2 , against a background gas of N 2 for the former two and O 2 for the latter.…”

Section: Methodsmentioning

confidence: 99%

Constraining the Thickness of TRAPPIST-1 b’s Atmosphere from Its JWST Secondary Eclipse Observation at 15 μm

Kempton

Whittaker

et al. 2023

ApJL

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Recently, the first JWST measurement of thermal emission from a rocky exoplanet was reported. The inferred dayside brightness temperature of TRAPPIST-1 b at 15 μm is consistent with the planet having no atmosphere and therefore no mechanism by which to circulate heat to its nightside. In this Letter, we compare TRAPPIST-1 b's measured secondary eclipse depth to predictions from a suite of self-consistent radiative-convective equilibrium models in order to quantify the maximum atmospheric thickness consistent with the observation. We find that plausible atmospheres (i.e., those that contain at least 100 ppm CO2) with surface pressures greater than 0.3 bar are ruled out at 3σ, regardless of the choice of background atmosphere, and a Mars-like thin atmosphere with surface pressure 6.5 mbar composed entirely of CO2 is also ruled out at 3σ. Thicker atmospheres of up to 10 bar (100 bar) are consistent with the data at 1σ (3σ) only if the atmosphere lacks any strong absorbers across the mid-IR wavelength range—a scenario that we deem unlikely. We additionally model the emission spectra for bare-rock planets of various compositions. We find that a basaltic, metal-rich, and Fe-oxidized surface best matches the measured eclipse depth to within 1σ, and the best-fit gray albedo is 0.02 ± 0.11. We conclude that planned secondary eclipse observations at 12.8 μm will serve to validate TRAPPIST-1 b's high observed brightness temperature, but are unlikely to further distinguish among the consistent atmospheric and bare-rock scenarios.

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Section: Methodsmentioning

confidence: 99%

Constraining the Thickness of TRAPPIST-1 b’s Atmosphere from Its JWST Secondary Eclipse Observation at 15 μm

Kempton

Whittaker

et al. 2023

ApJL

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“…A previously published version of the Planetary Atmospheres, Crust, MANtle (PACMAN) geochemical evolution model was adopted to simulate the evolution of the Trappist-1 planets (Krissansen-Totton & Fortney 2022). PACMAN captures the key processes that shape atmospheric evolution from magma ocean origins to potentially more temperate states (Krissansen-Totton et al 2021b;Krissansen-Totton & Fortney 2022). The model explicitly couples the time evolution of mantle oxidation state, atmosphere-interior volatile exchange-including degassing, surface weathering, and oxidation reactions-parameterized mantle convection, and thermal and nonthermal atmospheric escape to compute how atmospheric composition and surface climate evolve on Gyr timescales.…”

Section: Methodsmentioning

confidence: 99%

Implications of Atmospheric Nondetections for Trappist-1 Inner Planets on Atmospheric Retention Prospects for Outer Planets

Krissansen‐Totton

2023

ApJL

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JWST secondary eclipse observations of Trappist-1b seemingly disfavor atmospheres >∼1 bar since heat redistribution is expected to yield dayside emission temperature below the ∼500 K observed. Given the similar densities of Trappist-1 planets, and the theoretical potential for atmospheric erosion around late M dwarfs, this observation might be assumed to imply substantial atmospheres are also unlikely for the outer planets. However, the processes governing atmosphere erosion and replenishment are fundamentally different for inner and outer planets. Here, an atmosphere–interior evolution model is used to show that an airless Trappist-1b (and c) only weakly constrains stellar evolution, and that the odds of outer planets e and f retaining substantial atmospheres remain largely unchanged. This is true even if the initial volatile inventories of planets in the Trappist-1 system are highly correlated. The reason for this result is that b and c sit unambiguously interior to the runaway greenhouse limit, and so have potentially experienced ∼8 Gyr of X-ray and extreme ultraviolet–driven hydrodynamic escape; complete atmospheric erosion in this environment only weakly constrains stellar evolution and escape parameterizations. In contrast, e and f reside within the habitable zone, and likely experienced a comparatively short steam atmosphere during Trappist-1's pre-main sequence, and consequently complete atmospheric erosion remains unlikely across a broad swath of parameter space (e and f retain atmospheres in ∼98% of model runs). Naturally, it is still possible that all Trappist-1 planets formed volatile-poor and are all airless today. But the airlessness of b (and c) does not require this, and as such, JWST transit spectroscopy of e and f remains the best near-term opportunity to characterize the atmospheres of habitable zone terrestrial planets.

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“…However, due to the low abundances of Ca and Al compared to Fe, Mg, and Si, these planets (if they exist) will mostly be very small. Lastly, water worlds have been predicted, i.e., terrestrial planets with a thick layer of water and ice on the surface (e.g., Kuchner 2003;Unterborn et al 2018;Acuna et al 2021;Krissansen-Totton & Fortney 2022). If such a layer of water is sufficiently large, it could fundamentally alter the planetary interior dynamics by suppressing mantle melting and crust formation (Unterborn et al 2018).…”

Section: Devolatilizationmentioning

confidence: 99%

Plausible Constraints on the Range of Bulk Terrestrial Exoplanet Compositions in the Solar Neighborhood

Spaargaren

Wang

Mojzsis

et al. 2023

ApJ

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Rocky planet compositions regulate planetary evolution by affecting core sizes, mantle properties, and melting behaviors. Yet, quantitative treatments of this aspect of exoplanet studies remain generally underexplored. We attempt to constrain the range of potential bulk terrestrial exoplanet compositions in the solar neighborhood (<200 pc). We circumscribe probable rocky exoplanet compositions based on a population analysis of stellar chemical abundances from the Hypatia and GALAH catalogs. We apply a devolatilization model to simulate compositions of hypothetical, terrestrial-type exoplanets in the habitable zones around Sun-like stars, considering elements O, S, Na, Si, Mg, Fe, Ni, Ca, and Al. We further apply core–mantle differentiation by assuming constant oxygen fugacity, and model the consequent mantle mineralogy with a Gibbs energy minimization algorithm. We report statistics on several compositional parameters and propose a reference set of (21) representative planet compositions for use as end-member compositions in imminent modeling and experimental studies. We find a strong correlation between stellar Fe/Mg and metallic-core sizes, which can vary from 18 to 35 wt%. Furthermore, stellar Mg/Si gives a first-order indication of mantle mineralogy, with high-Mg/Si stars leading to weaker, ferropericlase-rich mantles, and low-Mg/Si stars leading to mechanically stronger mantles. The element Na, which modulates crustal buoyancy and mantle clinopyroxene fraction, is affected by devolatilization the most. While we find that planetary mantles mostly consist of Fe/Mg silicates, the core sizes and relative abundances of common minerals can nevertheless vary significantly among exoplanets. These differences likely lead to different evolutionary pathways among rocky exoplanets in the solar neighborhood.

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Predictions for Observable Atmospheres of Trappist-1 Planets from a Fully Coupled Atmosphere–Interior Evolution Model

Cited by 24 publications

References 124 publications

Constraining the Thickness of TRAPPIST-1 b’s Atmosphere from Its JWST Secondary Eclipse Observation at 15 μm

Constraining the Thickness of TRAPPIST-1 b’s Atmosphere from Its JWST Secondary Eclipse Observation at 15 μm

Implications of Atmospheric Nondetections for Trappist-1 Inner Planets on Atmospheric Retention Prospects for Outer Planets

Plausible Constraints on the Range of Bulk Terrestrial Exoplanet Compositions in the Solar Neighborhood

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