Modeling climate diversity, tidal dynamics and the fate of volatiles on TRAPPIST-1 planets

Abstract

TRAPPIST-1 planets are invaluable for the study of comparative planetary science outside our Solar System and possibly habitability. Both Time Transit Variations (TTV) of the planets and the compact, resonant architecture of the system suggest that TRAPPIST-1 planets could be endowed with various volatiles today. First, we derive from N-body simulations possible planetary evolution scenarios, and show that all the planets are likely in synchronous rotation. We then use a versatile 3-D Global Climate Model (GCM) to explore the possible climates of cool planets around cool stars, with a focus on the TRAPPIST-1 system. We look at the conditions required for cool planets to prevent possible volatile species to be lost permanently by surface condensation, irreversible burying or photochemical destruction. We also explore the resilience of the same volatiles (when in condensed phase) to a runaway greenhouse process. We find that background atmospheres made of N 2 , CO or O 2 are rather resistant to atmospheric collapse. However, even if TRAPPIST-1 planets were able to sustain a thick background atmosphere by surviving early X/UV radiation and stellar wind atmospheric erosion, it is difficult for them to accumulate significant greenhouse gases like CO 2 , CH 4 or NH 3 . CO 2 can easily condense on the permanent nightside, forming CO 2 ice glaciers that would flow toward the substellar region. A complete CO 2 ice surface cover is theoretically possible on TRAPPIST-1g and h only, but CO 2 ices should be gravitationally unstable and get buried beneath the water ice shell in geologically short timescales. Given TRAPPIST-1 planets large EUV irradiation (at least ∼ 10 3 × Titan's flux), CH 4 and NH 3 are photodissociated rapidly and are thus hard to accumulate in the atmosphere. Photochemical hazes could then sedimentate and form a surface layer of tholins that would progressively thicken over the age of the TRAPPIST-1 system. Regarding habitability, we confirm that few bars of CO 2 would suffice to warm the surface of TRAPPIST-1f and g above the melting point of water. We also show that TRAPPIST-1e is a remarkable candidate for surface habitability. If the planet is today synchronous and abundant in water, then it should always sustain surface liquid water at least in the substellar region, whatever the atmosphere considered.

Transition from eyeball to snowball driven by sea-ice drift on tidally locked terrestrial planets

“…For a spin-orbit resonance orbit, such as 3:2 like Mercury, all longitudes receive stellar radiation but time mean instellation at the equator is much lower than that at the permanent substellar point of a 1:1 orbit. In the experiments without sea-ice drift, Proxima b and TRAPPIST-1e exhibit a climate with tropical waterbelts 6,[8][9][10] 14 & 15). Rotation period determines the length of day and night (L !…”

“…In the 3D atmosphere-only climate experiments (the roles of ocean and sea ice dynamics are excluded), the planets have an ocean of open water under the substellar point while the rest regions are ice-covered, called an eyeball climate state [5][6][7][8][9] (Fig. 1 and Supplementary Figs.…”

TRAPPIST-1 Habitable Atmosphere Intercomparison (THAI): motivations and protocol version 1.0

“…However, Bolmont et al (2017) and Bourrier et al (2017) argued that depending on their initial water contents, the TRAPPIST-1 planets could have retained some water presently. Assuming that this water has remained in sufficient quantity, TRAPPIST-1e may be able to maintain habitable conditions (locally or globally around the planet) through a very large set of atmospheric configurations (Wolf, 2017;Turbet et al, 2018;Grootel et al, 2018, and references therein). The first attempt to characterize those planets through transmission spectroscopy has been conducted by de Wit et al (2016de Wit et al ( , 2018 using the Hubble Space Telescope (HST) for the six innermost planets.…”

On the stability of a climate model for an Earth-like planet with land-ocean coverage

“…Notwithstanding its simplicity, the EBM model proposed here represents, indeed, a useful tool to study the climate of Earth-like exoplanets. Of course, the model has some substantial limitations with respect to more detailed and complete models which include, for example, a description of the planet's atmosphere, such as GCM models [15][16][17][18][19][20][21][22][23]. On the other hand, taking into account that the physical information available on exoplanetary systems is typically limited, the present model represent a useful complementary tool for the study of exoplanets, as it allows to investigate in a simple way a large set of possible cases obtained for different irradiance values, ocean-land fractions, and vegetation coverages.…”

“…The first climate model used to investigate exoplanet climate was those by [14] including a single column atmosphere. More recently, also general circulation models (GCMs) have been used to characterize the possible habitability [15][16][17][18][19][20][21][22][23]. Although containing strong simplifications, EBMs allow to explore a large set of variables inside the parameter space and, in particular, they include the effects of vegetation that distinguishes them from the more complex and complete GCMs.…”