Climate diversity on cool planets around cool stars with a versatile 3-D Global Climate Model: the case of TRAPPIST-1 planets

TRAPPIST-1 planets recently discovered by Gillon et al. (2016,2017) are the closest known transiting temperate Earth-size exoplanets. The TRAPPIST-1 system hosts at least seven planets that are similar in size (at +/- 30%) and irradiation to Solar System rocky planets. Therefore, TRAPPIST-1 planets are invaluable for the study of comparative planetary science outside our Solar System and possibly habitability. Here we used N-body dynamical simulations and 3-D global climate simulations to investigate this fascinating planetary system in details.

Artist view (on left) of the possible TRAPPIST-1 planets, depending on the composition of their atmosphere; Phase diagram (on right) of the volatile species that could be present in the atmosphere/surface of the TRAPPIST-1 outer planets, Credit: M. Turbet et al.

First, we show that all the seven TRAPPIST-1 planets should be tidally locked, i.e. with one side permanently facing their host star. Then, using a 3-D Global Climate Model, 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.

Even though background gases such as N2, CO or O2 are resistant to atmospheric collapse, it should be difficult for TRAPPIST-1 planets to accumulate significant greenhouse gases like CO2 , CH4 or NH3 . CO2 can easily condense on the permanent nightside, forming CO2 ice glaciers that would flow toward the substellar region. A complete CO2 ice surface cover is theoretically possible on TRAPPIST-1g and h only. However, the CO2 ice deposits 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 1000 Titan's flux), CH4 and NH3 should be photodissociated rapidly and thus be 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 CO2 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.

By Martin Turbet | Design by Andreas Viklund | Inspired by Aymeric Spiga