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The discovery of seven Earth-sized planets around the ultracool dwarf star TRAPPIST-1 in 2017 brought a new type of planetary system to our attention. Until new discoveries, the TRAPPIST-1 system serves as a prototype of an ultracool M dwarf with a planetary system of Earth-like planets. Due to their proximity to the parent star and eccentric orbits, the TRAPPIST-1 planets are heated from sunlight but also from within by tidal dissipation. They can be habitable only if they avoid entering a runaway greenhouse state, in which all of their surface water would irreversibly evaporate into a thick atmosphere. We constrain the possible compositions and interior structures of each planet, and calculate the tidal heating arising from the star. Our results show that any surface H2O on planets b and c has likely evaporated to form an atmosphere, and rock mantles in these planets are partially molten, which may result in surface volcanism. Planet d might avoid the runaway greenhouse state, if its Bond albedo is ≳0.3, implying that planets d and e might be habitable. Planets f and g are too far from the star to experience significant tidal heating and likely have solid ice surfaces with possible subsurface liquid water oceans.

Tidal heating and the habitability of the TRAPPIST-1 planets (ExoplanetsIII conference)