Water loss from Earth-sized planets in the habitable zones of ultracool dwarfs: Implications for the planets of TRAPPIST-1
Bolmont et al
Ultracool dwarfs (UCD) encompass the population of extremely low mass stars (later than M6-type) and brown dwarfs. Because UCDs cool monotonically, their habitable zone (HZ) sweeps inward in time. Assuming they possess water, planets found in the HZ of UCDs have experienced a runaway greenhouse phase too hot for liquid water prior to entering the HZ. It has been proposed that such planets are desiccated by this hot early phase and enter the HZ as dry, inhospitable worlds. Here we model the water loss during this pre-HZ hot phase taking into account recent upper limits on the XUV emission of UCDs and using 1D radiation-hydrodynamic simulations. We address the whole range of UCDs but also focus on the planets b, c and d recently found around the 0.08 M⊙ dwarf TRAPPIST-1.
Despite assumptions maximizing the FUV-photolysis of water and the XUV-driven escape of hydrogen, we find that planets can retain significant amounts of water in the HZ of UCDs, with a sweet spot in the 0.04-0.06 M⊙ range. With our assumptions, TRAPPIST-1b and c can lose as much as 4 Earth Ocean but planet d - which may be inside the HZ depending on its actual period - may have kept enough water to remain habitable depending on its initial content. TRAPPIST-1 planets are key targets for atmospheric characterization and could provide strong constraints on the water erosion around UCDs.