Many outstanding questions remain concerning the ultimate fate of terrestrial planets that pass through a runaway greenhouse phase, such as Venus is thought to have in its past. Developing a better understanding of this process is critical, given the concerns about the long pre-main sequence super-luminous phases of smaller host stars (Luger & Barnes, 2015; Ramirez & Kaltenegger, 2014) which may desiccate many of the exoplanets found within the conventional habitable zone. One way to begin exploring this transition is to identify low-density targets orbiting small stars; one such planet is TOI-1266c, a super-Venus orbiting an early M dwarf that resides in the ‘radius valley’ (Fulton et al., 2017). Its moderate instellation (~2.4 times what the Earth receives), combined with its low apparent density (ranging from 2.2-9.2 g/cm2), suggests that the planet may have retained a substantial volatile reservoir over the course of its lifetime. The pre-main sequence super-luminous phase of TOI-1266 is roughly 400 Myr, which in combination with the enhanced extreme ultraviolet and X-ray fluxes would drive substantial atmospheric loss. This could remove up to a few percent of the planet’s mass in hydrogen within ~4 Gyr (see Luger et al., 2015). In the context of the uncertainties in the planet’s density, this scale of atmospheric loss largely precludes a H2-dominated atmosphere at present. If TOI-1266c is then a failed ice giant without a substantial primordial atmosphere, its ice-rich core would continually supply volatiles to the “atmosphere”, leaving the planet in an effectively perpetual runaway greenhouse state.
Here, we outline the potential atmospheric states of TOI-1266c using combined radiative-convective and photochemical modeling to explore what a potential steam atmosphere might look like. Additionally, we have worked to incorporate the uncertainties in the planet’s mass and the host’s age and activity through a comprehensive suite of atmospheric escape simulations. Together, several interesting outcomes emerge (predominantly driven by the size of the assumed volatile envelope) that either would suggest the planet is a super-Venus or super-Mercury, or an unexpected window into the evolution of steam atmospheres. As such, TOI-1266c represents a unique proving ground for theories related to the evolution of sub-Neptunes and Venus-like worlds, which in turn would ground future observations of exoplanets that may have undergone similar processes, as well as Earth-like planets that may be in the process of becoming uninhabitable.
Fulton, B.J., Petigura, E.A., Howard, A.W., Isaacson, H., Marcy, G.W., Cargile, P.A., Hebb, L., Weiss, L.M., Johnson, J.A., Morton, T.D. & Sinukoff, E., 2017. The California-Kepler survey. III. A gap in the radius distribution of small planets. ApJ, 154(3), p.109.Luger, R. and Barnes, R., 2015. Extreme water loss and abiotic O2 buildup on planets throughout the habitable zones of M dwarfs. Astrobio., 15(2), pp.119-143.Luger, R., Barnes, R., Lopez, E., Fortney, J., Jackson, B. & Meadows, V., 2015. Habitable evaporated cores: transforming mini-Neptunes into super-Earths in the habitable zones of M dwarfs. Astrobio., 15(1), pp.57-88.Ramirez, R.M. & Kaltenegger, L., 2014. The habitable zones of pre-main-sequence stars. ApJL, 797(2), p.L25.