A Snowball in Hell: The Potential Steam Atmosphere of TOI-1266c

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.

References:

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.