Limits to Habitability of Exoplanets From Tidal Heating

Tidal Venuses: Triggering a Climate Catastrophe via Tidal Heating

Authors:

1. Rory Barnes (a,b)
2. Kristina Mullins (a,b)
3. Colin Goldblatt (a,b,c)
4. Victoria S. Meadows (a,b)
5. James F. Kasting (b,d)
6. René Heller (e)

Affiliations:

a. Astronomy Department, University of Washington, Seattle, Washington, USA

b. NASA Astrobiology Institute–Virtual Planetary Laboratory Lead Team, USA

c. Department of Earth and Ocean Science, University of Victoria, Victoria, Canada

d. Department of Geosciences, The Pennsylvania State University, State College, Pennsylvania, USA.

e. Leibniz Institute for Astrophysics Potsdam (AIP), Potsdam, Germany

Abstract:

Traditionally, stellar radiation has been the only heat source considered capable of determining global climate on long timescales. Here, we show that terrestrial exoplanets orbiting low-mass stars may be tidally heated at high-enough levels to induce a runaway greenhouse for a long-enough duration for all the hydrogen to escape. Without hydrogen, the planet no longer has water and cannot support life. We call these planets “Tidal Venuses” and the phenomenon a “tidal greenhouse.” Tidal effects also circularize the orbit, which decreases tidal heating. Hence, some planets may form with large eccentricity, with its accompanying large tidal heating, and lose their water, but eventually settle into nearly circular orbits (i.e., with negligible tidal heating) in the habitable zone (HZ). However, these planets are not habitable, as past tidal heating desiccated them, and hence should not be ranked highly for detailed follow-up observations aimed at detecting biosignatures. We simulated the evolution of hypothetical planetary systems in a quasi-continuous parameter distribution and found that we could constrain the history of the system by statistical arguments. Planets orbiting stars with masses less than 0.3 MSun may be in danger of desiccation via tidal heating. We have applied these concepts to Gl 667C c, a 4.5 MEarth planet orbiting a 0.3 MSun star at 0.12 AU. We found that it probably did not lose its water via tidal heating, as orbital stability is unlikely for the high eccentricities required for the tidal greenhouse. As the inner edge of the HZ is defined by the onset of a runaway or moist greenhouse powered by radiation, our results represent a fundamental revision to the HZ for noncircular orbits. In the appendices we review (a) the moist and runaway greenhouses, (b) hydrogen escape, (c) stellar mass-radius and mass-luminosity relations, (d) terrestrial planet mass-radius relations, and (e) linear tidal theories.