Tidal Venuses: Triggering a Climate Catastrophe via Tidal Heating

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. Planets orbiting stars with masses <0.3 solar masses may be in danger of desiccation via tidal heating. We apply these concepts to Gl 667C c, a ~4.5 Earth-mass planet orbiting a 0.3 solar mass star at 0.12 AU. We find 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 non-circular 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. [abridged]

• The paper demonstrates that tidal heating from low-mass stars can induce runaway greenhouse effects, leading to the desiccation of terrestrial exoplanets.
• It employs simulations of orbital dynamics, including varying eccentricities, to identify critical heat flux thresholds jeopardizing habitability.
• The findings advocate for revising traditional habitable zone definitions by incorporating tidal influences in exoplanet assessments.

The Impact of Tidal Heating on Exoplanetary Habitability: Insights from "Tidal Venuses: Triggering a Climate Catastrophe via Tidal Heating"

The paper "Tidal Venuses: Triggering a Climate Catastrophe via Tidal Heating" explores the potential for tidal heating to influence the habitability of terrestrial exoplanets orbiting low-mass stars. The primary focus is on how tidal forces can generate sufficient heat to induce a runaway greenhouse effect, thereby desiccating the planet and rendering it uninhabitable. The authors introduce the term "Tidal Venuses" to describe planets that have lost their water through such heating and propose the term "tidal greenhouse" to denote this phenomenon.

Key Concepts and Methodology

Traditionally, stellar radiation has been the dominant heat source considered in determining exoplanet climates over long timescales. However, this paper emphasizes that tidal heating, particularly for planets orbiting low-mass stars, can be significant enough to trigger a runaway greenhouse effect. This occurs when tidal forces generate heat at a rate that induces a persistent greenhouse state, allowing all hydrogen to escape the atmosphere, thus eliminating any chance for liquid water and, consequently, life.

To substantiate their hypothesis, the authors simulate the evolutionary paths of hypothetical planetary systems around low-mass stars. These simulations consider various orbital parameters, mass distributions, and initial eccentricities to determine whether the planets could have maintained habitable conditions. The results suggest that for stars with masses less than approximately 0.3 solar masses, there is a substantial risk of planets becoming desiccated through tidal heating.

Numerical Results and Implications

The paper provides a detailed analysis showing that tidal heating can, in certain cases, produce heat fluxes greater than critical limits necessary for inducing a runaway greenhouse. It is noted that the susceptibility to tidal heating is closely tied to the planet's eccentricity and proximity to its host star. The authors further apply their model to an exoplanet in the Gliese 667C system, concluding that it likely did not lose its water through tidal heating, given its orbital stability and low eccentricity.

Theoretical and Practical Implications

Theoretically, these findings prompt a reconsideration of the habitable zone (HZ) concept when applied to planets with non-circular orbits, especially those orbiting low-mass stars. The traditional insolation-driven HZ may not account for the significant role of tidal influences in such systems. Practically, the paper suggests that exoplanets previously considered candidates for life due to their position in the HZ might actually be inhospitable if past tidal heating has already desiccated them.

Future Directions

The research underscores the need for comprehensive models that integrate both stellar and tidal heating effects when evaluating exoplanet habitability. Future work could benefit from tighter constraints on tidal dissipation parameters and improved models of planetary atmospheres under intense heating conditions. Additionally, as observational capabilities advance, such as with the James Webb Space Telescope, there will be opportunities to refine these models with empirical data, enhancing our understanding of exoplanetary environments.

In summary, the paper provides significant insights into the role of tidal forces in shaping exoplanetary climates. By identifying a new category of potentially uninhabitable planets—Tidal Venuses—it challenges existing frameworks and calls for a more nuanced approach to assessing exoplanet habitability, particularly in systems with low-mass stellar hosts.