Tidal locking of habitable exoplanets
Abstract
Potentially habitable planets can orbit close enough to their host star that the differential gravity across their diameters can produce an elongated shape. Frictional forces inside the planet prevent the bulges from aligning perfectly with the host star and result in torques that alter the planet's rotational angular momentum. Eventually the tidal torques fix the rotation rate at a specific frequency, a process called tidal locking. Tidally locked planets on circular orbits will rotate synchronously, but those on eccentric orbits will either librate or rotate super-synchronously. Although these features of tidal theory are well known, a systematic survey of the rotational evolution of potentially habitable exoplanets using classic equilibrium tide theories has not been undertaken. I calculate how habitable planets evolve under two commonly used models and find, for example, that one model predicts that the Earth's rotation rate would have synchronized after 4.5 Gyr if its initial rotation period was 3 days, it had no satellites, and it always maintained the modern Earth's tidal properties. Lower mass stellar hosts will induce stronger tidal effects on potentially habitable planets, and tidal locking is possible for most planets in the habitable zones of GKM dwarf stars. For fast-rotating planets, both models predict eccentricity growth and that circularization can only occur once the rotational frequency is similar to the orbital frequency. The orbits of potentially habitable planets of very late M dwarfs (
- Publication:
-
Celestial Mechanics and Dynamical Astronomy
- Pub Date:
- December 2017
- DOI:
- arXiv:
- arXiv:1708.02981
- Bibcode:
- 2017CeMDA.129..509B
- Keywords:
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- Dissipative forces;
- Planetary systems;
- Rotation;
- Extended body dynamics;
- Astrophysics - Earth and Planetary Astrophysics
- E-Print:
- 40 pages, 11 figures, 3 tables (including the online tables). Accepted to Celestial Mechanics and Dynamical Astronomy. Source code to integrate the tidal evolution equations and to generate the figures and tables are available at https://github.com/RoryBarnes